jon_r_mc asked in PetsFish · 1 decade ago

can sharks really live in fresh water?

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  • 1 decade ago
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    No they can't. Here's why:

    In order to carry out the complex chemical reactions that sustain life, all living things -- including sharks and people -- have a supply of water and salts in their bodies. Although skin and other living tissue looks solid, it actually has tiny pores in it. Small molecules such as water and salts can pass readily back and forth through the skin. But if there is more of one kind of molecule on one side of the skin than on the other, some of the molecules will move from an area of higher concentration to an area of lower concentration, through a natural process called 'diffusion'. Diffusion will occur until the concentration of that molecule is equal on either side of the skin. Diffusion is the same process that allows a sugar cube to completely dissolve in a cup of tea or coffee: the sugar molecules spread out from an area of high concentration (the cube) to an area of low concentration (the tea or coffee); eventually, all parts of the tea or coffee are equally sugary.

    The sea is composed mostly water, but dissolved in the water are also various salts. The concentration of salts in seawater is usually about 3 to 4%. The living tissue of human beings and most fishes are considerably less salty than this. As a result, there is more fresh water inside the human or fish than outside in the sea. In response, water naturally diffuses from the body across the skin, as though attempting to dilute the outside sea. (The diffusion of water across a semi-permeable membrane is a special case of diffusion, usually termed 'osmosis'; in the interests of simplicity, I'll continue to use diffusion here.) Human skin is relatively water-tight, but fish skin in rather leaky. As a result, the bodies of most marine fishes are constantly losing fresh water to the surrounding sea. But all living things need a supply of water inside their bodies in order to function properly. What most fishes must do to restore the water their bodies need is drink lots and lots of seawater You've heard the expression, "Drinks like a fish"? Well, it's true: marine fishes drink seawater almost constantly. In order to get rid of the excess salt contained in the seawater, many fishes have specialized salt-secreting structures in their gills called "chloride cells".

    But sharks have hit upon a different strategy. Instead of being less salty than the sea, sharks store certain metabolic wastes (namely, urea and trimethylamine oxide, or TMAO for short) so that their overall 'saltiness' is actually slightly greater than that of the sea. As a result, sharks do not continually lose their bodily supply of freshwater to the sea. Instead, any fresh water they need diffuses gently into their bodies through the mouth, gills, and other exposed membranes. Any excess water in a shark's body is filtered by the kidneys and excreted out an opening called the 'cloaca', located between the pelvic fins (the rearmost paired fins, behind the shark's belly). It's a very elegant solution to a significant environmental challenge. But it has its limitations.

    If a typical sharks were to swim its very 'salty' body into fresh water, so much fresh water would diffuse into its tissues that the kidneys would have to work overtime in order to get rid of it all. This is a very energy-demanding process, and explains why most sharks do not enter fresh water: it's simply too much effort to keep excreting all that freshwater. But some sharks, such as the Bull Shark (Carcharhinus leucas), are able to enter fresh water for prolonged periods. They achieve this neat trick by greatly reducing their bodily concentrations of urea and TMAO. Even so, a Bull Shark in fresh water is slightly saltier than its surrounding environment, so that it must continually excrete excess water in the form of dilute urine. In total, some 43 species of sharks and rays (which are essentially flattened sharks) spend at least part of their lives in fresh water. But one family of South American stingrays -- the so-called River Stingrays (Potamotrygonidae) -- evolved from a marine ancestor to become thoroughly adapted to living in fresh water. So much so, in fact, that their bodies have lost the ability to manufacture urea and -- if placed in full-strength seawater, they quickly die. These freshwater stingrays are thus trapped by their biochemistry.

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  • 4 years ago

    Bull sharks have been known to swim up rivers. They have a higher tolerance for fresh water than most other sharks.

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  • Anonymous
    1 decade ago

    Not all of them but the Bull Shark can live in fresh water even for extended periods of time. They are a very prolific shark and live almost anywhere and yes, we are on the menu with the Bull Shark.

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  • 1 decade ago

    Bull Sharks can live in fresh water and several species can stay in brackish waters. Most sharks , however need salt water to live

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  • 1 decade ago

    Sharks do not drink seawater; instead they retain high concentrations of waste chemicals in their body to change the diffusion gradient so that they can absorb water directly from the sea. This adaptation prevents most sharks from surviving in fresh water, and they are therefore confined to a marine environment. A few exceptions to this rule exist, such as:

    The river sharks are six rare species of shark in the genus Glyphis, are members of the family Carcharhinidae, and swim in fresh water.

    The bull shark, Carcharhinus leucas, also known as the bull whaler, Zambezi River shark or colloquially Zambi, is the best known of several species to swim in both salt and fresh water (it is found in Lake Nicaragua, in Central America) and in deltas.

    Source(s): Wikipedia, the free encyclopedia
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  • Anonymous
    1 decade ago

    Only the bull shark is capable of "living" in fresh water. They however can only stay in fresh water for short periods of time( a few months).They must return to saltwater to have their babies. The young will not survive in fresh water.

    Source(s): Spent last three years on sport fishing boat as deck hand. You get asked a lot of odd questions with this job. I always research on any question that I can't answer
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  • 1 decade ago

    Many "sharks" sold in pet stores and fish stores are not true sharks in the sense like a great white. They just have an appearance of sharks and are labeled as such. These shark look alikes can survive in freshwater or slightly salted.

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  • 1 decade ago

    Depends on the species. Many 'freshwater' sharks actually need brackish (slightly salty) water rather than fresh.

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  • Anonymous
    1 decade ago

    Not in an aquarium but there are plenty of documented cases of sharks being hundreds of miles up river in a freshwater river.

    Source(s): This information and detailed profiles of over 2,500 aquarium fish along with over 11,000 photos of aquarium fish can be found at FishGeeks - http://www.aquaria.info Get your 2007 tropical fish calendars: Freshwater http://www.cafepress.com/fishgeeks.91944067 saltwater http://www.cafepress.com/fishgeeks.91939978
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  • 1 decade ago

    No, there are many fish species that are sold as freshwater sharks, but most of them are not actually sharks at all, for example, bala sharks and iridescent sharks. Those that are actually sharks are actually not freshwater, they are brackishwater and will not survive in salt water.

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  • 1 decade ago

    only 1 bull shark

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