Hello good people,
fairly short question, answers welcome,
Peter
I’m not aware of any obvious reason why octopuses and squid have failed to adapt to freshwater. Other mollusks like snails and clams do just fine.
The correct English plural, by the way, is octopuses. “Octopi” is pseudo-Latin, the actual Latin plural being octopodes. (Octopods is sometimes used in scientific articles.)
Octopi, along with Squid, have copper-based blood.
This transports oxygen better, under very cold, high-pressure environments.
Could fresh water screw this up, somehow?
Cephalopods never developed a sodium pump thus osmosis would try to equalize salt concentrations between its body and the water, this would result in a loss of salt in the body and an increase in water.
The same compounds are found in the blood of other mollusks, and it doesn’t seem to be a problem for them. Many octopuses are found in warm shallow water, so their blood chemistry doesn’t limit them to cold deep water.
I wonder if we can introduce them into Lake Superior and see how they fare? 
I believe that is actually a Greek plural (which is what you want, “octopus” being from Greek, not Latin).
Is this true? I find it hard to believe,as this is a basic element of cellular machinery.
It is essential for nerve conduction (at least in most animals), and the squid axon is used as THE model for nerve conduction.
Furthormore, seawater is not iso-osmolar with intracellular fluid, either.
Sorry, Rat Avatar-
I found a cite that supports your statement. Shoulda looked first, before shooting my mouth off. :smack:
http://www.abc.net.au/science/articles/2013/01/16/3670198.htm
“It is probable that they never developed a sodium pump that would help them cope with osmotic change in freshwater,” Norman explains.
I still find it incredible.
Curious about the plural,
my latin and Greek teachers were both ex Oxford. One of their dislikes was Octopuses rather than Octopi. I don’t do ‘dog’ Latin.
Still fighting general ignorance etc.
So is it an essentially physiological difference with Squid and other eight limbed marine invertebrates?
Peter
Hydra close enough?
Cephalopods are found in all the oceans of Earth. None of them can tolerate freshwater, but the brief squid, Lolliguncula brevis, found in Chesapeake Bay, may be a notable exception in that it tolerates brackish water
Bartol I.K., Mann R., Vecchione M. (2002). “Distribution of the euryhaline squid Lolliguncula brevis in Chesapeake Bay: effects of selected abiotic factors”. Mar. Ecol. Prog. Ser. 226: 235–247. doi:10.3354/meps226235.
http://www.int-res.com/abstracts/meps/v226/p235-247/
Distribution of the euryhaline squid Lolliguncula brevis in Chesapeake Bay: effects of selected abiotic factors
Thank you Captain Haddock for your cite, ignorance fought.
Deep joy, the best of SDMB, short, sharp and precise.
P.
Fedex me one and I’ll introduce it to Lake Michigan. I’ve always wanted to play a prank and scare the shit out of some swimmers.
Please can we rename the Brief Squid the ‘Squ’ seems so apt.
P
No wonder they were “ex” Oxford. 
Straight Dope Science Advisory Board member bibliophage has written a masterful staff report on Latin plurals.
If that fresh-water mutant cephalopod you just caught in Lake Michigan has 25.13274 legs (or arms or whatever), it’s an octopi.
Are they Vulcans?
I’m not sure that’s entirely right. From what I can gather, oktṓpodes (ὀκτώποδες) is the Greek plural. Were the word native to Latin, the singular would be octōpēs and the plural would therefore be octōpedes.
(Another common Latin plural “mistake” is the usually jocular use of “penii” for “penises,” where “penes” would be the correct Latin plural.)
At any rate, I think “octopi” or “octopodes” or “octopedes” all sound dumb in English and the plural should be the normal “octopuses” (or perhaps I can allow for just “octopus,” but I’d prefer to keep it regular.)
Also, I didn’t realize this, but “octopodes” is not pronounced as I thought it would be (that is, “octo” + “podes.”) It’s something like “ock-TOP-uh-deez,” which sounds like something you’d order at a Greek restaurant. (And, actually, Greek style octopus is quite good.)