cthulhu77;98624 said:
It is simply a matter of respiration and food.
If you look at the complex of cephalopods living in the oceans (and trees, for those of you with fanciful natures), it is easily seen that the larger species live in deep water...the closer you get inshore (less absorbable oxygen), the smaller.
For a squid, or more likely, an octopus, to adapt to freshwater, it would have to me extremely small...and the food sources in freshwater aren't even close to what is available in the tidal areas.
Doubtful that it will ever happen.
I'm with you on food, might add salinity, but I'm not sure I buy the respiration thing: many cephs live in deep, low oxygen conditions, and do just fine; in fact, Gilly's work with Humboldts suggests that they're much better at coping with that than folks had previously thought. Ward's book
Out of Thin Air I read recently posits (among many other oxygen-related animal evolution theories) that cephs evolved driven more by advanced respiration, and that jet propulsion and free swimming were side effects of just pushing lots of water over the gills... Although cephs need more oxygen for their active lifestyles, there are plenty of examples of freshwater bivalves and even terrestrial gastropods, so it's demonstrably not inherent in mollusc physiology that they can't survive in low-oxygen environments... and cephs tend to have much better coping mechanisms for low oxygen than other molluscs, like the separate hearts for the gills.
I suspect that vulnerability is an issue, too: cephs don't do well drying out, and they're not well-suited to living in small pools (since we know how much food they require and waste they produce!), and they're vulnerable to predation by birds in shallow freshwater.
if it were true though!
