Aquatic environment : Water contains a much lower concentration of oxygen than the atmosphere. Some freshwater organisms breathe air. Many waterbucks and certain beetles come to the surface entrap air bubbles under their wings cases and swim down in water. Water scorpion (Ranatra), mosquito larvae and rat-tailed maggots possess breathing tubes that extend the surface.
Aquatic animals have several adaptations to obtain sufficient supply of oxygen from the medium. Sessile animals wave their respiratory organs in the surrounding water. Echiuroid worms keep a current of water flowing through their burrows by the rhythmic contraction of their bodies. Most fishes and higher crustaceans possess special mechanisms for pumping water continuously over their gills. Certain other fishes produce a flow of water over the fills by swimming rapidly through water with their mouth open. The metabolism of species is so low that a small oxygen supply is sufficient. Certain species possess special respiratory pigments that aid in the absorption of oxygen at very low pressure.
It is rater difficult to determine the minimum concentrations of oxygen required by various aquatic animals as it is influenced by temperature pH and other factors. In those ponds and lakes in which the oxygen concentration of the lower levels falls to zero either in summer or in winter, the organisms migrate out of the oxygen less zone or they enter upon some sort of anaerobic existence. Copepods and other zooplankton leave the hypolimmon layer of lakes as the summer stagnation begins. Fish and other motile fauna undertake vertical migrations. Certain species of worms, crustaceans, insect larvae, mollusks , and other invertebrates remain in the bottom of lakes and survive a period of many weeks with no oxygen in the bottom water.
Water problem in Aquatic Environment
Depending upon the concentration of salts, the aquatic medium can be divided into marine, estuarine and freshwater habitats. Animals living in the above mentioned habitats have the problem of water balance inside their body tissues, except if the osmotic pressure inside the tissue of the organisms is equal to that of surrounding medium. The freshwater animals suffer from endosmosis, where the water tends to enter the body through all the possible surfaces which are exposed to the surrounding medium and tends to dilute the tissue fluid. Various methods are exercised by different animals to get rid of this excess water. Protozoans possess contractile vacuoles which collect excess of water from body fluid and burst out of the body releasing water to the exterior. The fresh water fishes have an exoskeleton of chitinous scales which prevents absorption of water through general body surface. These excrete hypotonic ruin with excess of water. On the contrary, most of the marine vertebrates have their body fluids even more watery than sea-water.
Aquatic organisms are commonly adapted to live within a specific salt concentration. Usually the limitations of these adaptive mechanisms restrict the animals either to fresh or sea-water. These could tolerate only narrow fluctuations in salt concentration. Such animals are known as stenohaline. But animals are found to tolerate wider ranges of salt concentration. These are known as euryhaline. Animals of esturarine water (tidal water) are euryhaline and can withstand greater fluctuations in salt concentration. Certain animals are stenohaline as well as euryhaline during different periods of their life span (e.g. salmon and eels). Sa1nons hatch in the head waters of certain rivers, where salt concentration is very low. As juveniles they migrate to the ocean. At maturity, they again enter the rivers of origin.