Do Fish Sleep?: Fascinating Answers to Questions about Fishes

Two: Where Fishes Live

TWO Where Fishes Live Question 1: Where are fishes found? Answer: Almost any body of water—fresh, salty, cold, hot (to a point), clean, polluted—can support fish life. Surprisingly, although fresh waters comprise only a tiny proportion of the available water on the earth (0.01 percent), they contain 41 percent of the known fish species. It is thought that teleost fishes evolved in fresh water. The proportionately great fish diversity in fresh water may also be partly due to the fact that freshwater habitats, unlike marine ones, are largely isolated from one another; geographic isolation permits populations to become more different from each other because they cannot interbreed, allowing evolution to occur more rapidly. Question 2: Can the same species live in both fresh and salt water? Answer: An aquatic animal faces very different challenges if it is living in salt water compared to fresh water. In fresh water, its tissues have a higher salt concentration than the surrounding water, so water tends to enter the body (through osmosis) and the animal becomes bloated. To counteract this, freshwater fishes excrete a lot of dilute urine. Gills are vital for their ability to regulate the chemicals in their blood. They take up ions from the surrounding water to maintain delicate ion balance in their blood. Salt water presents the opposite problem: the salts R5367.indb 12 R5367.indb 12 10/26/10 7:14:13 AM 10/26/10 7:14:13 AM WHERE FISHES LIVE 13 outside the body are more concentrated than those in the body, so if too much water leaves the body, the animal could shrivel up like a prune. Marine sharks and rays do not excrete much nitrogen-containing wastes in urine, but rather store them in a nontoxic form (as urea) in their tissues, allowing them to be in osmotic balance with the sea water. Bony fish, such as cods, solve it another way—they consume lots of salt water and then excrete the salts through specialized chloride cells in their gills. Their kidneys produce a small amount of very concentrated urine. Humans don’t have a way to excrete the salts; that is why people die if salt water is all they have to drink. While most species of fishes live either in salt water or in fresh water, those that migrate back and forth between salt and fresh water (like salmon, striped bass, and eels) switch strategies as they move up or down the estuary (the bay-like area where salt water and fresh water mix and create intermediate salinity). Fishes in estuaries, where the salinity changes a great deal, must tolerate big swings in salinity . Some species, like the mummichog, Fundulus heteroclitus, can live in full-strength sea water as well as fresh water. Fish that tolerate a wide range of salinities are called euryhaline, and those that tolerate only a small range are called stenohaline. Question 3: Do the same species live in freshwater streams as in lakes? Answer: Freshwater habitats are very diverse, with different temperatures, depths, and currents. They also vary in the amount of suspended materials, oxygen, and nutrients in the water and in the makeup of the substrate (bottom). All of these variables affect the species that are most at home in each habitat . Two major types of freshwater habitats are running water (streams and rivers, termed lotic habitats) and still water (lakes and ponds, or lentic habitats). Fishes in streams are affected by the stream width, the velocity of the water, and the sediments. Upstream water tends to be shallower with swifter currents, rocky substrate, lower temperature , and more dissolved oxygen than further downstream. R5367.indb 13 R5367.indb 13 10/26/10 7:14:14 AM 10/26/10 7:14:14 AM 14 DO FISH SLEEP? Headwater fish must be able to cope with these factors and find food, which is less abundant here than downstream. In small, exposed, rocky streams there is little chance for rooted plants to grow, so algae are the main food. Asian hill-stream loaches, Homalopteridae, for example, have flattened bellies and their fins form sucker discs to cling to algae-covered rocks from which they scrape and eat algae. Other fishes in this habitat feed on insects or crustaceans that graze on the algae or on streamside vegetation that falls into the water. As the water proceeds downstream , the land slopes less steeply, reducing...