Marine animals such as hag fishes, bony fishes, elasmobranch fishes, sea birds (gulls and petrels), marine reptiles (turtle, crocodiles, snakes, lizards) and marine mammals (whales, porpoises and seals) are hypoosmotic to their surrounding environment because their body fluids have an osmotic concentration about one half of that of sea- water and so they tend to lose water by osmosis and conserve salts by diffusion.
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Their need is to conserve water and remove salts and thereby keep their body fluids from becoming too concentrated. This problem has been solved in different ways by above mentioned animals.
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In hag fishes, for example, body fluids are about as salty as the salt waters of the surrounding ocean, and so are isotonic with them.
Hence they do not tend to lose water by osmosis. In bony fishes the body fluids are hypotonic in relation to the surrounding environment having an osmotic pressure only about one-third that of sea water.
Thus they have the problem of losing so much water to their environment that the solutes in the body fluids become too concentrated and the cells die.
This problem has been solved by the evolution of special gland cells (called chloride secretory cells) in the gills that excrete excess salt.
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Reduction or loss of the renal corpuscles, poorly developed glomeruli and loss of the distal convoluted tubules from the nephrons in the kidneys also serve to eliminate the water in the urine to a minimum.
Thus the urine becomes more concentrated and is more scanty. These fishes, therefore, can take in salt water.
The water molecules are diffused into the gut, while the excess salts are removed from the body through the salt secreting cells in the gills.
The marine cartilaginous fishes (sharks, rays) have solved this problem in a different way. These forms have about the same amount of salts in their body fluids as marine bony fishes but in addition they have developed an unusual tolerance for urea and trimethylamme the fresh water forms, and they excrete large amounts of dilute or isotonic urine.
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Sea birds such as gulls and petrels live at sea generally feed on marine animals which have high salt concentration and occasionally drink sea water.
They, therefore, have an identical problem of marine fishes, i. e., osmotic loss of water from the body and influx of salts into the body.
Similarly there are marine reptiles such as turtles, crocodiles, snakes and lizards which are hypoosmotic to their environment, i. e., sea water.
They also have the same problem of loss of water from the body and infusion of high salts into the body.
The kidneys of both marine birds and reptiles are not so efficient that they could excrete high concentrated urine but they have solved the problem of excess salt elimination by means of specialized salt glands (K. Schmidt-Nielsen and Fange, 1958; K. Schmidt-nielsen, 1960).
In marine birds these salt glands are located in the head while in sea turtle and other reptiles close to the eyes.
These glands are capable of desalting. The salt glands, containing a rich supply of capillaries, consist of thousands of branching tubules which remove salt from the blood and drain the resulting concentrated salt solution into the nasal cavities or directly to the exterior.
Marine mammals, such as whales, porpoises and seals also have the same problem of water loss.
They lose water through excretion and through moisture in the exhaled air, but this cannot be replaced by drinking sea water.
Instead the water is obtained through oxidation of food, since water is a by-product of such oxidation.
Also, water is conserved through a great concentration of the urine. The females have an additional water loss in their milk when they are nursing young, but this loss is counterbalanced by a concentration of milk. The milk of these mammals is about ten times as concentrated in fat as is cow’s milk.
So in general the following physiological adaptations are found among marine animals to meet the osmotic problems:
1. To prevent the loss of water due to exosmosis the entire animal body remains covered by a thick cuticle (invertebrates) or scales or thick body wall (vertebrates).
2. Reduction or lose of the renal corpuscles, poorly developed glomeruli and loss of the distal convoluted tubules from the renal system serve to conserve large amount of water in marine bony fishes.
3. Marine bony fishes also have special gland cells in their gills which remove excess salts from the body.
4. The marine elasmobranch fishes are capable to retain a large concentration of urea and other dissolved salts in their body fluids which keep the body fluids either isotonic or hypertonic to the sea water and thus, do not tend to Jose water by osmosis.
If there is any excess of water in the body that is removed by the action of well developed glomeruli of the kidneys.