Re: how do fish maintain homeo staisis when going from salt to fresh water??

Hi Peter, GREAT QUESTION!

They do it by a process called osmotic regulation, or often referred to as 
asmoregulation for short.  This is the maintenance of proper internal salt 
and water concentrations.  When they move into freshwater they must keep 
their body fluids higher than that of the water.  Water enters their 
bodies osmotically, and salt is lost by diffusion outward, so they have 
developed ways to maintain salt levels higher, and minimize water intake 
through the gills and by using a special apparatus in their kidneys.  

There are some other animals that live in estuaries which have to contend 
with daily changes in salinity due to the tide!  Some do it by 
osmoregulation (like the fish) and keep their internal fluid levels at a 
constant rate, while others (generally soft bodied organisms, like worms) 
let their internal fliud levels change with the water around them - these 
guys are called "osmotic conformers".

Rochelle.