Anatomy of finfish and shellfish

Unit 8 - Urino – Genital System

8.5. Excretory system
The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.
The kidney, gills, and skin play an important role in maintaining a fish's internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies.
Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion).
The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but that of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable (like those of the kidney) of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water in their food.
Marine fishes must conserve water, therefore their kidneys excrete little water. To maintain their water balance marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. By reabsorption of needed water in the kidney tubules, they discharge a more concentrated urine than do freshwater fishes. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Some marine fishes, at least, can excrete salt by clusters of special cells in the gills and intestine.
There are several teleosts-for example, the salmon-that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment.
Marine lampreys, hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood.