how some aquatic organisms must respond to variations in their environment with particular reference to salt and water balance.

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Aquatic environments are generally split into three categories; fresh water, marine (salt water) and estuarine (a combination of salt and fresh water). The organisms that live in each of these environments have specific adaptations that allow them to respond to changes in their surrounds. Fish generally prefer either fresh water or salt water and are unable to cope with a change from one to the other. For example the cells in fresh water fish are saltier than the surrounding water, so due to osmosis (the diffusion of water through a semi-permeable membrane) water is constantly flowing into its body. For this reason fresh water fish require very efficient kidneys to remove all of the extra water, and methods of maintaining and collecting the salt lost through urination. Random fact: Some fresh water fish are said to never drink and be constantly peeing... In contrast salt water fish have the opposite problem. Their cells tend to be less salty than the environment in which they live, so the water is trying to escape. As salt water fish do not have access to fresh water to replace the water they are losing (due to osmosis), they have to constantly be drinking salty water in order to stay hydrated. To cope with the excess salt being held in their bodies, the kidney of the salt water fish has had to evolve, to produce very concentrated urine, and some even have other mechanisms to excrete excess salt. Because the internal structures of each fish is set up to cope with its specific environment (either salty of fresh) it is unable to adjust when placed in an unfamiliar environment. For this reason most freshwater fish can only live in fresh water and vice versa. However some fish like salmon and eels are able to cope with the changes between salt and fresh water and are thus able to survive in estuaries. An estuary is a body of water where a creek or river

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