Marine Biology

Adaptation of Nekton

i. Buoyancy : Most fish have a gas-filled swim bladder. Most fish can regulate the amount of gas in the bladder and thus control their buoyancy. Gas filled cavities (lungs) help all air-breathing nektonic animals to float. Other means used by marine mammals to increase buoyancy are bone reduction and the presence of a layer of lipids (fats or oils). Large amounts of lipids are also present in nektonic fish that do not have swim bladders (sharks, mackerels, bonito). In addition to these static means of producing buoyancy, some nektonic animals have hydrodynamic mechanisms for producing buoyancy during movement. Examples are pectoral fins and flippers and the presence of a heterocercal tail.

ii. Musculature: The fishes those living in the deep sea areas have suitable adaptations to withstand the prevailing high pressures and to the dark conditions. These fishes are fragile and weak with soft and loose muscle.

iii. Colour: They develop black or dark brown colourations to minimise the problem of predation.

iv. Eyes: Many deep sea nekton have reduced eyes or no eyes.

v. Bioluminescence: Some fishes have bioluminescent organs in their body to attract their prey as well as to find their mates (e.g. angler fish)

2. Reptiles : Turtles possess a special adaptation to marine life by having buccal respiration in which a highly vascularised mucous epithelium takes up oxygen from water in the mouth. As in the case of turtles, the mucous epithelium of the buccal cavity  in sea-snakes is known to be supplied with numerous capillaries which enable these snakes to take oxygen form the water. This would explain the records of sea snakes seen resting in the bottom and remaining submerged for several hours.

3. Seabirds : Seabirds swim at the sea surface and under water; they use their webbed feet, their wings, or a combination of wings and feet. They float by using fat deposits in combination with light bones and air sacs developed for flight. Their feathers are waterproofed by an oily secretion called preen, and the air trapped under their feathers helps keep the birds afloat, insulates their bodies, and prevents heat loss. When diving, the birds reduce their buoyancy by exhaling the air from their lungs and air sacs and pulling in their feathers close to their bodies to squeeze out the trapped air. The underwater swimmers such as cormorants and penguins have thicker, heavier bones, and penguins have no air sacs