Kelvin
K P Lim and Peter K L Ng
|
Kelvin
K P Lim and Peter K L Ng
|
INTRODUCTION 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).
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).
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).
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. |
Introduction Freshwater habitats Fishes in Singapore Conservation 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 |
