Greg is joking. We had a big discussion of this some time ago. There are freshwater versions of many saltwater species - shrimp and lobster, for instance. Yet there are no freshwater octopuses....too bad!
Megas is existing off an almost completly freshwater Crayfish diet.
every third day or so i make him eat a thawed shrimp but thats about it
i catch the crayfish right out of the creeks near where i live.
Megas actually ate one that was 4 times his size or so. it was very interesting to watch Megas tactics.
and Megas is getting BIG hes grown a lot and like i said on and almost exclusive freshwater Crayfish diet.
I don't think you'd want to risk the possible long-term effects of feeding freshwater creatures to an octopus whose natural diet is crustaceans and other animals found in the ocean. Jean mentions that freshwater animals contain too much fat for an octopus.
Octos seem to enjoy crawfish and an occasional one probably won't hurt - the crawfish move fast and the octopus enjoys hunting. If the crawfish do manage to hide, they won't live more than a couple of hours. Be careful to remove the uneaten parts as soon as possible.
Plus the fw animals could have parasites that the marine critters are not set up to deal with (it happens!)
Good rule of thumb.....if you have an exclusively marine animal like an octopus it should be fed marine food. (and visa versa for freshwater critters) There are nutrients and trace elements etc that these animals can ONLY get from a natural diet.