field guide to the freshwater fishes of singapore

Index of fishes
General parts of a fish
Kelvin K P Lim and Peter K L Ng

Fishes are vertebrates, i.e., animals with backbones. They are ectothermic (= poikilothermic or cold-blooded) like the reptiles and amphibians, their body temperatures being dependent on the external environment. They are the most successful of all the higher aquatic life forms. Of the five established classes of vertebrates, fishes (at least most of them) are the only ones which breathe with gills as adults. They have a great diversity, and there are more fishes than any other vertebrate class - with over 20 000 described species. Scientists who study fishes (ichthyologists) however, are still discovering many new species each year.

Of the 8 000 or so freshwater species, over 1000 species are found in South East Asia, a very high density and richness by any standard. Of the many groups of fishes, only the bony fishes (as opposed to those with cartilaginous or "soft" bones like sharks and sting rays) are present in Singapore freshwaters. These fishes also have typical fins which have many spines and rays.

Although most fishes are either completely freshwater or marine, there are many that live in estuarine areas with brackish water (mixture of salt and fresh water) (e.g., in mangroves or at the mouths of drains). Many of these are able to live in both completely fresh or completely marine waters. They are termed euryhaline fishes. Eels and salmon are two better known examples.

Mastery of the aquatic habitat requires solutions to several major problems. Oxygen is one of the most serious. Life-sustaining oxygen dissolves very poorly in water, only about 5% (varying with temperature, acidity, amount of dissolved matter etc.), in sharp contrast to the 21% in the atmosphere. The gills of the average fish are made up of many layers of thin-walled filaments, richly supplied with blood vessels. In the water, the gill-filaments open out (like a feather duster under water), exposing a very large surface area for oxygen absorption. In the higher (or bony) fishes, the gills are even more efficient as water is made to flow only in one direction (opposite to the blood flow) past the gill chamber with the aid of the mouth and gill cover. This countercurrent system ensures sufficient oxygen can be extracted. A hard gill cover or operculum is only present in the higher fishes, serving as protection for the delicate gills as well as controlling the water flow.

In many parts of Asia however, pristine water is hard to come by, and oxygen concentrations can get very low. Many fishes have evolved an alternative method for respiration - to breath atmospheric air. They do this in many ways. Some modify the inner layers of their mouths (e.g., snakeheads), others have a special structure just above their gills (e.g., fighting fishes, gouramies and catfishes), while a few even use part of their digestive system to trap air (e.g., swamp eels). As a result, air-breathing fishes are very successful in padi fields and stagnant pools. Many of them however, are so dependent on their makeshift "lungs" that their gills have degenerated and if kept under water for too long, they will drown, even if the water is well oxygenated! Many eels are also able to breath through their thin and slimy skins (cutaneous respiration).

The scales present in most fishes are mainly protective in function, although they also help in osmoregulation by reducing the amount of water coming in. They are made of a chitinous material. Some fishes like catfishes and eels do not have them, their skins appearing smooth. In almost all fishes, the scales contain chromatophores, i.e., cells which contain colour pigments. When these cells allow the pigments to disperse throughout the scale, the fish appears more colourful or darker. When they are concentrated into one small area, the overall fish appears light-coloured. Fishes often have many types of chromatophores arranged in various patterns, which explains the beauty of many of them. The coloration of a fish is affected by the environment, health, fear, reproductive cycle (when it is used for courting the females), etc.
Fish scale



A serious problem facing freshwater fishes is the entry of water into their bodies and the loss of vital body salts to the outside by the process called osmosis. The control of body salt and water by the fish is termed osmoregulation. Freshwater fishes have a well developed kidney which they use to remove the extra water and conserve salts.

Swimming is brought about usually by several sets of fins. Propulsion is usually provided by the caudal or tail fin, aided by the powerful muscles in the "tail" part of the fish. The dorsal, ventral and anal fins are for balance, ensuring that the fish does not roll and turn uncontrollably when it is swimming. For control of movement, the fish relies on its head movements as well as its two side or pectoral fins (located beside its gill covers).

Some fishes (e.g., certain catfishes and eels) have the dorsal, caudal, ventral and anal fins fused to varying degrees, and it is not possible to separate them. These fishes adopt a more snakelike locomotion when swimming. Most fishes have one dorsal fin. In catfishes however, there is often a small fleshy fin (without rays) behind the dorsal fin.

One can usually tell how a fish lives by the shapes of the body and fin. Fishes with crescent-shaped caudal fins, high dorsal fins, and long and slender pectoral fins are usually fast swimmers. Those with broad and truncate tails are often stouter fishes relying on sudden bursts of speed to catch their prey or escape enemies, but lack stamina.

Various types of caudal fins

Various types of pectoral fins

Most fishes have a gas or swim bladder inside their bodies (usually just above the gut) which help them maintain their balance as well as the depth they choose to live in. The fishes are able to pump in gases to inflate it (to float higher) or absorb gases into their blood to deflate it (to sink).

Feeding habits vary a great deal. Some are herbivores, feeding on water plants, like "underwater goats" of sorts (e.g., the grass carp). Other herbivores scrape algae off rocks (e.g., the armoured sucker catfish). A few are filter-feeders, filtering the water for plankton (small plants and animals drifting in the water) (e.g., the big head carp). Carnivores (meat eaters) (e.g., the snakeheads) have powerful jaws and numerous sharp teeth for gripping their prey. All fishes swallow their food whole without chewing, although some will tear the prey into smaller pieces first. Some carnivores are so specialised that their jaws are able to extend forwards to catch their prey. Many fishes (e.g., barbs) are like man, feeding on both plant and animal matter (omnivores). Not surprisingly, these fishes are common and quite successful.

The majority of fishes lay eggs and practise external fertilisation. Many however, are ovo-viviparous, i.e., they nurse the eggs inside the bodies until they hatch. They thus effectively give birth to live young (e.g., guppies). These fishes practise internal fertilisation and the males' ventral and/or anal fins are modified to enable the transfer of sperm into the female's body. Egg-laying fishes however, have also many unusual methods to ensure a higher survival rate. Some build nests of air bubbles, others with weeds. Others brood their eggs and young in their mouths (e.g., Tilapia and some species of fighting fishes). Others guard their offspring with determined ferocity (e.g., snakeheads).
Skulls of carnivorous fish

Hampala macrolepidota
Cyprinidae, Malaysia

Golden Dragon Fish
Scleropages formosus
Osteoglossidae, Malaysia

Wallago leerii
Siluridae, Malaysia

Channa micropeltes
Channidae, Malaysia
Fishes have adapted extremely well to the rigours and challenges of the aquatic environment. Since they first appeared in the fossil record between 500 and 600 million years ago (they are the oldest vertebrates), they have diversified greatly. Some forms are believed to have given rise to all tetrapods (four-legged or -limbed vertebrates) including man.

The classification and nomenclature of fishes have changed greatly over the years, and the scheme proposed below is based on what we believe is a useful one. The naming of fishes follows that used for all animals, i.e., using the binomial (two names) system first proposed by Linnaeus in the 18th century. All scientific names are in italics.
Freshwater habitats
Fishes in Singapore
Amazing Fishy Facts
About the guidebook
From A Guide to Common Freshwater Fishes of Singapore by Kelvin K P Lim and Peter K L Ng
Published by the Singapore Science Centre and sponsored by BP

@Raffles Museum of Biodiversity Research and Singapore Science Centre