Anatomy: Animals Inside

Excretory System

Do marine fish drink water?

Marine bony fish such as tuna, flounder, and halibut drink seawater almost constantly to replace water lost by osmosis and through their gills. It is estimated that they drink an amount equal to 1 percent of their body weight each hour, an amount comparable to a human drinking 1.5 pints, or nearly 3 cups (700 milliliters), of water every hour around the clock. The gills eliminate most of the excess salts obtained by drinking large quantities of seawater. The fish excrete small quantities of urine that is isotonic to their body fluids. By contrast, cartilaginous fish (e.g., sharks and rays) do not need to drink water to maintain the balance of water (osmotic balance) in their bodies. They reabsorb the waste product urea, creating and maintaining a blood urea concentration that is one hundred times higher than that of mammals. Their kidneys and gills thus do not have to remove large quantities of salts from their bodies.


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