A guide to larvae and juveniles of some common fish species from the Mekong River Basin

This guide provides descriptions and illustrations for larval and juvenile stages of 64 indigenous Mekong fishes, most of which are important in fisheries and some of which have high conservation significance.

We would also like to thank the biologists of the Department of Fisheries, Thailand, who collected specimens from hatcheries, and colleagues in the Thai Department of Fisheries who have graciously allowed the senior author to work on preparing material for this document over several years. Also, many thanks to the Fisheries Programme of the MRC, which supported several field trips to collect wild specimens for preparing the illustrations.
We would like to thank the former staff of the Assessment of Mekong Fisheries component of the MRC Fisheries Programme for their support and encouragement. Special thanks are due to the Mekong Fish Database team members in Udon Thani, who provided valuable support, which included scanning pictures and entering data. In particular, we would like to thank Ekkapon Udommongkhonkit who prepared the first draft of this report based on the species information and drawings. We also thank Ms Siriwan Suksri, Ms Juthamas Jivaluk and Ms Apiradee Hawongkittkul who proofread and improved the text. Dr Tom Trnski of the Australian Museum is gratefully acknowledged for assistance with technical editing. All line drawings and pictures of adult fish were reproduced from various sources, but mainly from Fishes of the Cambodian Mekong (Rainboth, 1996) and are reproduced here with the permission of the FAO. All larvae illustrations in this publication were drawn by the senior author. Next to each plate or colour photograph its author is individually acknowledged.
The preparation of this paper was facilitated by the MRC Fisheries Programme with funding from DANIDA and SIDA. The guides for each species include hand-drawn figures of the stages of development of each species from early larvae, through pre-larvae and post-larvae to juvenile fish. The descriptions and tabulations cover important diagnostic features, including morphology, meristics and pigmentation. The guide also summarises some basic information on classification, size, ecology, biology and conservation status for each species.
The book will be useful for anyone involved in monitoring or surveys of the Mekong basin's fishes. Accurate identification is required in all ichthyological studies. In many studies, for example of migration and spawning, it is particularly important to be able to identify larvae and juveniles. This guide will also support those involved in applied research, such as on the impacts of hydroelectric and irrigation dams on fish spawning and recruitment, as well as in aquaculture and other fields. Much basic work will be facilitated by the availability of this manual and it is hoped that many similar guidebooks will be produced to enhance the quality of research on Mekong fisheries.

Introduction
The Mekong River originates in China in the upper Mekong Basin, then flows through five other countries (Myanmar, Lao PDR, Thailand, Cambodia and Viet Nam) in the Lower Mekong Basin to discharge to the South China Sea. It is one of the world's largest river systems, with a catchment area of about 795,000 km 2 and a mean annual discharge of about 475 km 3 (MRC, 2010). The Mekong River basin supports one of the world's largest inland capture fisheries, a resource that provides food and livelihoods for millions of people (MRC, 2010). Maintaining the productivity of the system requires a good understanding of fishes' life cycles, their migratory habits, as well as their dependence on different habitats at different stages in their lives.
Accurate identification of fish species at all stages from larva to adult is necessary to support the ichthyological studies which provide basic information for management. Several guides have been recently published for the identification of the adult or sub-adult stages of fishes of the Mekong Basin (e.g. Kottelat, 2001;Rainboth, 1996 andVidthayanon, 2008). By contrast, there is little or no published information to assist in identification of larvae or juveniles, as is the case generally for fishes of inland tropical waters. Existing regional guides to larvae and juveniles (e.g. Leis and Carson-Ewart, 2000) cover mainly marine species, so they are useful only for identifying some of the coastal fishes that penetrate inland waters, or for identifying to family level some freshwater representatives of marine fish families. There are about 850 fish species recorded from the Mekong basin, and about two thirds of these (including most of the common species) are from purely freshwater families (Hortle, 2009), so there is a very large gap in the information that can be used to identify larvae and juveniles of Mekong basin fishes.
The Mekong River Commission has actively sponsored basin-wide fisheries research since the mid-1990s, including local ecological knowledge surveys, logbook monitoring of fisher catches, household surveys, catch assessment surveys, sampling of larvae and juveniles and research on aquaculture of indigenous species. The results have been widely publicized and as a result the importance of fisheries in the Mekong basin is now well-recognised. At the same time, many counterpart staff from the fisheries agencies of the Lower Mekong Basin countries have been trained, including in identification of fish larvae and juveniles. This guidebook includes in a systematic form much of the diagnostic information used during the MRC-sponsored studies of larvae and juveniles.
This guide is primarily the result of studies by Dr Apichart Termvidchakorn based on samples of fish collected in the Mekong basin in Cambodia, Thailand and Viet Nam. For each species, a series of specimens at various stages was built up, and then for each stage the important diagnostic features were measured and/or counted, and drawings were made of each stage to provide a representation of a typical fish at that stage. For most species, a series of specimens was also obtained from aquaculture fish for which the identity was certain so that there would be no doubt as to the identity of the immature stages. Measurements of smaller specimens or characters (< 1 cm) were made using an eyepiece micrometer (accurate to 0.01 mm), and to measure larger specimens or characters (> 1 cm) a dial calliper (accurate to 0.1 mm) was used. Drawings were made using a camera lucida where necessary to ensure accurate depictions of shape and proportion. This publication covers 64 species known from the basin, as well as one introduced species. It is intended to be the first in a series of publications on the larvae and juveniles of Mekong basin fishes. The guide is expected to be widely used in the Mekong basin for ichthyological research, which is expected to become increasingly important as the basin becomes more developed. In particular, information is needed to manage the impacts caused by dams that block fish migration pathways and modify rivers.
The study of fish larvae and juveniles is also necessary in development of aquaculture and in many other applied research fields. The Mekong is a regional hotspot for biodiversity, and several of the fish featured in this guide are listed on the IUCN Red List of threatened species; these are the giant barb, Catlocarpio siamensis (listed as critically endangered in 2006 but not currently listed), Jullien's barb, Probarbus jullieni (endangered) and Bocourt's catfish, Mystus bocourti (vulnerable). Unfortunately, as a result of lack of basic research, the conservation status of most of the species covered in this manual and many more Mekong species cannot be evaluated at present, highlighting the need for manuals of this type to support basic research.
All information contained in this publication and more is available in electronic format in the Mekong Fish Database 2003(MRC, 2003 available from the Mekong River Commission Secretariat. Various guides to fish larvae in Thai language are produced by the senior author and colleagues (e.g. Piamthipmanus et al., 2004;Termvidchakorn, 2003Termvidchakorn, , 2005Termvidchakorn et al., 2005). Reports on fish distribution with various biological observations are also published regularly in Thai language (e.g. Tungmas et al., 2004).

Fish reproduction and development
Fish have a wide array of reproductive behaviours, but they can be broadly classed as (1) nonguarders, (2) guarders or (3) bearers, as summarised in Moyle and Cech (2004). The majority of Mekong species are non-guarders, i.e. after their eggs are spawned they are not protected by the parents. Within this group, Mekong fishes may be classed as pelagic or benthic (demersal) spawners.
Some of the common lowland river fishes spawn pelagic eggs, which can drift with rising waters. Pelagic eggs are buoyant or semi-buoyant as they contain oil globules and have high water content. Pelagic eggs are very small (about 0.5-1.2 mm diameter when spawned) and typically hatch within 1-2 days. The newly hatched larvae continue to drift with the current as they develop. Most species spawn early in the flood season when the eggs and larvae may drift with the rising waters to colonise floodplains where food is abundant. However, there are risks; pelagic spawned eggs may be eaten by predators while they are drifting or may be dispersed into unfavourable environments.
near the spawning locale, or they may become pelagic and drift with the current. Many mainstream fish are benthic spawners, and benthic spawning is also common among floodplain spawners and in some tributary fishes that are relatively non-migratory (e.g. Tor spp.).
Guarders are so-called because the eggs and/or young are guarded by one or both of the parents. They produce relatively few eggs which are larger than those of non-guarders. In the Mekong system, guarders include featherbacks (Notopteridae), snakeheads (Channidae) and gouramies (Betta spp. and Osphronemus spp.).
Bearers are fish that carry eggs on or in their bodies during development. In the Mekong system bearers include Ariid catfishes, in which the male parent broods the eggs in his mouth until after hatching, and rice-fishes (Oryzias spp.), in which the fertilised eggs are carried internally or externally (between the pelvic fins) by the female before being laid on vegetation at an advanced stage of development.
Each species description in this manual includes notes on the basic breeding ecology of each species, which can be updated from FishBase (www.fishbase.org). When considered with environmental data (on flow rates and habitats), as well as estimates of the likely age of specimens, field workers may be able to draw some conclusions about the likely time and place of spawning of the fishes. For example, pelagic eggs are likely to drift downstream immediately after spawning, whereas benthic adhesive eggs are more likely to remain where they are spawned until they hatch.
Although some inferences may be drawn based on the sampling location and stage of development of the early life stages of fishes, little is known about the distribution of fish larval drift within river channels in this region, so it should not be assumed that larvae drift passively with the current. Rather, as they develop they may move vertically or laterally in the water column, resting at times on the bed or edges. Much research still needs to be pursued in this area.

Development of fish
Fish pass through several stages and change greatly in size and appearance as they develop from an egg to an adult. There are many variations in schemes used to classify the early life stages of fish. The simple scheme referred to in this manual follows the nomenclature developed by several earlier workers (Hubbs, 1943;Balon, 1975;Russell, 1976 andKendal et al., 1984). It should be noted that some species do not develop through the stages precisely as described below. For example, longtoms (Xenentodon spp.), half-beaks (Hemiramphus spp.) and rice-fishes (Oryzias spp.) develop for an extended period within the egg, so that when they hatch they are already at a post-larval stage.
Note that the term 'fry' is widely used to refer to advanced larvae or juveniles.

Egg, embryonic phase or incubation period
This phase covers the period from fertilisation to hatching of the egg. During incubation, the embryo cannot feed, but is nourished by the egg yolk and other food stores. The embryo's cells divide and differentiate to produce body somites (forerunners of muscle blocks), a beating heart and circulatory system and various other organs or their precursors. Hatching involves the breaking of the chorionic membrane or 'egg shell', usually by thrashing movements of the embryo's tail and body, to release the larva.

Gut
All fish have a rudimentary straight gut (alimentary canal) as pre-larvae, when most fish feed on easily digestible microscopic zooplankton. The gut folds or coils as the digestive tract develops and as the diet changes, with the timing and shape differing between species. The anus tends to move closer to the head as a fish develops and its position is a useful diagnostic feature.

Gas bladder
By the pre-larval stage, most species develop a visible gas bladder, whose shape, size and position may be useful characteristics for identification. The larvae of Clupeiformes and Gobiidae always have visible swim-bladders. As a fish develops it becomes more opaque, so that as a juvenile or adult its gas bladder is usually not visible. A few fishes (e.g. glass perchlets Ambassis spp.) are transparent as adults, but once fixed in formalaldehyde their internal features are not visible.

Head spination
Some fish larvae have on their head and operculum spines which are important as armour against predators. Spination is useful diagnostically for most marine fishes that have pelagic larvae. Spines are present on the pre-larvae of all Perciformes (perch-like fishes). In this manual, spines are important diagnostically for Lobotidae (head spines) and for Cobitidae (spines below the eye).

Eyes
All of the fish larvae in this guide have round eyes except for some Clupeoid larvae which have oval eyes. Most early pre-larvae (i.e. immediately post-hatching) have no pigment in their eyes; the pigment appears later, typically after one day. In some families, (Belonidae and Adrianichthyidae) development is to an advanced stage in the egg, so that when the fish hatches it is a post-larva in which the eyes are already developed and densely pigmented.

Fin formation
The size and position of fins and the number of spines and rays are diagnostically important. The median fins (dorsal, caudal and anal) begin to form from a finfold which is present in the pre-larva; dorsal and anal fins first begin to differentiate as anlages, which are the bud-shaped initial clustering of embryonic cells from which a body part or an organ develops. The paired fins (pectoral and pelvic) develop later than the median fins. The pectoral fins become visible in pre-larvae and begin to develop their spines and rays at the late post-larval stage. Pelvic fins usually develop last. Where fin spines are present they develop before fin rays.

Meristics
Meristics refers to counts of features and the most important are shown for each fish as follows.
DFC -Dorsal fin ray count AFC -Anal fin ray count P 1 FC -Pectoral fin ray count P 2 FC -Pelvic fin ray count Note: for each fin, the number of spines is denoted by Roman numerals and the number of rays by normal numbers. For example, a fin with one spine and six rays is denoted as I, 6.

Morphometrics
Morphometrics refers to measurements that relate to the shape of the fish, which changes as it grows. Body lengths are expressed in this guide in mm (millimetres) as total length or as standard length, as shown in Figure 4.
The approximate total length is noted next to each developmental stage, together with its typical age in days. Standard length is used for morphometric tables because total length cannot be accurately measured if fins are damaged. Important measurements are shown in Figure 4 as follows.   Figure 8 Types of fish mouths (Rainboth, 1996) Terminal Sub-Terminal Inferior Superior Protracted Retracted Figure 6 Form of fish teeth (Rainboth, 1996) Incisors

Figure 10
Terms used in describing melanophore pigmentation and fin structure of fish larvae A crepuscular, omnivorous species found in standing and sluggish water from the Mekong Delta to at least as far upstream as Chiang Saen. It undertakes localised lateral migrations from the main river to floodplains during the flood season. At several places it is reported to move into tributaries during the flood season. It carries eggs in May and June, and is reported to spawn from May to August; the eggs are laid in small clumps on submerged vegetation in seasonally inundated areas, although it may breed in both riverine and standing water habitats. It is sexually mature at a weight of 250 g; a female measuring 21-25 cm usually lays 1,200-3,000 eggs. It is an important commercial food fish and is caught by seines, lift-nets, weirs and barrages. It can reach 60 cm in length and is commonly about 25 cm.

Morphological characteristics Pigmentation
Yolk-sac larva Can be found in swamps.
The yolk-sac is ovoid, with homogenous yolk. No row of pigment on tail.
Melanophores on head and trunk.

Pre-larva
Develops in swamps; feeds on zooplankton.
Compressed body with large terminal mouth and a triangularshaped gut; 70-75 myomeres and 14-18 pre-anal myomeres; no spines on head. Short dorsal fin, very long anal fin and very small pelvic fin.
Melanophores on head and trunk in young and old larvae. No pigmentation on peritoneum, pectoral or pelvic fins. A carnivorous, nocturnal species, found in rapids and pools in large and medium-sized rivers throughout the Mekong Basin. It migrates locally and moves into smaller tributaries and flooded areas including inundated forest during the flood season, and returns to main river channels when the water starts to recede. It spawns from March to July, attaching eggs to submerged wood. At least one of the parents guards the eggs and fry. It is an important species in the fishery, caught with a variety of gear, and it is also seen in the aquarium trade. It reaches 100 cm standard length.

Meristics and morphometrics of larvae and juveniles
Melanophores on head.

Pre-larva
Larvae develops on floodplains, first feeding at around 12.5-17.3 mm, feeding on zooplankton.
Melanophores on head and trunk in early and late larvae.
Peritoneum covered with melanophores, pectoral fins without melanophores, pelvic fins not present in larvae.

Cyprinidae
Adult Opsarius koratensis (Smith, 1931) Stream barilius An insectivorous species with a preference for trichopterans. Found over gravel substrate in fast current. Spawning and rearing grounds are on the floodplain, juveniles move to the main stream with the flow of water at the end of the flood season. It is of limited commercial importance; caught with seines and cast-nets. It grows to a length of about 10 cm.

Yolk-sac larva
Occurs on the floodplain.
Melanophores on head and trunk.

Pre-larva
Develops on floodplains, first feeding on zooplankton at 4.4-5.6 mm.
Melanophores on head and peritoneum Melanophore series along dorsal mid-line from head to end of tail, along lateral mid-lines of trunk and tail, over gut and along ventral mid-line of tail.

Post-larva
Omnivorous Body elongated, with small superior mouth and long gut shape.
Melanophores on head and peritoneum; in older post-larvae on the trunk. Mid-lateral stripe, with short vertical bands through it in larger juveniles. Occasionally this fish feeds on poisonous fruit which makes it behave strangely and makes the meat toxic to eat; it is an important food fish in part of the basin; caught with nets, traps and hooks. It grows to a maximum length of 70 cm, more commonly 50 cm.

Pre-larva
Develops on the floodplain, first feeding at 4.8-5.9 mm when it feeds on zooplankton.
Body elongate with a terminal mouth and long gut; slightly hunch-backed nape has 38-41 myomeres and 26-29 pre-anal myomeres. Large gap between dorsal and anal fins.
Larvae have pigmentation on head and trunk. No pigmentation on peritoneum, pectoral nor pelvic fins. Dorsal mid-line head to tail, lateral mid-line of trunk and tail, dorsally along gut and ventral mid-line of tail.

Post-larva
Omnivorous. Body with superior mouth and elongated gut.
Mid-lateral stripe that intensifies with growth and the pigment is dense on the dorsal part of body. It migrates upstream at the Khone Falls in May-July; as flow volumes increase rapidly in June migratory activity intensifies and become more regular. This migration is mainly for dispersal and feeding. Local fishers claim that the small individuals that move upstream at this time do so to feed on eggs released by native spawners. It spawns in spring and summer from January to June sometimes extending to August. Spawning is typically in shallow water among aquatic plants. It seems to be capable of reproducing in cooler waters within the Mekong basin. A 47-cm female releases about 300,000 sticky eggs. It is important as a food fish and is caught with nets and hook-and-line. It can grow to a maximum length of 120 cm, but is more commonly caught in the range of 30-50 cm.

Pre-larva
Starts feeding on zooplankton at 6.2-7.0 mm in length.
Melanophores on head and trunk in young and older larvae. Peritoneum covered with melanophores.
Elongated body with small terminal mouth and elongated gut.
Over entire body except for ventral margin of head and gut. A very large, mainly herbivorous fish, which is widely distributed in the Mekong system. It is a long-lived species which reaches sexual maturity late in life and adult specimens are increasingly rare. It is migratory: juveniles enter inundated areas during the rainy season. It spawns in the rainy season between June and August in swamps which receive water from the river. Eggs are seen from January to August, but fishers mostly report eggs from May to July. Juveniles 2-4 cm long appear from July to November. The giant barb reaches sexual maturity at an age of 7 years, at a body weight of 9 kg. A 60 kg female sheds about 400,000 semi-buoyant eggs that are dark brown in colour and have an initial size of 1 mm expanding to 3 mm after water absorption. Hatching occurs within 20-22 hours after fertilization at 28-29ºC. Occasionally caught with middle or large scale gears and sold for food. Maximum length is 300 cm, more commonly 100-200 cm.

Ecology Morphological characteristics
Pigmentation Yolk-sac larva Develops in floodplain and riverine habitats.

Pre-larva
Develops in floodplain and riverine habitats, starts feeding on zooplankton at 6.9-7.6 mm in length. A large omnivorous freshwater species that occurs basin-wide in the Mekong basin and in large tributaries with hard substrates. It migrates to spawning grounds in the dry season, often in association with P. labeamajor. Spawning grounds are shallow rapids with strong currents in the mainstream of large rivers with sand and gravel substrates. Juveniles often migrate downstream with small cyprinids for dispersal and feeding. These trophic migrations by juveniles and sub-adults occur mainly at the onset of the flood season and are reported throughout the distributional range. In captivity, both males and females mature when five years old. A 4-kg female was recorded as producing about 500,000 eggs. The eggs are buoyant or semi-buoyant, but slightly heavy and adhesive and about 2 mm in diameter. Hatching occurs in 32-72 hours at 23ºC. It is an important commercial species that is caught mainly during the spawning period with large meshed gill-nets, seines and hook-and-line. It has a maximum length of 165 cm, more commonly 28-30 cm.

Meristics and morphometrics of larvae and juveniles
Melanophores on posterior ventral part of trunk.

Pre-larva
Starts feeding on zooplankton at 8.6-9.2 mm length.
Pigmentation on caudal fin. Dorsally and laterally on head, dorsally along posterior half of gut, along ventral mid-line of tail, dorsal mid-line of caudal peduncle and series mid-laterally on tail along myotomes.
Slightly inferior mouth, elongated gut. An omnivorous species that is found in undisturbed rivers and is sensitive to human activities. In central Thailand the species moves downstream at the onset of the flood and returns upstream after one or two months to spawn in July near the mouths of small streams which the young subsequently ascend as larvae.
Collection of young fish in Cambodia showed that the migration pattern and timing is probably similar to Thailand. Juveniles enter streams and feed on floodplains. Caught with nets and hooks; it is eaten, but the meat is sometimes toxic when it feeds on certain kinds of fruits. Its maximum length is 80 cm.

Meristics and morphometrics of larvae and juveniles
Melanophores on head.

Pre-larva
Small clear streams, move to floodplain for feeding. It starts feeding on zooplankton at 10.7-11.8 mm length.
Dorsolaterally on head and body, some laterally on gut, absent over gut, large triangular melanopore laterally over hypural bones.  (Bleeker, 1850) Soldier river barb A common omnivorous species, that is found basinwide. It is migratory and spawns in the mainstream and on floodplains and in inundated riparian forests in the early flood season. Females reach sexual maturity at a length of 10.3 cm, males at 9.7 cm. The average fecundity of a 15 cm fish is 3,000-4,000 eggs and it is a total spawner. Eggs and larvae are pelagic and they drift from spawning grounds into flooded areas or quiet, shallow parts of the mainstream. It is an important food fish caught with small, medium and large-scale gears. It grows to a maximum length of 74 cm, commonly 45 cm.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in main river channels and on floodplains.

Pre-larva
Develops in shallow parts of main river channels and on floodplains. Starts feeding on zooplankton at 6-7.3 mm length.
Dorsally on head and irregular series along dorsal mid-line of body, lateral mid-line of trunk, mid-laterally on tail with an elongated melanopore on each myoseptum.
Dorsally over gut, dark melanophore laterally over hypural plate started from 13.9 mm. An omnivorous species, which is found basin-wide in large and medium-sized rivers at mid-water depths. It undertakes annual upstream and downstream nonreproductive and reproductive migrations and colonises flooded areas during the inundation period. Matures at a length of 13 cm. Females of 19-21 cm length may carry up to 25,000 eggs. It spawns in July-August, however the species appears to have a protracted spawning season with a peak in the late rainy season in September. It spawns in midwater in floodplains and flooded forest, or among flooded vegetation. It has buoyant or semi-buoyant eggs, with an initial diameter of 0.74 mm. The eggs hatch 12 hours after fertilization at 28ºC. Although the species appears to thrive in standing water, it has a requirement to return to flowing water to spawn. It is important in the fishery, has good aquaculture potential and is also used in the aquarium trade; maximum length is 20 cm.

Meristics and morphometrics of larvae and juveniles
No melanophores.

Pre-larva
Starts feeding on zooplankton at 4.8-5.6 mm length.
Dorsally and laterally on head, scattered melanophores dorsally on trunk and tail, lateral mid-line of trunk and tail from 16.0 mm. Ventral mid-line of tail and dorsally along posterior of gut. Large, dense melanophores laterally. An omnivorous species, which lives basin-wide in a variety of freshwater habitats, but with a preference for standing water. It is a migratory species, however most reports indicate that it is a "local migrant" migrating from the Mekong up into small streams and canals and onto flooded areas during the rainy season and back again when the water recedes. It is an opportunistic spawner: it spawns in the rainy season between May and August depending on environmental factors such as rainfall and water current. The species normally reach maturity after one year, but it can reach this stage only eight or ten months at a body length of 8-8.5 cm and a weight of 9-20 g. The species is known to have a high fecundity between with 200,000 and 800,000 eggs/kg. The eggs are semi-buoyant, with an initial size of 0.5-0.8 mm. The maximum size after water absorption is 2.5-3.5 mm. Hatching occurs 12 hours after fertilization at 25ºC. The total length of the larva at hatching is 2-3 mm. It is an important food fish and is important both in fisheries and aquaculture. Maximum length is about 33 cm.

Pre-larva
Starts feeding on zooplankton at 5.2-6.1 mm length.
Dorsally on head. From 6.0 mm melanophores spread to dorsal surface of trunk and long dorsal mid-line of tail. Series dorsally on posterior-half of gut along ventral mid-line of tail. Scattered melanophores laterally over hypural plates. Mid-lateral series spreads anteriorly from tail onto trunk. Peritoneum is covered with melanophores.

Post-larva
Herbivorous. Elongated body with terminal mouth shape and long gut.
Concentrated on dorsal half of head and body. An omnivorous fish that occurs in a variety of flowing waters in rivers, streams, canals and ditches; it seems to be slightly less common than B. altus. It feeds on aquatic macrophytes, submerged land plants, filamentous algae, small fishes, insects, worms, crustaceans and zoobenthos. It has some importance as a food fish and is usually marketed fresh. It is caught with gill-nets, cast-nets, lee traps and fence filter traps (tone traps). It grows to a maximum length of 35 cm and is commonly about 25 cm.

Meristics and morphometrics of larvae and juveniles
No pigmentation.

Pre-larva
Develops on floodplain, starts feeding on zooplankton at 5-5.8 mm length.
Melanophores dorsally on head over gut and along ventral mid-line of tail. A few expanded melanophores laterally over hypural plates develop in larvae from 9.5 mm.  (Smith, 1945) Goldfin tinfoil barb An omnivorous species, that is found in large and medium-sized rivers and floodplains. A migratory species that moves between large and medium sized rivers, displays both longitudinal and lateral movements. It is a mainstream pelagic spawner that produces buoyant or semi-buoyant eggs. Spawning takes place during late the wet-or early dry-season in December or January as water levels fall. Three spawning grounds have been identified just below the Khone Falls. Vocalisations appear to be closely associated with spawning behaviour. It becomes mature at a length of about 30 cm. It is an important food fish which is marketed fresh. It is caught with seines, gill-nets and traps. It can grow up to 50 cm.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in streams.
No melanophores on head or trunk.

Pre-larva
First feeding on zooplankton at 6.4-7.6 mm.
Melanophores on operculum, dorsally on head and on gas bladder.
Melanophores dorsally on head and over gut in post-flexion larvae from 8.8 mm. Series along ventral mid-line of tail. Concentration of melanophores laterally on caudal-fin base. Series on lateral mid-line of tail.  Smith, 1934 Eye-spot barb An endemic carnivorous species which is found basin-wide in slowly moving or standing waters and which adapts readily to reservoirs. It displays both longitudinal and lateral movements. It breeds in the beginning of the rainy season and the young are found in seasonally flooded habitats in June. It is of some importance as a food fish and is caught with various nets and traps. It is grows to maximum length of 35 cm.

34-35 myomeres and 22-24 pre-anal myomeres.
Pigmentation on head and trunk in older larvae; dorsally on head. Series along dorsal mid-line of body. Band dorsolaterally from origin of dorsal fin and on caudal peduncle, on lower jaw from 8.2 mm. Mid-laterally on trunk from 9.7 mm.
Serrate dorsal-fin spine. Band below dorsal fin and on peduncle intensify mid-laterally, fine melanophores dorsally on head and body. Main references: Rainboth, 1996.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in flood plain.
Elongated yolk sac with homogenous yolk.
Melanophores dorsally on head and trunk.

Pre-larva
Start feeding on zooplankton at 4.8-5.7 mm.
Large dark spot on caudal peduncle. Dorsally on head and nape extending laterally and posteriorly with growth, midllaterally series along trunk and tail. Along dorsal-fin spine which extends dorsolaterally to trunk by 18.5 mm.
Dense melanophore at anal-fin origin from 8.9 mm.
Elongated body with terminal mouth and long gut.
Dorsal and lateral surface of head and body, dark mid-lateral spots above the lateral line. A dense spot at the origin of the dorsal and anal fins. An omnivorous species, which is found basin-wide in rivers of all sizes. It migrates onto floodplains where it breeds in areas covered with vegetation. during the wet season. Adults leave floodplains as the water subsides in December and January. The exact timing of spawning varies according to environmental factors such as rainfall and current. The eggs are adhesive with an initial diameter of 0.7 mm and will hatch 18-19 hours after fertilization at 27-28ºC. The larvae is about 3 mm long at hatching. Fish mature at 10-11 cm; fecundity varies in fish in different environments, with 20,000-100,000 eggs for fish weighing 15-80 g. In culture, fish with a total length of 10.9-12.3 cm and a body weight of 18-23 g contain about 7,300-15,000 eggs. Red-cheeked barb are caught with nets and traps and may be processed into fish paste. Maximum length is about 25 cm.

Pre-larva
Starts feeding on zooplankton at 4.2-4.8 mm length.
Peritoneum covered with melanophores. Caudal fin is pigmented. Dorsally on head and along dorsal mid-line of body. Mid-lateral series on body. Dorsally over gut and along ventral mid-line of tail. Laterally on caudal-fin base. On the leading edge of dorsal fin from 11.9 mm.

Morphological characteristics Pigmentation
Yolk-sac larva Develop in riverine habitats.
No information available.
Melanophores on head and body both in early and older larva, peritoneum covered with melanophores.

Post-larva
Herbivore. Elongated body with inferior mouth and elongate gut. The gas bladder is visible until juvenile fish of 30.5 mm.
Dorsally on head and body. An abundant herbivorous species occurring basinwide in large and small rivers. It is highly migratory. During reproductive migrations, fish feed very little, relying on fat deposits around their viscera. Mature eggs are reported from April to July with a strong peak during May-June and the species spawns at the beginning of the flood. The species is extremely important in the dai fisheries and is also caught basinwide with other gears. This fish is commonly processed by fermenting. It occurs in the aquarium trade.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in floodplain habitats.
Melanophores on ventral part of the head.

Pre-larva
Starts feeding on phytoplankton at 5.1-6.3 mm in length.
Larvae have melanophores dosally on the head that extend onto the trunk from 12 mm. Mid-lateral series posteriorly on trunk and tail, about 1 melanophore per myomere. Over gas bladder and dorsally over gut. Ventral mid-line of tail.

Post-larva
Herbivore. Elongated body with terminal mouth and long gut, the gas bladder is visible until 15.0 mm.

Dorsally and laterally on head and body.
Proximally on dorsal and anal fins. An omnivorous migratory riverine species, which occurs in large rivers in the Middle and Upper Mekong Basin. Engages in upstream dry-season migrations in the Mekong mainstream of Southern Lao PDR and either enters tributaries, or remains in the Mekong mainstream for spawning during the wet-season months. It spawns on floodplains and in large rivers during wet-season. It is a pelagic spawner, producing buoyant or semi-buoyant eggs. It is an important commercial species, which has good potential for aquaculture. Its maximum length is 50 cm; more commonly it reaches 25 cm.

Yolk-sac larva
Develops in floodplain and mainstream habitat.
Melanophores on ventral part of the head.

Pre-larva
Starts feeding on phytoplankton at 5.5-6.4 mm length.
Dorsally on head and body which extend to lateral surfaces by 8.5 mm. Dorsally over posterior, portion of gut, extending anteriorly with growth. Ventral mid-line of tail. Dense arrangement of melanophores on caudal-fin base. Dorsal-fin spine, distally on dorsal fin membrane, proximally on anal-fin from 3.0 mm, along caudal-fin rays.
Elongated body with terminal mouth long gut and the gas bladder is visible with 10.0 mm.
Diffuse melanophores dorsally and laterally on head and body, along dorsal-fin spine and melanophores proximally on anal-fin membranes, along caudal-fin rays. A predominantly herbivorous species occurring in flowing and standing waters including reservoirs throughout the Mekong Basin. At the onset of the rainy season it starts longitudinal migration that continue into tributaries, small streams and canals and seasonally flooded areas. Spawning begins at the onset of the rainy season and peaks in June-July in the Mekong River and in July-October in reservoirs. It is reported to spawn in swamps, flooded areas, or upstream of shallow sandbars in the mainstream. It can reproduce when two years old at 40 cm length. Fecundity estimates are highly variable from 10,000 to 1,000,000 for 49 cm female. Eggs are semi-buoyant and hatch in 14-16 hours at 28ºC. The fry move into inundated grasses or other shelter along the bank and continue to follow the leading edge of the advancing water as floodwaters spread. It is very important commercially and is marketed fresh or dried and salted. The fish is caught with various small to large-scale gears. Maxinum length is 50 cm.

Pre-larva
Starts feeding on zooplankton at 5.3-6.3 mm in length.
Long dorsal-fin base.
Dorsally on head and along lateral mid-line of trunk and tail. Dorsally over gas bladder. By 13.4 mm, melanophores over head and body except for ventral margin of head and gut. Dorsal fin spine by 13.4 mm, heavy pigment on proximal portions of caudal-fin rays.
Elongated body with terminal mouth and long gut, the gas bladder is visible until 10.4 mm.
Long-based dorsal fin.
Entire head and body except ventral margin of head and gut. Proximally on dorsal, pelvic, anal and caudal fins, expanding with growth. An herbivorous white fish species found in floodplains and lowland rivers with fairly fast currents and it also adapts to reservoirs. It enters flooded forests, preferring areas with at least some current. It spawns pelagically in the mainstream in the wet season; the eggs are buoyant or semi-buoyant. The fish are marketed fresh or processed by fermenting (e.g. to prahoc). It is caught with nets and traps. This species has good aquaculture potential. Maximum length is 45 cm.

Pre-larva
Starts feeding on phytoplankton at 5.9-7.4 mm in length.
Melanophores dorsally on head. Dorsally and laterally on body by 12.6 mm. Along ventral mid-line of tail by 15.0 mm. Dorsally on posterior of gut. Denser patch laterally on caudal-fin base.

Post-larva
Herbivore. Elongated body, superior small mouth and long gut.
Dorsally on head and body, extending laterally with growth. Dark patch laterally on caudal-fin base. Diffuse melanophores along dorsal-fin spine and on dorsal-fin membrane. An omnivorous species found in fast-running water where there is a rocky substrate. Fish migrate longitudinally in schools from Cambodia to Lao PDR in December and January and laterally to the tributaries and floodplains during high water in June-July and back to the mainstream at the end of the flood season. This species has some importance as a food fish and is becoming common in the aquarium trade. It is caught with a variety of gears, including seines, set-nets, cast-nets, gill-nets, lee traps and tone traps. It grows to a maximum length of 17 cm.

Pre-larva
Starts feeding on zooplankton at 4.3-4.7 mm in length.
No melanophores in small larvae to 9.0 mm, melanophores on snout and dorsally on head by 15.0 mm. Opposing pair of melanophores on dorsal and ventral margin of caudal-fin base which intensify with growth. Melanophore on ventral margin of tail started from 10.6 mm. A mainly herbivorous fish, which is found in streams and rivers over sandy substrates. It migrates from Cambodia to southern Lao PDR in January-February and moves into seasonally flooded habitats and returns to rivers as floodwaters recede. It spawns in floodplains. Important in the aquarium trade and caught with various gears including seines, set-nets, cast-nets, gill-nets, lee traps and tone traps. It grows to 12 cm.

Pre-larva
Starts feeding on zooplankton at 4.8-5.5 mm in length.
On snout and dorsally on head. Dorsally over gut, with dark patch above pectoral fin. Laterally along trunk and tail, with melanophores along myosepta, caudal-fin base.

Post-larva
Herbivore. Elongated body, with inferior mouth and long gut; gas bladder visible until 8.4 mm.
Entire head and body pigmented except for ventral margin of head and gut. Dark line through snout and onto operculum from 12.5 mm. Caudal-fin base patch intensifes with growth. Dorsal-fin, pelvic-fin and anal-fin membranes. An omnivorous species with a preference for algae. Found in fast flowing, small and medium sized streams with rocky substrates. Migrates to rice fields and floodplains in July-August for spawning and feeding. It feeds on periphyton, phytoplankton and some insects. Of limited use as food fish, but used in the aquarium trade. Caught with cast-nets, gill-nets, scoop-nets, tone traps and occasionally taken with seines. It attains a maximum length at 15 cm.

Pre-larva
Starts feeding on phytoplankton at 4.6-6.1 mm in length.
No pigmentation in early larvae until 4.1 mm. Melanophores dorsally on head and body from 4.5 mm. Stripe through snout, operculum, dorsally over gut and along ventral margin of tail. Mid-lateral series along trunk and tail, intensities with growth. Large spot laterally on caudal-fin base. A nocturnally active carnivore, which is widely distributed in flowing waters of the Mekong Basin, where the substrate is covered with rocks or logs. It feeds on molluscs, worms, benthic insect larvae and planktonic crustaceans. A white fish species, which moves into flooded areas during the rainy season and returns to rivers during November and December. It is caught with artisanal and large scale gears, cast-nets, set-nets, traps, seines and drifting hook-with-float. It is also a common element in the dai (bag net) catches of the Tonle Sap. It is important as a food and also in the aquarium trade. Maximum length is about 25 cm.

Pre-larva
Starts feeding on zooplankton at 3.3-3.8 mm in length.
Elongated body, with small superior mouth and long gut; 32-35 myomeres and 21-25 pre-anal myomeres. Barbels form by 14.2 mm. Scattered melanophores on head and trunk in both early and older larvae, peritoneum with a row of melanophores. Melanophores are arranged in 6 bands from 13.3 mm. Dorsal fin and anal fin pigment develops on distal margins by 13.3 mm.

Post-larva
Carnivore. Elongated body, with small superior mouth at the larval stage and modified to terminal in juvenile and long gut; gas bladder is visible until 10.5 mm.
Suborbital spine forms at 10.5 mm.
Horizontal stripe through snout. Melanophores dorsally on head, 6-10 transverse bands along body, some extend to dorsal and anal fins. Juveniles have more than 10 transverse dark bands from nape to caudal fin. A highly migratory nocturnal carnivore, which is found in large rivers with sand-mud substrates. It is primarily a mollusc eater, but also eats benthic insect larvae, worms and crustaceans. It migrates in large schools; it moves into small streams and inundated areas during high water periods and returns to rivers as water levels drop. Eggs are reported to be in fish from February to July throughout the distribution range.
There is a strong peak around May-June, indicating that spawning takes place during this period. It is believed to spawn on or near floodplain areas. The eggs are grey-green and are semi-buoyant. After hatching, larvae live and forage near the hatching area, but will move to small canals and then to large rivers. A brood fish of about 130 g and 19.5 cm spawns 60,000-80,000 eggs. Taken by seines, traps, push-nets, cast-nets and drifting hook-with-float; it also forms a substantial part of the dai catches in the Tonle Sap in November and December and is one of the most important species in the tone trap fishery at the Khone Falls. It is very important both as a food and aquarium fish. Maximum length is 25 cm.

Pre-larva
Starts feeding on zooplankton at 3.3-3.7 mm in length.
Horizontal stripe on snout. Scattered melanophores on head and body form bands by 11.3 mm.

Post-larva
Carnivore. Elongated body, with small superior mouth and long gut; gas bladder is visible until 5.7 mm. Suborbital spine forms at 11.8 mm.
Horizontal stripe on snout, 6 vertical bands on trunk and tail, the third band extends onto dorsal-fin spine. Fine melanophores in middle of dorsal fin. Adult Yasuhikotakia nigrolineata (Kottelat and Chu, 1987) Black-lined loach An omnivorous schooling species found in small muddy lakes and other standing water habitats; it feeds on aquatic invertebrates including worms, crustaceans, insects and also plant matter. It is an uncommon species. Not fished commercially, but highly regarded as aquarium fish. Maximum length is 5.5 cm.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in streams and on floodplain.
No melanophores on head or trunk.

Pre-larva
Starts feeding on zooplankton at 3-3.2 mm in length. An omnivorous species found basin-wide on hard surfaces in tributaries, where it holds on to fixed objects with its sucker-like mouth. Its diet consists largely of mud detritus, algae, periphyton and phytoplankton, but it also feeds on insect larvae and zooplankton. It is caught with seines, trawls, set-nets, traps, or gill nets. It is of some importance as a food. Large individuals are sold in markets; smaller ones are used to make prahoc (salted fish). Its maximum length is 20 cm.

Yolk-sac larva
Develops in floodplain and riverine habitats.
No information available.

Pre-larva
Starts feeding on phytoplankton at 4.5-5.4 mm in length.
Elongated body, with terminal mouth and long gut; 32-35 myomeres and 24-28 pre-anal myomeres. Anterior insertion of pectoral fin, posterior placement of anal fin. Body depth rapidly reduce at midgut. An omnivorous species that does not undertake longitudinal migrations, but migrates into flooded areas during the high water season. Found in rivers and streams and seasonally in flooded areas. It feeds on insects, aquatic insect larvae, including odonatans, fish and large crustaceans, earthworms, snails, roots, fruits and detritus. It probably spawns throughout the rainy season; adult females have welldeveloped ova in February and spawning takes place at the beginning of the rainy season, with the young being caught during August. However the presence of eggs in adult fish in July and August indicates that the spawning season is protracted. It is sexually mature at 13 cm and 20 g. It is caught with seines, gill-nets and traps. Maximum length is about 17 cm.

Meristics and morphometrics of larvae and juveniles
Melanophores on head and trunk.

Pre-larva
Starts feeding on zooplankton at 5.8-7.6 mm in length.
Short dorsal fin. Barbels form by 4.0 mm.
Head and body heavily pigmented except for ventral portions of head and gut. Caudal-fin heavily pigmented. As larvae develop, pigment spreads to all fins and into patches of dense pigment as they approach the juvenile stage.

Post-larva
Carnivore. Elongated body, with inferior mouth and moderaterly long gut.
Long adipose fin.  Roberts, 1994 Striped mystus An omnivorous species known from flowing and standing waters especially around submerged woody vegetation. It feeds on insect larvae, including chironomid worms, as well as zooplankton and fishes. It spawns just prior to, or at the onset of the rainy season and its young are first seen in July and August. It is caught with nets and traps and is brought to the market fresh or smoked forms. Maximum length is 25 cm.

Morphological characteristics Pigmentation
Yolk-sac larva Develops on floodplain.
Melanophores on dorsal part of head and trunk.

Pre-larva
Starts feeding on zooplankton at 4-4.6 mm in length.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in freshwater on floodplain.
Melanophores on dorsal part of head and trunk.

Pre-larva
Starts feeding on zooplankton at 3.7-4.2 mm in length.
Evenly-distributed melanophores over entire head and body, except ventrally on head and gut. Body pigment spreads to dorsal adipose, caudal and anal fins.
Elongated body with small terminal month.
Entire head and trunk, heaviest dorsally and absent ventrally on head and gut. Dark spot above pectoral fin. Unpigmented stripe along lateral mid-line.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in wetland areas.
Melanophores on head and trunk.

Pre-larva
Starts feeding on zooplankton at 6.5-7.4 mm in length.
Evenly-distributed melanophores dorsally and laterally on head and trunk and entire tail.

Post-larva
Carnivore. Elongated body, with small slightly inferior mouth; gas bladder visible until 13.9 mm. A common carnivore, in both rivers and standing waters; occurs in rapids, deeper slow reaches and standing waters. It feeds on fishes and crustaceans including crabs, shrimps and planktonic crustaceans and it also eats aquatic insect larvae, mud and plant fragments. Undertakes shorter local reproductive migrations within the Mekong mainstream at the onset of the flood season; as the water level rises it continues into small seasonal tributaries and flooded forests. When water levels start to recede at the end of the flood season, fish return to the main river channel where they spend the dry season. This species probably breeds through most of the year with a peak in the rainy season when it spawns on the floodplain. It is caught basin-wide with seines, hook-and-line, gill-nets, set-nets and in tone, chan, kha and lee traps at the Khone Falls. It has some importance as food and is marketed fresh.

Meristics and morphometrics of larvae and juveniles
Melanophores on head and trunk.

Pre-larva
Starts feeding on zooplankton at 5.0-5.7 mm in length.
Melanophores evenly distributed over head and body. Pigment dorsally over gut.
Pigment spreads to adipose, dorsal and anal fins as larvae develop.

Post-larva
Carnivore. Elongated body with inferior mouth. Entire head and body, plus dorsal and adipose fins. Adult Hemibagrus wyckioides (Fang and Chaux, 1949) Red-tail catfish A carnivorous species that occurs in large upland and lowland rivers with an irregular substrate. Occurs most commonly in areas with irregular depths, tree roots and rocky substrates. It feeds on shrimps, fishes, crabs, insects, earthworms and snails. It is, at least locally, a migratory fish; although it is indicated as a black fish species it has been found to migrate upstream, it also enters inundated areas. The migration seems to be for dispersal and feeding. It is usually caught by hook-and-line, less often by drift gillnets and seines; the hooks are often baited with live fish such as Gyrinocheilus pennocki or Clarias batrachus. It is also one of the most important species in the lee trap fisheries in southern Lao PDR. It is commercially high-valued species which is marketed fresh. Maximum length is about 65 cm.

Ecology Morphological characteristics Pigmentation
Yolk-sac larva Develops in floodplain habitats.
Round yolk-sac with homogenous yolk.
Melanophores on head and stomach.

Pre-larva
Starts feeding on zooplankton at 4.0-4.7 mm in length.
Scattered melanophores dorsally and laterally on head. Series along dorsal mid-line and mid-laterally along trunk. Pigment over anterior half of gut. Dorsal pigmentation intensifies with growth.

Post-larva
Carnivore. Elongated body, with a wide inferior mouth and gut.
Pigment on head and body intensified to all surfaces of head and except ventrally on head and gut. A carnivorous non-migratory species, common in the Tonle Sap near the Great Lake. It feeds on crustaceans and benthic invertebrates. It is caught with seines, cast-nets and gill-nets; of some importance as food and usually marketed fresh. It grows to a maximum length of 24 cm.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in wetland areas.
Melanophores on head and trunk.

Pre-larva
Starts feeding on zooplankton at 8.3-9.2 mm in length.
Diffuse melanophores dorsally on head and trunk and over entire tail.

Post-larva
Carnivore. Elongated body, with a small, looks inferior mouth and long gut. The gas bladder is visible until 10.2 mm. The dorsal spine is not serrate.
Melanophores intensify with growth but do not extend onto the fins. Siluridae Adult Belodontichthys truncatus Kottelat & Ng, 1999 Sharp-tooth sheat fish A carnivore with a preference for fish. It is widely distributed in large rivers of the Mekong Basin, found from middle depths to the surface in deeper parts of large rivers. The dry season habitat appears to be rapids and rocky areas. It feeds mainly on smaller fishes near the water surface, but also takes ants, leaves and bark. It is classified as a white fish species, which migrates for feeding. Fishers report that this species enters flooded forests and streams in the high-water season. The length at first maturity is around 55 cm. It apparently spawns in the early flood although it is not known where it spawns. It can be taken by hook-and-line, but is usually caught by cast-nets, gill-nets, or seines. individuals 10-50 cm long are abundant in the dai fishery. It is one of the most important fishes in the lee trap fishery in southern Lao PDR; an excellent game fish which may be caught using hook and line. It is a very important species in the fisheries. It is marketed fresh or dried and salted; around the Great Lake it is put on ice for export to Thailand. Maximum length is about 70 cm.

Pre-larva
Starts feeding on zooplankton at 6.8-7.7 mm in length.
Elongated body, with terminal mouth and triangular gut; 60-65 myomeres and 16-20 pre-anal myomeres. Barbels form by 6.0 mm. Very short dorsal fin and long anal fin.
Early larvae only have melanophores on head, whereas older larvae have melanophores on head and laterally on the gut. Stripes along dorsal and lateral mid-lines form at 10.8 mm.

Post-larva
Carnivore. Elongated body, with superior mouth and short gut.
Dorsal mid-line stippled from head to tail. Lateral mid-line stippled from trunk to tail. Ventral mid-line stippled. A carnivorous species that occurs throughout the mainstream and in some large tributaries with turbid waters and adjacent streams and canals. It feeds on pelagic fishes and large crustaceans in mid to surface water. This is a white fish that moves into flooded riparian forests and probably into floodplains during high water levels. It spawns throughout the flood season. Young fish begin to move back to the rivers in October, where they remain common until January. It is caught with seines, gill-nets and hook-andline. It is an important food fish sold fresh, dried and salted. It is one of the principal economic species in Cambodia. It grows to a maximum length of 77 cm.

Ecology Morphological characteristics Pigmentation
Yolk-sac larva Develops in floodplain and riverine habitats.

Pre-larva
Starts feeding on zooplankton at 6.7-8.0 mm in length.
Elongate body, with terminal mouth and short gut; 60-65 myomeres and 11-16 pre-anal myomeres. Barbels form by 5.0 mm. Dosal-fin form by 9.6 mm and reduce their size or disappear in juvenile, long anal fin.
Dorsal on head and diffuse laterally. Dorsolateral stripe on trunk and most of tail. Ventro-lateral stripe on tail.

Post-larva
Carnivore. Elongate body, with terminal mouth and short gut to 33% SL percent of standard length.
Dorsally and laterally on head and body. Ventral mid-line series on tail. A predominantly carnivorous species, occurring in both flowing and standing water throughout the LMB in rivers, streams, lakes and impoundments, with a preference for rapids and faster flowing waters.
It feeds on small fishes, shrimps, aquatic insect larvae and leaves. Deep pools are important dry season habitats. It migrates upstream in May-July, mainly for reproduction. It also undertakes lateral migrations from the main river into smaller tributaries and inundated riverine habitats at the onset of the flood season. It returns to the main river channel when water starts to recede at the beginning of the dry season. Below Khone Falls, these lateral migrations are followed by a longitudinal migration within the mainstream. The purpose of this upstream migration is reportedly to find a deep pool where it can spend the dry season. It mainly carries eggs in May and June, corresponding to a spawning period in May-August. Eggs have been reported in females from 320 g (length about 22 cm). It is taken by seines, gill-nets, or hook-and-line and mainly sold fresh. It attains a maximum length of 60 cm.

Pre-larva
Starts feeding on zooplankton at 5.5-6.3 mm in length.
Melanophores dorsally and laterally on head and body and ventrally on tail. A dorsolateral and ventro-lateral stripe on the posterior half of the tail. A row of melanophores dorsal to the gut. A carnivore, which lives in large streams and rivers throughout the basin; less common than Wallago attu. It enters flooded forest in high water and stays near the edge of the forest in low water. Schooling behaviour has also been observed outside the Mekong basin in the Kapuas River where its congener W. leerii used to form large migratory schools. Its apparently migrates into smaller streams to spawn when the water level starts rising after strong rain. Eggs have been observed in fish from April to October, with the majority of observations between May and July. It is reported to spawn in flooded grassland in July, at night, and it breeds in deeper water than Wallago attu. During spawning the fish swim in pairs and the eggs are spawned near the surface. It is caught with seines, gill-nets, hooks, and by explosives in northern Cambodia. It is usually marketed fresh. It attains a maximum length of 145 cm.

Meristics and morphometrics of larvae and juveniles
Melanophores on head.

Pre-larva
Starts feeding on zooplankton at 6.4-7.8 mm in length.
Elongated body, with large and terminal mouth, triangular gut, with gas bladder visible; 58-61 myomeres and 14-16 pre-anal myomeres. Short dorsal fin and long anal fin. Barbels form by 5.4 mm.
Early larvae have melanophores on head and on tail, older larvae from 7.4 mm have melanophores on head and trunk. Pectoral fin has fine melanophores by 17.7 mm. Mid-lateral stripe along trunk and tail. Along base of anal fin there is a longtitudinal stripe which is absent by 29.1 mm.

Post-larva
Carnivore. Elongated body with large and terminal mouth.
Uniform pigment over head and body. Pectoral fin pigmented. Dorsal and anal fins become pigmented by 32.5 mm. An omnivorous species found in large rivers with turbid and slow or standing waters. It feeds on forest fruits and vegetation, along with insects, fish, zooplankton and detritus. A possible migrant that can be found in medium-sized rivers and may be found in flooded riparian forests where the water has a pronounced current. It is reported to inhabit deep pools for at least part of the year. It is caught with seines, cast-nets, traps, lift nets and hook-and-line. It is marketed fresh. It grows to a maximum length of 17 cm.

Meristics and morphometrics of larvae and juveniles
No melanophores on head or trunk and gut.

Pre-larva
Starts feeding on zooplankton at 5.7-6.2 mm in length.
Elongated body with terminal mouth and triangular gut; 40-45 myomeres and 12-16 pre-anal myomeres. Short dorsal fin and adipose fin, long anal fin barbels by 4.8 mm.
Gut is covered with melanophores dorsally and laterally-pigment develops dorsally on head and nape by 27.7 mm.

Post-larva
Carnivore. Elongated body, with terminal mouth and triangular gut. Long anal fin, flat barbells, maxillary barbel longest and reaches to anus.

Meristics and morphometrics of larvae and juveniles
On peritoneum.

Pre-larva
Starts feeding on zooplankton at 5.8-7.2 mm in length.
Melanophores dorsally and laterally on head and trunk; patches of pigment mid-laterally on the tail, which gradually merge to form a mid-lateral body stripe; anal-fin base is heavily pigmented; caudal-fin base and longest rays of caudal fin are pigmented. Heavy pigmentation dorsally on gut.

Post-larva
Carnivore. Long body, wide terminal mouth and moderately short gut.
Snout, dorsally on head and body, mid-lateral stripe on body and pigment dorsal to gut persists. A molluscivorous catfish that occurs basin-wide in large rivers. It stays in permanent river channels and does not move into flooded forests. Above the Khone Falls it migrates upstream during the late dry season and/or early flood season for spawning, for sub-adult disperal and feeding. Below the Falls, the pattern is reversed, with a downstream migration at the onset of the flood season. Some populations migrate into major tributaries. Eggs occur from March to July with a peak in May-June, which probably is the main spawning season, but the species has also been reported to spawn from January to April. Juveniles of 2-4 cm have been reported both below (downstream to Can Tho and Dong Thap) and above Khone Falls (upstream to Nong Khai Province, Thailand). It can be caught with seines, gill-nets, cast-nets, traps, driftnet and hook-and-line; it has some importance as food and is marketed fresh. It grows to a maximum length of 79 cm.

Morphological characteristics Pigmentation
Yolk-sac larva Develops mainly in streams, but also in floodplain habitats.
Melanophores on head and trunk.

Pre-larva
Starts feeding on zooplankton at 7.0-8.5 mm in length.
Melanophores dorsally on head and body in both early and older larvae; a few melanophores on caudal-fin base; mid-lateral body series forms by 15.3 mm. a few melanophores dorsally on gut.
Elongated body with a large and inferior mouth. Pectoral-fin I,7. Maxilla barbels reach to behind the operculum. An omnivorous species with a very broad diet; it occurs basin-wide in large rivers and floodplains. Juveniles feed on insects. A white fish species that is highly migratory in the mainstream in the beginning of the flood season; migrates both for feeding and reproduction. In some areas it is reported to move between the Mekong and smaller streams, it is also known to migrate into floodplains. It is reported to spawn both at the beginning of the flood season and in August-October. In Cambodia, it spawns on floodplains and/or at mainstream spawning grounds near Stung Treng from where larvae drift to the Bassac region in six to eight days. It is caught with seines, gill-nets, traps, hook-and-line and trawl; one of the most important species in the wet season lee and tone trap fishery at Khone Falls. It is one of the most esteemed food fish; it is also used as an aquarium fish. It has a maximum length of 150 cm, more commonly at 90-100 cm.

Meristics and morphometrics of larvae and juveniles
Melanophores on ventral part of the trunk.

Pre-larva
Starts feeding on zooplankton at 6.4-7.0 mm in length.
Early larvae have melanophores dorsally on head and dorsally and laterally on gut. Large patches laterally on trunk. Large patch over urostyle. Caudal-fin base pigmented along margin. Laterally on head and on snout by 20.6 mm.

Post-larva
Omnivore. Elongated body, wide inferior mouth and short gut.
Dorsally and laterally on head and trunk. A schooling, omnivorous species, which is found basinwide in the mainstream of the Mekong. It migrates into the flooded forest during high water and spawns in the mainstream most of the year with a peak late in the flood or early in the dry season. Eggs have been observed in the abdomen all year round except for February, but most often reported from April to June. It spawns in rapids at the beginning of the rainy season; in Viet Nam it is reported to spawn in August to September in the main river. Females are sexually mature at 13 cm and 25 g. At least locally a very important species in the fisheries, it is caught with various small-to large-scale gears. It is marketed fresh and sold in the aquarium trade. Its maximum length is 35 cm SL.

Meristics and morphometrics of larvae and juveniles
No melanophores on head or trunk.

Pre-larva
Starts feeding on zooplankton at 6.1-7.5 mm in length.
Patch of melanophores on head and nape of post-larvae, peritoneum with row of melanophores.
Elongated body with moderate size, inferior mouth.
Head and nape patches expend to cover all of dorsal surface of head and body by 38.8 mm. Heteropneustidae Adult Heteropneustes kemratensis (Fowler, 1937) Stinging catfish A medium-sized carnivore which mainly occurs in stagnant water and swamp forest habitats; found mainly in ponds, ditches, swamps and marshes and sometimes in muddy rivers. It can tolerate slightly brackish water and is adapted to survive in oxygen-poor water. It is a predator of small fishes and insects. It breeds in small standing water bodies during the flood season, including ponds, derelict ponds and ditches. It can be caught with cast-nets and by dry pumping, but is rarely seen in markets. It can be kept alive out of water for long periods of time as long if the skin is kept damp. The demand of this species is great due to its medicinal value. It has a maximum length of 30 cm TL.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in floodplain habitats.
Melanophores on head.

Pre-larva
Starts feeding on zooplankton at 4.4-6.4 mm in length.
Gut < 50% BL Small larvae have melanophores only on head, whereas larvae from 4.4 mm are evenly covered in small melanophores except for the gut ventrally. There is a dark patch of pigment above the pectoral-fin base. This is a small carnivorous species, found in rivers, rivulets, canals, drains, ponds and lakes. It mainly occurs at the surface in quiet waters where there are aquatic plants. It feeds on aquatic insects, crustaceans and worms. It is a minor element of commercial catches, but is commonly caught by subsistence fishers using seines, dip-nets, or cast-nets. It is sold fresh for food and in Cambodia as smoked fish eaten for snacks. It is also found in the aquarium trade. It grows to a maximum length of 7 cm. This is a live-bearing species; fish hatch as post-larvae because they complete earlier stages of development in the egg.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in riverine and wetland habitats.
Melanophores on head and trunk.

Pre-larva
Starts feeding at 4.2-4.5 mm in length, feeds on zooplankton.
On snout, dorsally and laterally on head; dorsal mid-line of head and body; laterally and ventro-laterally on trunk and tail.

Post-larva
Carnivore. Elongated body with superior mouth and elongate lower jaw.
Extends to most parts of head and body, with a dark series along the lateral mid-line of the trunk and tail. On lower jaw. Main references: Krachangdara, 1994;Rainboth, 1996;Talwar and Jhingran, 1992.

Yolk-sac larva
Develops in riverine habitats.
Ovoid yolk-sac with homogenous yolk. Embryo has a further development and hatch at the post-larvae stage.
Melanophores on head.

Pre-larva
Starts feeding on zooplankton at 9.7-10.3 mm in length.
Dorsally on head and along dorsal mid-line of body; mid-lateral body series apparent from 13.6 mm; ventro-lateral and ventral surfaces of trunk and tail. Row of melanophores dorsally over gut.

Post-larva
Carnivore. Very elongated body with a long gut and very long jaws. Gas bladder is visible until 35.7 mm. Jaws; dorsal mid-line of head and body; mid-lateral body stripe, thick ventro-lateral stripe; caudal peduncle.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in wetland areas.
Melanophores on trunk; one dorsoventral cluster or band of melanophores on tail.

Pre-larva
Starts feeding on zooplankton at 7.2-8.5 mm in length.
Posteriorly placed dorsal and anal fins.
Dark stripe from snout to operculum, through level of eye; mottled pattern over whole body except for ventral surfaces of head and gut; the pigment patch extends onto dorsal and anal fins.
Pigment patch intensified and resembles adult coloration. A carnivorous species that feeds on aquatic invertebrates. It is an oral-booder, the only one in the family Ambassidae. It is found in sluggish rivers and floodplains in the Lower Mekong Basin. It is caught with seines, set-nets, cast-nets and traps. It is sold for food and occasionally seen in markets. Its bright coloration makes it a desirable aquarium fish. It has large 2nd and 3rd anal spines, of which the 2nd anal spine is the larger. Its maximum length is 10 cm.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in swamps and wetland areas.
No melanophores on head or trunk.

Pre-larva
Starts feeding on zooplankton at 3.4-3.8 mm in length.
Dorsally on gas bladder.
Compressed body, with large mouth and triangular gut; gas bladder is visible until 12.1 mm.
On head, dorsal and anal fins starting from 18.1 mm.

Developmental stages of Parambassis apogonoides
Adult Parambassis siamensis (Fowler, 1937) Siamese glass perchlet A carnivorous species which is found in sluggish and standing water including reservoirs in the Middle and Lower Mekong Basin. It is a common species that proliferates in impoundments. It feeds on invertebrates. It is caught with seines, traps, lift-nets, cast-nets and set-nets. It is caught mostly by artisanal and subsistence fishermen for family consumption; occasionally seen in markets and often sold in the aquarium trade. It has a maximum length of 6 cm SL.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in swamps and wetland areas.
No melanophore pigment on head and trunk.

Pre-larva
Starts feeding on zooplankton at 4.3-5.4 mm in length.
Dorsally on gas bladder.

Post-larva
Carnivore Body, with large mouth and triangular gut; gas bladder is visible until 10.0 mm.
Dorsally on head and distally on membrane of dorsal and anal fins. As juveniles develop, on the dorsal, dorsolateral and ventral surface of the trunk and tail.

Datnioididae
Adult Datnioides undecimradiatus (Roberts & Kottelat, 1994) Mekong tiger fish A carnivore that occurs in the Middle and Lower Mekong Basin. The family is marine, but this species has adapted for freshwater life. It feeds on fish and shrimps. It spawns at the end of the dry season; females examined in February were full of eggs and males examined in early March were in spawning condition. It has a maximum length of 40 cm SL.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in wetland areas.
Melanophores on head.

Pre-larva
Starts feeding on zooplankton at 4.6-5.8 mm in length.
Melanophores on head and trunk.
Head and body uniformly covered in large, expanded melanophores. Banding begins to develop by 15.7 mm. Pigment spreads to spinous dorsal fin by 9.0 mm, to pelvic fin by 15.7 mm and to anal fin by 12.7 mm. A non-migratory, slow-moving species, which occurs in flowing and standing, fresh and brackish waters, including reservoirs in the Middle and Lower Mekong Basin. It prefers standing water with dense aquatic plants. Juveniles are found in running streams with sand and rock substrates. It feeds on fishes, shrimps, molluscs, aquatic insects, crabs, insect larvae and detritus. It spawns at the beginning and end of the dry season. This fish becomes sexually mature at about 8 cm in length and fish measuring 15-30 cm have 6,800-90,000 eggs. The eggs are yellow, pear-shaped and they measure 0.6 x 2.2 mm. It is caught with seines, trawls and gill-nets and is a very important commercial species, marketed alive or fresh and considered a delicacy over much of eastern Asia, where exported fishes command a high price. It attains a maximum length of 50 cm, but more commonly reaches 30 cm.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in swamps and floodplain habitats.
Ovoid yolk-sac with homogenous yolk; contains multiple oil globules at anterior.
No information available.

Pre-larva
Starts feeding on zooplankton at 3.7-4.1 mm in length.
Patch below pectoral fin base; dorsally on head and behind operculum by 4.5 mm. Along anal fin base and anterior half of caudal peduncle; band from posterior of dorsal fin to posterior of anal fin; band at caudal-fin base.
Moderately long body with only in juvenile mouth; gas bladder is visible until 11.0 mm.
Pigment distributed in solid patches on body, spreads onto dorsal and caudal fins. Head is uniformly pigmented, becoming darker with development. A small zooplanktivorous species that inhabits estuaries and the lower courses of rivers and ascends the Mekong as far as Phnom Penh. It is caught with fine-meshed seines, trawls, set-nets and cast-nets; not seen in markets. It grows to a maximum length of 3 cm.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in wetland areas.
Short yolk-sac with segmented yolk; contains a single oil globule.
No melanophores on head or trunk.

Pre-larva
Starts feeding on zooplankton at 4-4.5 mm in length.
Elongated body with large superior mouth; gas bladder conspicuous.
Series along anal-fin base and ventrally on caudal peduncle. A common inhabitant of all types of water bodies that have slow currents, such as canals, lakes, ponds and swamps. It can occupy seasonal water bodies because it has an accessory air-breathing organ, which allows it survive for long periods out of water. It is an omnivore with a very broad diet, ranging from macrophytic vegetation to fish and detritus. A black fish species, it does not migrate longitudinally within the Mekong and other large rivers, but migrates laterally from large rivers or other permanent water bodies to flooded areas during the wet season. It spawns from the end of the dry season until the beginning of the flood typically in floodplain habitats. It is sexually mature when it is ten months old. The eggs are buoyant with a diameter of 0.75 mm. It is an important food fish in southeast Asia; usually it is sold in markets where it is kept alive for several days by keeping it moist. It grows to a maximum length of 23 cm, but is more commonly 10 cm.

Yolk-sac larva
Develops in swamps and in floodplain habitats.
Melanophores on head and trunk.

Pre-larva
Starts feeding on zooplankton at 3.2-3.6 mm in length.
Melanophores dorsally on head, nape and along dorsal & ventral mid-line of trunk and tail and ventrally on gut. Row of melanophores mid-laterally on trunk and tail. Large melanophores laterally on middle of hypurals. Just after flexion, melanophores uniformly distributed over head and body except for ventral surface of gut and caudal-fin base. Hypural spot persists. Dark patch above pectoral-fin base.
Deep body with large mouth and triangular gut. Serrated preopercle by 16.9 mm.
Uniformly distributed melanophores differentiate into lateral body bands by 10.1 mm. Patch above pectoral-fin base and over hypurals persist. Melanophores on head and trunk.

Pre-larva
Start feeding on zooplankton at 3.1-3.7 mm in length.
Moderately deep body with oblique superior mouth and triangular gut; short-based dorsal fin over middle of long-based anal fin 32-36 myomeres and 10-11 pre-anal myomeres.
Melanophores on entire head and dorsally on trunk; mid-lateral stripe from snout to operculum; dorsally over gut.
Slightly compressed body with superior mouth and triangular gut. An omnivorous species found in sluggish and standing waters including reservoirs. It can breathe atmospheric air and live in oxygen-poor standing-waters, typically with dense aquatic plants. It is non-migratory; found in flooded forests and moves back to rivers and permanent waterbodies as floodwaters recede. The male spawner makes a bubble nest; the eggs are buoyant, with an initial diameter of 0.91-2 mm and they hatch after 27 hours at 28ºC, or after 18-20 hours at 28-30ºC. The larvae measure about 2.7 mm at hatching. Fish reach maturity at 7-12 months at a length of 8-10 cm. Fecundity is around 18-36,000 eggs depending on the size of fish; an 18 cm-female can lay around 30,000 eggs. It is caught with seines, set-nets, cast-nets and traps and is an important food fish sold fresh, dried or grilled. It grows to a maximum length of 25 cm, but is more commonly about 18 cm.

Meristics and morphometrics of larvae and juveniles
Melanophores on head and trunk.

Pre-larva
Starts feeding on zooplankton at 3.4-3.9 mm in length.
Hatchet-like body with superior, oblique mouth and triangular gut; short-based dorsal fin over middle of long-based anal fin; 38-40 myomeres and 9-12 pre-anal myomeres.
Melanophores dorsally and laterally on head and trunk. Peritoneum covered with melanophores; a row of elongated melanophores on the ventrolateral surface of the trunk and tail. Patch on hypural plate. By 13.2 mm, dorsal and lateral surfaces of head and trunk covered in fine melanophores, and a ventro-lateral stripe beginning to form on the trunk and tail by 24.4 mm.
Deep compressed body with superior mouth and triangular gut.
Small, uniformly-distributed, melanophoresdorsally and dorsolaterally on the head and body. Mid-lateral stripe intensified from operculum to caudal base. Caudal spot remains separate from mid-lateral stripe. This fish is found in shallow sluggish or standingwater habitats from Chiang Saen to the Mekong Delta. It feeds on zooplankton, crustaceans and insect larvae. It is a black fish species that does not undertake longitudinal migrations within the Mekong, but migrates from the main river, or other permanent water bodies to flooded areas during the wet season and returns to permanent water bodies at the beginning of the dry season. It nests in rain-fed paddy fields in June-July. It is caught with seines, cast-nets, set-nets and traps. It is in marketed fresh, salted and dried. It is also a popular aquarium fish. It attains a maximum length of 15 cm, more commonly 8.5 cm TL.

Pre-larva
Starts feeding on zooplankton at 3.6-4.1 mm in length.
Elongated body with superior mouth and triangular gut; 34-38 myomeres and 7-11 pre-anal myomeres. Short dorsal fin over middle of long anal fin.
Melanophores dorsally on head and series along dorsal mid-line of body. Series of melanophores mid-laterally on trunk and tail; series along ventral mid-line of tail disappears by 19.7 mm. Dorsal surface of gut covered with melanophores which extends as internal pigment onto tail.
Compressed body with small superior mouth and triangular gut. An omnivorous fish that can breath air and is found in sluggish and standing-waters with dense vegetation, such as swamps, lakes and rivers. It feeds on submerged terrestrial plants and aquatic macrophytes, fruits, seeds, filamentous algae, insects, fish, frogs, earthworms and sometimes dead animals. After hatching, the fry feeds on the yolk sac for 5-7 days. After the yolk sac has been absorbed, fry feed on zooplankton. After 1 month it becomes omnivorous, first feeding mainly on animals and then changing gradually to vegetable matter. It spawns in a nest in the dry season and has floating eggs, which hatch in 36 hours at 25-30ºC. After hatching, the larvae attach to the nest materials and start feeding after eight days. Fish reach sexual maturity at the age of 2½ years, a body weight of 1.2-1.5 kg and a length of 37.5-40 cm. Fecundity is 500-5,000 eggs/kg of bodyweight. It is caught with seines, lift-nets, cast-nets and baited hooks; utilized fresh and eaten after steaming, pan-frying or baking. It reaches a length of 70 cm and is more commonly caught at 40-50 cm SL.

Yolk-sac larva
Develops in riverine and floodplain habitats.
Round yolk-sac with homogenous yolk; contains one pigmented oil globule at posterior.
Melanophores on head and trunk.

Pre-larva
Starts feeding on zooplankton at 6.3-7.0 mm in length.
Moderately long dorsal fin over long anal fin.
Melanophores distributed over entire head and body; black band on caudal peduncle; by 8.5 mm a dark patch develops dorsally on head above eye; darker vertical bands from mid-laterally from 9.6 mm.
Compressed body with small mouth and triangular gut.
Banding intensifies on body and extends from dorsal to ventral body margins; melanophores extend onto dorsal and anal fins. Main references: Kottelat, 1998;Rainboth, 1996.

Yolk-sac larva
Develops in swamps and floodplain habitats.
Melanophores on head and trunk.

Pre-larva
Starts feeding on zooplankton at 3.5-4.2 mm in length.
Melanophores on snout, dorsally on head and trunk; series along ventral mid-line of tail; ventro-lateral melanophores on tail by 10.5 mm.
Slightly compressed body with small oblique, superior mouth and triangular gut.
Dorsally and laterally on head, body and ventrally on tail. Two dark stripes along body differentiate by 24.6 mm. This gouramy is found in shallow sluggish or standing-water habitats with dense vegetation in the Middle and Lower Mekong basin. It feeds on zooplankton, crustaceans and insect larvae. It is not fished commercially, but larger individuals are sometimes taken by seines, cast-nets or set-nets. It is occasionally sold as part of mixed catches in markets and is regularly seen in the aquarium fish trade. It attains a maximum length of 7 cm and is more commonly seen up to 4 cm TL.

Morphological characteristics Pigmentation
Yolk-sac larva Develops in swamps and floodplain habitats.
Melanophores on head and trunk.

Pre-larva
Starts feeding on zooplankton at 3.3-3.9 mm in length.
Stripe mid-laterally on head from snout to posterior edge of operculum. Short vertical bars dorsally along body, intensifying with growth.
Slightly compressed body with oblique superior mouth and triangular gut. Elongate dorsal fin. Ovoid yolk-sac with homogenous yolk.
Melanophores on head, trunk and yolk sac.

Pre-larva
Starts feeding on zooplankton at 6.4-6.9 mm length.
Dorsally on head. Series on dorsal and lateral mid-lines of trunk and tail and on ventral mid-line of tail. Dorsally on gut and extends as internal pigment on tail.

Meristics and morphometrics of larvae and juveniles
Melanophores on head and trunk.

Pre-larva
Starts feeding on zooplankton at 6.8-7.7 mm in length.
Melanophores on snout and dorsally on head and trunk, ventrally on gut and tail; gut and pectoral fin covered with melanophores.

Pre-larva
Develops on floodplains. Body elongate and bilaterally symmetrical Stomach long and bag-like.
Melanophores on head and trunk.

Post-larva
Occurs on floodplains and moves to main streams in flowing water and feeds on bottom invertebrates after metamorphosis. A benthic feeder on molluscs, crustaceans and other invertebrates as well as some vegetable matter. The body is inflatable and the skin has small prickles that may be confined to the belly. It has four fused teeth. It has no pelvic fins and the caudal fin is truncate to round in shape. Nostrils have a tubular nasal tentacle that is distally divided into two tips that are less than one half its length. Found in slowly flowing streams throughout the LMB. Not fished commercially and possibly poisonous. Sometimes taken accidentally in seines, cast-nets, set nets and traps. It grows to a length of about 6 cm.

Morphological characteristics Pigmentation
Yolk-sac larva Occurs on floodplains and in slow-flowing water.
Body short, balloon-like and with few myomeres.
Melanophores on head above the eye.

Pre-larva
Occurs on floodplains and in slow-flowing water.
Balloon-like body, large eyes and few myomeres.
Melanophores on head above the eye, snout on ventral part of the head.

Post-larva
Occurs on floodplains and in slow-flowing water.
Small single gill opening on each side, teeth fused, beak-like, no pelvic fin, dorsal and anal fins on the posterior part of body. Nostrils with tubular nasal tentacle.
Brownish background on dorsal part of body with several large dark sports and a large mid-lateral ocellus below the dorsal fin.
Developmental stages of Tetraodon cochinchinensis from wild collections Larva that has used all or most of its yolk and is capable of capturing prey. fish larva A term covering developmental stages in the early life history of a fish between the time of hatching and transforming to a juvenile, the latter a miniature replica of the adult; [young fish between time of hatching and attainment of minimum adult fin ray counts, which at birth or hatching is fundamentally unlike its parents and must pass through metamorphosis before assuming adult characters; (pl. larvae)]. fish larva identification Determining which species a fish larvae or juvenile belongs to based on both meristic characteristics (measurements of morphological characteristics) and ontogenic development. flexion larva A fish larva at the stage of development when the posterior end of the vertebral column starts to bend upwards, as a preparation for the development of the caudal fin; marks a mid-point in larval development when caudal fin rays start to appear. flexuose Winding or having turns or undulations. fossil A part or record of behaviour of an organism that has been preserved from past time, usually in sedimentary rock. front In morphology, an anterior position. fry A young fish, usually a late post-larva or juvenile. fusiform Spindle-shaped; used in reference to the body shape of a fish which is cylindrical or nearly so and tapers toward the ends. gastrula Stage in embryonic development between blastula and embryonic axes. geography A branch of science that deals with describing the Earth and its life.
geological time 1. The period of time from the formation of the earth to the beginning of recorded history; prehistoric time.

A very long span of time extending over millions of years. gill filaments
The threadlike structures on the posterior edge of each gill arch; [The soft, red, fleshy part of the gills]; contain a network of blood vessels that facilitate the exchange of gasses between the blood and the external environment. gill opening The opening behind, and sometimes under, the head where the water of respiration is expelled. Bony fishes have a single such opening on each side. globiform Formed like a globe, or sphere. granular yolk Yolk consisting of discrete units of finely to coarsely granular material. gular fold A transverse fold or membrane across the neck. gut The alimentary canal. A term used when describing fish larvae. hatching The process when a fish larva frees itself from the egg membrane. histology The study of the fine structure of body tissues. hypurals In fish larvae, flattened bones along the ventral side of the urostyle. From these bones the caudal fin rays arise. ichnotaxon A taxon based on the fossilized work of an animal, including fossilized trails, tracks and/or burrows. ichthyologist A person who studies fish. ichthyoplankton Eggs and larvae of fish drifting in the water column. identification The art of assigning the correct scientific name to a specimen [based on published descriptions; an important tool for the proper identification of a fish is a taxonomic key]. imago The last or adult stage of insect metamorphosis; the perfect insect. incubation period The time taken for development from the time of fertilization to hatching of the larva; varies according to the temperature and the size of the egg and amount of yolk it contains; can reach days to months. indented Refers to a structure with a small notch in the middle. interdorsal The space on the dorsal surface between the first and second dorsal fins; measured from the point of insertion of the first to the origin of the second. interorbital distance The shortest distance between the eyes. interorbital space The area on top of the head between the eyes; measurements may be taken of the least width, either fleshy (to the edges of the orbits) or bony (between the edges of the frontal bones which rim the orbits). interradial membrane The soft membrane between rays and spines in the fins of fishes. interspinous membrane Skin between the spines of, e.g., the dorsal fin. intestine Part of the alimentary canal, from the end of the stomach to the anus. invertebrate drift Stream and terrestrial invertebrates that float with the current. juvenile Young fish after attainment of minimum adult fin ray counts but before sexual maturation. juvenile stage Young fish, fundamentally like the adult in meristic characters (excluding escalation) but smaller and reproductively inactive. Kupffer's vesicle A small, vesicular, ventro-caudal pocketing which forms as blastopore narrows. The origin and function of this vesicle is still not clear, however there are indications for metabolic function. The vesicle is a transitory structure. larva A juvenile animal which does not have the characteristics of the adult e.g. fish and some insects (cf. fry, nymph). [pl. larvae] larval development After hatching, fish larval development can be regarded as a sequence of three major phases: yolk-sac stage, pre-larval stage and post-larval stage. Undergoing metamorphosis, a fish larva turns into a juvenile. Larval development varies according to temperature and food availability. late embryo Stage prior to hatching in which the embryo has developed external characteristics of its hatching stage. lateral scale rows The number of near-vertical scale rows between the upper end of the gill opening and the base of the caudal fin; sometimes called vertical scale rows. This count is taken on fishes which lack a lateral line or on which the lateralline scales are difficult to enumerate. leptocephalus The elongate highly compressed transparent, ribbon-like and pelagic larval stage of some primitive teleost fishes such as the tarpon (Megalops), and eels (Anguilla); (pl. leptocephali). lozenge-shaped Shaped like a rhombus. median predorsal scales The number of scales running in a median row anteriorly from the origin of the dorsal fin. median septum The ventral septum, between the left and right side of muscle tissue. membrane The thin layer of tissue covering a part of an animal or connecting the fin elements. metamorphosis Change of body shape, e.g., the change from a larval form to a juvenile adult form.
[The process is especially dramatic in soles (Soleidae) and tongue fishes (Cynoglossidae) where the larvae are pelagic and symmetrical while the later stages are benthic and asymmetrical; (alt. transformation)]. microencapsulated diets Special feeds developed for the rearing of larvae or some forms of animal where their normal food item are microscopic and of a particular size. microphagous Feeding on microscopic particles; [eg. filter feeders]. migration Movement that results in an alternation between two or more separate habitats, occur with a regular periodicity, and involves a large proportion of the population. [The direction of the migration can be lateral, longitudinal or vertical and can be repeated either diurnally or seasonally. Fish migrate for several reasons e.g. feeding, spawning or to avoid desiccation, deoxygenation and predation. Migration can occur at all stages in a fish's life, and can be either active or passive (eggs and larvae) and cover distances from a few metres to several hundred kilometres]. modal Relating to the statistical mode, which is the value that occurs most frequently in a series of numbers; e.g., if, in a species of fish, two individuals have 14 pectoral-fin rays, seven have 15 rays, and four have 16 rays, 15 is the modal count. morphology The scientific study of the form and structure of organisms, especially their external form or the form and structure of a given organism, considered as a whole. morphometric Relative to measurements of the shape of an individual; body proportions; the size relationships of various morphological characteristics of an animal. morphometrics The scientific measurement or analysis of the shape or form of organisms. mosquito Mosquitoes are parasitic, blood sucking insects of the family Culicidae; the larvae re aquatic and are important food items for many fishes. In tropical countries some mosquitoes carry dangerous diseases and a lot of effort is put into eradicating them including spaying water bodies with DDT and introduction of mosquito-eating fishes, potentially with severe environmental side effects. myoseptum or myomere Connective tissue partitions separating myomeres. nauplius Early egg-shaped larval stage of prawns and other crustaceans. Nauplii are unsegmented and bear three pairs of appendages. Important fish food, especially for fish larvae. nictitating eyelid or nape A transparent, moveable membrane or inner eyelid that protects and helps to keep the eye clean. notochord length Straight-line distance from anterior most part of head to posterior tip of notochord; used prior to and during notochord flexion. nursery area Growth areas for juvenile fishes, most often on the floodplain in the case of the Mekong. nursery habitat The place where a juvenile fish first feeds and grows. nymph In insects a juvenile form, which is similar to the adult; some nymphs are important food items for fishes.

Odonata
The order of dragonflies; The larvae of dragon flies are entirely aquatic and constitute part of the food of some fishes, however the larvae are rather large and may sometimes feed on fish larvae and fry. oil globule Discrete sphere(s) of fatty material within the yolk-sac of a fish larva; the size and number of globules are used for species identification. ontogenetic characters Those characters associated with developmental stages. ontogeny The early life history of an organism, i.e., the succession of stages it passes through from the zygote to the adult. oophagy Embryos feeding on eggs produced by the ovary while still inside the mother's uterus, as is the case with some sharks. opisthonephrus The embryonic or larval kidney. optic vesicles Embryonic vesicular structures which give rise to the eyes. organogenesis The relatively advanced period of embryonic development characterized by formation of the organ system. otocyst A cyst that contains the otoliths. Sensory organ for the perception of gravity and acceleration. oviphagous Method of embryonic nutrition where one embryo feeds on unfertilized eggs or other embryos within the uterus. passive migration Also called larval drift. The relatively passive movement of fish larvae from the spawning ground to the nursery areas. palaeohistory Fossil history. periblast A layer of tissue between the yolk and cells of blastoderm which is observed as a thin border around blastula. peritoneum The membranous lining of the body cavity; the pigmentation of the peritoneum is an important character in the identification of fish larvae. phylogeny Evolutionary relationships and history; the sequence of events that make up the evolutionary past, o ften depicted by a branching tree-like diagram. placental Viviparous reproduction in which embryos are connected to their mother's blood supply by a placenta. plankton Passively floating or weakly swimming aquatic organisms; planktonic organisms vary considerably in size and include algae, crustaceans, and larval fishes among others. posterior The tail end; at or towards the rear end of the fish; the opposite of anterior; situated farther back than something else. post-larva Animals that have changed from the larval form to juvenile or adult form; usually refers only to the stage immediately following the larvae and which shows already juvenile characters such as fin rays; [(pl. post-larvae)]. preanal length The measurement from the snout tip to the origin of the anal fin; [equivalent to snout to vent length in larvae]. preanal myomeres The number of myomeres between the anterior most myoseptum and the posterior margin of anus. Pre-juvenile Developmental stage immediately following acquisition of minimum fin ray complement of adult and before assumption of adult-like body form; used only where strikingly different from juvenile. primordium Rudimentary form of an anatomical structure; anlage. production capacity The capacity of a water body or production facility to produce fish. protolarva Yolk sac type of fish larvae, also called pro-larvae. pupa A resting stage in the life cycle of some insects; the larval insect is enclosed in a protective case where it changes into the adult form, e.g., a butterfly (pl. pupae). quaternary The recent prehistoric past which has been dominated by the arrival of Man. querimana stage Silvery pelagic juvenile stage of mullets (Mugilidae). reflexed Bent or turned backwards. saggital 1. Relating to the imaginary mid-line of the body dividing it vertically into two symmetrical halves. 2. Relating to the line of function of the two halves of the skull. Secchi depth The depth at which a Secchi disc is no longer visible. secretory Involved in producing a secretion, or exuding a substance. septum In anatomy, a dividing wall or partition. skeletogenesis The growth or formation of the skeleton. spinous With a spine or containing a spine. spiral valve A spiral structure in the intestines of some fish; a spiral fold of mucous membrane projecting into the intestines. somites Primitive, segmented, mesodermal tissue along each side of notochord. stellate Star-shaped; with radial form. swim-up fry Fry that is swimming in the water column in search for food. tail ring Body segments of a pipefish located between the anus and the base of the caudal fin. tail-bud stage Stage of embryonic development characterized by a prominent caudal bulge and marked development of cephalic region. tail-free stage Stage of embryonic development characterized by separation of the tail from the yolk. telolecithal eggs In contrast to most invertebrates (holoblastic cleavage, yolk incorporated into the dividing cells), teleosts have telolecithal eggs, with the yolk mass separate from the developing embryo. This egg type undergoes meroblastic or discoidal cleavage, where yolk is not incorporated during cell division. tides Periodic movement of water resulting from gravitational attraction between the earth, sun, and moon.
trailing gut A gut shape that protrudes from the body cavity downward like a sac. Can be found e.g. Pleurotidae larvae. transformation in fish larva The process (synonymous with metamorphosis) at the end of the larval stage, characterized by a marked change in form or structure and involving acquisition of juvenile or adult characters and loss of larval character; also refers to the stage where this process occurs; the term "transitional" is used sometimes for larvae that undergo a gradual transformation. [The transformation is especially dramatic in soles (Soleidae) and tonguefishes (Cynoglossidae) where the larvae are pelagic and symmetrical while the later stages are benthic and asymmetrical]. transverse groove A horizontal furrow across the head, such as it is seen in Bangana behri.

Triassic
From approximately 225 to 185 million years before present. tubular Having the form of a tube, such as the snout in sea horses. type horizon The geological stratum from which the name-bearing type of a nominal species or subspecies was collected. urostyle In fish larvae, a backwardly projecting process on the most posterior vertebra. The urostyle is usually included when counting vertebrae. Terminal vertebral element in higher teleosts, derived from the fusion and loss of several of the most posterior centra of the more primitive forms. uterine cannibalism The first hatchling derives nourishment in the mother's reproductive tract by consuming either younger hatchlings (embryophagy) or unfertilized eggs (oophagy). uterus An enlarged portion of the oviduct modified to serve as a place for development of young or eggs. ventral Describes a feature anatomically located on, in or near the belly. Relating to the underside or lower surface of an animal or body part. ventro-lateral Positioned or orientated between the ventral and lateral surfaces. vesicle In anatomy, a small cavity, sac or air space. vestibular In anatomy, relating to or designating an opening at the entrance to a canal. viviparous development Refers to development of an organism through the juvenile stage within a parent. yolk Food reserve of embryonic and early larval stages, usually seen as a yellowish sphere diminishing in size as development proceeds. yolk-sac A bag-like ventral extension of the gut containing nutritive materials that first appears in the fish embryo and is later absorbed by the larva during the stage after hatching and before feeding. yolk-sac larva Fish larva which has already hatched from the egg but has not started feeding yet and still absorbs the yolk in the ventrally attached sac. At the end of the yolk-sac stage the yolk and oil globule(s) have been used up and the major organ and sensory systems required to capture prey are functional. The mouth and gut have formed, the anus is open at the margin of the ventral finfold, the eyes are pigmented, and the primordial pectoral fins are present. zooplankton Small suspended and passively floating or weakly swimming aquatic animals. Zooplankton vary considerably in size and include such animals as bacteria, protozoans, crustaceans, molluscs and fish larvae. zygote Cell formed from the union of an egg and a sperm; fertilized egg.