There are many kinds of organisms on the planet Earth and they have different habitats. Some of the organisms are fresh water organisms and some are pool water, and some salty water. The bodies and the requirements of the body with respect to their habitat varies and they cannot survive in other atmosphere. Like a fresh water organism can never survive in Salty water. Sea Creatures Need Salt
Shifty37 is correct...for the most part. The use of the word Osmosis is\correct as that is the function of a cell wall to protect the single cell organism. I believe you are wanting to teach children about TIDE pools, not rock pools. A pool in a chunk of rock somewhere is not the same thing. Even if you just had a pool of saltwater in a rock, it would not be the same thing and would not likely support much life. Tide pools on the other hand support abundant life of many types but require tidal movement to support that life. As they said above however there is a physical law of equalization that states: The concentration of any two substances that can be mixed will equal out.... So if I put salt water in with regular water the salt will eventually even out and the salinity will eventually even out. So if you put a single cell organism that lives in freshwater in salt water the salt will invade the cell to try and "equal" out, and the same goes for sea creatures. However there are many sea creatures that can survive in freshwater. Dolphilns, whales, orca, salmon, steelhead trout, etc all can live in both salt and freshwater.
The reason comes down to evolution. First off, a lot of marine creatures can survive in fresh water for at least a significant period of time. The main reason organisms stick to the salt water is due to the fact that they don't need to venture into fresh water. They've evolved to eat substances or prey on creatures that live in the oceans, and so stay in ocean environments.
However your right, some marine creatures cannot survive in fresh water environments, and the reason for that is down to two main factors. First off there are more minerals than salt in salt water, which a lot of marine organisms need to survive.
The second reason is down to osmosis, the movement of water. All organisms expend a lot of energy and effort keeping their internal water at a constant amount. In salt water, water from organisms will more readily move out of organisms since the salt in the water makes it so theres a lower concentration of water outside, than inside the organism. In fresh water, theres the opposite problem, water more readily moves into organisms. Organisms inhabiting both these environments have their own mechanisms for keeping their water level constant. Therefore if you moved a salt water animal that needs salt water into fresh water, it would absorb too much water from its environment and die. Being a teacher, I'm sure you'll find a way to explain that to children lol.
