Why do some organisms survive in salt water but not others?
Some organisms' internal salt levels mimic the external, thus they are able to survive in a salty environment, without water loss. Some have gills and renal or kidney organs, to help concentrate and excrete salt. Fish living in a marine environment will always be in peril of losing water to the environment, as outward osmosis will occur, if not for their gills and kidneys helping to maintain homeostasis. Sometimes, marine creatures, like whales, eat...
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It is due to adaptation, some organisms have been in salt water for so long that they have developed mechanisms that helps them survive. While other organisms have been out of salt water so long that they only developed mechanisms that would help them survive in their current environment but not salt water.