The blood of freshwater fishes is typically more salty than the water in which they live. Osmotic pressure, the force that tends to equalize differences in salt concentrations, causes water to diffuse, or enter, into the fish's body, primarily through the gills, mouth membranes, and intestine. To eliminate this excess water, freshwater fishes produce a large amount of very dilute urine. Lampreys, for example, may daily produce an amount of urine equal to as much as 36% of their total body weight; bony fishes commonly produce amounts of urine equaling from 5 to 12% of their body weight per day. As these fishes are gaining water, they are losing salts. Salts contained in their foods are insufficient to maintain the proper salt balance. Freshwater fishes have therefore developed the capacity to absorb salts from water by means of their gills.
Marine bony fishes, in contrast, have blood that is less salty than sea water, and consequently they lose water and absorb salts. To offset this loss of fluid, marine fishes drink seawater and produce very little urine. The drinking of seawater, however, adds to the concentration of salts. These salts are eliminated in several ways. Calcium, magnesium, and sulfates are passed out through the anus along with wastes. Sodium, potassium, chloride, and nitrogenous compounds, such as urea, are excreted through the gills.
The hagfishes and the sharks have approached the problem of fluid balance in other ways. Hagfish blood has a total salt concentration approximately equal to that of seawater. Sharks' gills do not excrete the nitrogenous waste product urea, retaining it instead in the blood. The presence of urea and another waste product, trimethylamine oxide, as well as various salts, keeps the shark's blood at a slightly higher solute concentration than that of seawater.
Distribution - Anatomy - Circulation -
Body Temperature - Swimming - Gas Bladder
Lateral Line System - Evolution - Reproduction
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