Why can't crabs live in fresh water?
Photo: Flickr user nate steiner
Blue crabs thrive in salt water, but crabs placed in fresh water will soon die. Why?
It’s thought that life evolved in the sea, and most animals’ blood reflects this ancient history. The blood of most animals has a chemical composition not unlike sea water. For these animals, the salt ions in blood are critical for life. That’s why marine animals don’t do well in fresh water. In fresh water, a marine animal risks losing its internal salts to the outside environment. Very few marine animals have evolved ways to regulate their internal salt concentration in both salt and fresh water.
But blue crabs can do this, to some extent. They’ve evolved a kind of “salt pump” in their gills that allows the crabs to keep the salt level in their blood constant – even in water that’s almost fresh. That’s important – because young crabs develop in sea water, while adults live in bays and estuaries where the water is less salty.
But even the remarkable gill pumps of a blue crab can’t draw salt from fresh water. So, crabs will die in fresh water because they can’t keep enough salts in their blood.
More resources on “animals”…