I have always been particularly interested in the concept of the freshwater octopus - an unknown cephalopod reported from lakes in Oklahoma and surrounding areas of North America. Whilst the stories seem to be apocryphal, some seem to have a germ of truth. According to accepted wisdom freshwater octopuses are a complete impossibility, because cephalopods cannot live in freshwater. But scienetists have been wrong before...
One specimen found dead in a North American lake a few years ago proved to be a marine species Octopus burryi that had been dumped. Mark A Hall apparently suggested that these things could be eurypterids - an extinct group of arthropods related to arachnids, which include the largest known arthropods that ever lived. They were formidable predators that thrived in warm shallow water in the Ordovician to Permian from 460 to 248 million years ago. However, I have not read his argument, and on the surface this would seem to be highly unlikely.
Another suggestion is that they are freshwater jellyfish. There is indeed a species of freshwater jellyfish - Craspedacusta sowerbyi which appears to have an almost global distribution. It has been found in countries on almost every continent including parts of Yorkshire, London and nearly every state in America.
The medusa appearance is sporadic and unpredictable from year to year. It is not uncommon for C. sowerbyi to appear in a body of water where they had never been documented before, in very large numbers, and they may be even reported on the local news. In parts of the mid-west and the great lakes area of North America, seeing one is considered to be a sign of good luck.
The trouble is that it is only the size of a 5p peice. I, however, have been fascinated by the species ever since first reading about it in my childhood bible of Natural History The Hong Kong Countryside by G.A.K Herklots. I want to exhibit some one day in the CFZA museum.
Could there be a larger species, analogous to the larger jellyfish of the oceans? It seems unlikely, but still the stories of freshwater octopi continue.
We were recently sent this video which purports to show two nubile young women being attacked by one of these cryptic cephalopods. I think that it is dubious in the extreme, but include it for its entertainment value..