Aquatic realm is one of the most important habitats where a large number of animals live. It is especially important because it covers a large area of the universe. Adaptations to water habitat are of two types: primary aquatic adaptations which includes primitive gill-breathing vertebrates (fishes); secondary aquatic adaptations which are acquired as in reptiles and mammals.
I. Primary Aquatic Adaptations
In fishes the aquatic adaptations are perfect as they were evolved in water and never had terrestrial ancestory. The main changes took place are:
1. Body contour:
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The head, body and tail are compressed so as to form a stream-lined body. The head is sub-conical, the edges of the jaws and opercula fit precisely and even the eyes are according to the curvature of head. These are no protuberance of the animal through water.
2. Locomotion:
Locomotion is affected by undulations of the flexible body, having fins. The fins are paired or unpaired extension and serve as accessory locomotory organs.
3. Swim bladder:
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Presence of air bladder or siuim bladder in bony fishes serves as accessory respiratory organ as well as hydrostatic organ. It is a hollow structure and filled with gas or air and maintains the fish at a certain depth of flotation.
4. Gills:
Occurrence of conidial respiration as gills are well suited for gaseous exchange in water.