. . . don't exist.
But because it's finger squid season in Texas, I've been reading up on the closest approximation to a freshwater squid: the Atlantic brief squid, Lolliguncula brevis. It's a pretty great name for a pretty great squid. The latin name just rolls off the tongue: lolly-gunk-you-lah. And "brief"? Like these? Oh wait, those are squid briefs, not brief squids . . .
I think the brief squid was named for its diminutive size.
Who knew brevity could be measured in centimeters?
But seriously folks, the brief squid may be the most freshwater-tolerant of cephalopods. It lives all along the east coast of the Americas, from New Jersey through the Gulf of Mexico and all the way to Rio de Janeiro. It is particularly fond of bays and estuaries, where salinity is reduced due to freshwater input from rivers.
Our brief friends appear to prefer water of 17 ppt or higher (seawater is usually in the low 30's, and true freshwater is zero), but studies in the early 80's suggested that they could survive salinity of 8.5 ppt. Yikes!
However, L. brevis is still far from being a truly freshwater squid. So far, that exists only in fiction.
Freshwater Squid . . .