Princley said:
Sharks of many different types have been found in fresh water as far up the mississippi as MN.. , for a time a shark can in fact live in fresh water But not on a perminate basis.. Many Marine fish their body's take in Salt and need the salt
where as Sharks bodies block out the salt ..i am putting this as simple as i can so seeing sharks bodies block out salt they can go in to fresh water for a time
Always going back home to the Oceans..Slamon r a bit simlar by be able to keep out salts..they breed in fresh go ocean bound for years and return to fresh to lay eggs and die...most sharks breed in the ocean tho
Sort of... but they don't necessarily "block" out the salt, it's a little more complex
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 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. Though CS may start life in fresher waters, once they are older, they no longer to adapt as well to variations, and will need a more brackish setting. You need to acclimate them SLOWLY, raising the salinity very slightly over a period of several weeks for the best results. It's not too complicated, but you do need to be patient, as in the wild, they do not just up and leave the freshwater and jump into salt. They hang around inlets, and such, gaining a tolerance, and acclimating themselves. Just take your time, and do it slowly, and make sure you watch for any signs of stress. Good luck.
