I was a chef at a yacht club back in Maryland some years back. They had a large aquarium built into the wall near the bar, and because of my nonstop ceph-babbling, the head chef had taken some passing interest in the subject. After doing his AM checklist, he was at the bar waiting for the kitchen to open one morning when the aquarist stopped by for the regular upkeep and to check-out the health of the tropical freshwater species. The head chef (Mike) brought-up octopus to the aquarist, who suddenly became very excited and told him that he'd put one in the tank. When Mike mentioned that he thought they were only saltwater, the aquarist assured him up and down that such an animal did, in fact, exist. When Mike told me later what he had been told, I told him no. They do not exist. Cephs don't even like brackish water and are extraordinarily sensitive to saline levels. Nobody listened to me (just like when I tried to explain that there IS a difference between cuttlefish and squid), and one day about three weeks later, I arrived for work and was told that the aquarist had come by with an octopus in a sealed bag. According to multiple witnesses, he dumped the bag directly into the freshwater tank after submerging it - much too briefly - for the temperatures to reach a mean level. The octopus acted stunned for a few moments, then immediately attacked and killed one of the favorite fish. Then another. Then the octopus was removed and taken away, and the aquarist left, promising to pay for or replace the fish that were killed.
The two-spotted octopus common to inshore waters here in California - often popularly referred to as the California Mudflat Octopus - does live in intertidal zones, and has a much higher tolerance for salinity fluctuation and brackish conditions. I'm sure there are other varieties around the globe that live in similar zones and are similarly adjusted to life under these tough conditions, but as for a freshwater octopus? I sincerely doubt it. I wish it were true, but until a real expert comes forward with what, for me, would be an earth-shaking discovery, I don't believe in such a creature. Would be cool though. Too bad.
Maybe we could invent one, we let a octo live in a tank and for every batch of eggs that live in the tank, we add a single drop of freshwater into the tank, then we choose the stongest of the batch to live in it and we do so everytime they have eggs. Sounds interesting, right?
Erich, I too don't believe it's possible, though you know the biology of it a lot better than me. NTL, we haven't discovered every species in the world yet, and who knows what dwells deep in underground caverns etc... after all they recently found micro-organisms that eat minerals that no one dreamed they would and their dna is unique on this planet.
I wondered what happened to that poor octopus --- an aquarist should know better.