Fish adaption in the wild
Fish as a group are one of nature’s success stories. Fish can be found in nearly all bodies of water and on occasion can be found flopping onto land. Even the most inhospitable bodies of water such as suphurous thermal springs, ponds that dry up in summer, the deepest part of the ocean have all been colonised by fish.
It is estimated that there are currently 30,000 different species of fish on earth. That is more than any other vertebrate. There are thousands of species that live in freshwater tropical streams, rivers, ponds and lakes. The most abundant of these are in Africa, Asia and the Americas.
The most popular species of fish from these bodies of water have found their way into the aquarium trade and into people’s homes the world over. The most popular species in the hobby are usually the most colourful, hardy and easiest to breed or they may have an unusual shape or unusual behaviour.
Adapting to life
Fish have been on earth longer than reptiles, amphibians, birds and mammals, all of which have evolved from fish. It shouldn’t come as a surprise to note that fish have had longer to adapt to their environment than the other groups of animals listed. They are better adapted to their aquatic environment than most other life forms. They have found their way into many niches and developed many unusual behaviours that better allow them to cope with their environment.
Adaptation to life in rivers
The upper parts of river have fast flowing turbulent waters with little mineral content. The fish that live in these waters have adapted by becoming streamlined and fast swimmers and are able to mainatin a stationary position relative to the river bed while water flows past them. Because of the noise from the turbulent waters these fish have developed better eye-sight and lesser hearing ability. These adaptations help predator fish to better hunt their prey and also helps fish to avoid predation by larger fish and other predators.
Mid river, further downstream, is where the flow of the river is less and the river widens. Fish found in this part of the river are usually deeper bodied and less stream lined. Water in this part of the river is usually clear which aids fish with good eyesight.
In the river low lands the river becomes even slower and even wider. The water is usually cloudy with dissolved decaying vegetable matter and tannins from submerged logs, pieces of wood and roots.
Because of the water cloudiness in this part of the river fish cannot rely so much on their eyesight. They instead have developed better sense of smell, taste and hearing. Many shoaling fish in this part of the river have developed bright or even reflective scales to enable them to see each other in the murkiness.
Adaptation to life in ponds
There are other tropical fish that have adapted to live in lakes and ponds of Asia. During the hot and dry seasons the ponds and small ditches evaporate and reduce to small and stagnant bodies of water. Gouramis are a class of fish that have adapted to these low oxygen conditions by developing the ability to breathe air. They achieve this by gulping air into a specially adapted labyrinthian organ that absorbs the swallowed air. This allows them to survive in low oxygen conditions that would drown normal tropical fish.
These ponds and lakes are usually muddy or cloudy so having good eyesight is not such an advantage. Fish in these cloudy conditions have developed barbels near their mouth to help them find food. The gouramis have developed a pair of thin hair like fins that they use to feel and taste their surroundings and help them find their way through the thick vegetation. Fish in these cloudy conditions have also developed a better sense of smell.