In cephs, the cells have a greater salt concentration than the surrounding fresh water. Water enters the cells in an attempt to reach equilibrium and the cell walls burst - eventually killing the organism.
animals that move between fresh and salt water have ways of maintaining the water/salt content of their cells.
Ok, the ceph's cells need keep a salt concentration gradient, isn't it?
If you put it in a fresh water, the gradient changes, the fresh water get into the ceph cells for re-equibrate cell gradient, and the poor bug, finally bomb off!!!
The old myth of crocodiles shedding tears for their prey comes from the fact they have glands for removing excess salt near their eyes. Bull sharks often go hundreds of miles up river from the ocean here in Mississippi (one was caught on rod and reel north of St Louis).
And what do you want to say with it? ( is nice info but I think not relevant here). Cephs can have very different kind of cells. Maybe sharks can do it, and also the humans can do the oposite to it. Why have the cephs be the same like another animals?
Do you have another teory.
P.D: Sorry maybe I have sound too rude, but my english is limited. Sorry.
There are plenty of molluscs that have adapted to a fresh water existence, slugs and snails even made it on to land, but cephalopods for one reason or another over the almost 480 million years of their evolution have done neither. The same applies to echinoderms; an entire phylum sealocked. This could all change one day, but it simply hasn't yet. It may only take a few random quirks to the genome and Bob could very well be your uncle.