Can you give an approximate arm and mantle length and do you have any clue as to the body of water that was its original home? From the Caribbean there are two that are common nocturnal dwarfs, O.mercatoris and O.joubini. We see the O. mercatoris the most. The animal in the photo at the top of the website looks very similar to O. mercatoris (I don't know how to tell them apart) but it is the Pacific Octopus digueti. From the pictures, I can't see the more typical look of O. mercatoris though and my, "don't put any money on it" guess would be O. joubini if it is a Caribbean animal.
If that is its current size after two months and it was about the same size when it entered your aquarium, it is definitely not vulgaris and is a dwarf species. If two months ago its whole length was about the size of its mantle, then it is possible that it is a larger species but I am inclined to think dwarf (caveat: I have a limited but growing experience with Caribbean species and am still very much a hobbiest, learning as I go. Additionally octopuses are often difficult to ID from photos even by our experts). IME, young octopuses are all very shy and may appear nocturnal until they are 4-5 months old (give or take) but if you have not seen considerable growth in two months this will be an adult and not a juvenile.
Thanks guys.. He was about the same size when I bought him two months ago. He looks a lot like the Octopus bocki in the other thread, but his eyes stand a little taller. If he is a dwarf, I'll keep him in a 40b and finish up the 75 for a different species. I'm hooked after keeping this guy.
Definitely a dwarf then. I am pretty sure it is not O. mercatoris or any of the smaller octos I have kept. If it is Caribbean, then quite possibly O. joubini but I have no experience with any of the hundreds of other little guys
FYI, a 75 would ultimately be too small for most O. vulgaris (we have seen two that brooded small enough for this size tank but typically they would need a tank twice this size). It is a good size for an O.briareus though.
Do you at least know what ocean it's from? That would help decide between boki and joubini (Caribbean). O. joubini has a lot of iridescent blue, and is about twice the size of what I think was a boki. The "boki" was red and white and friendly after dark under red lights. Either way, they were pretty happy with hermit crabs and frozen, uncooked shrimp.
The standing rule is, one animal per tank. There have been a few exceptions and O. mercatoris of the same size (and preferably housed together in the wild) seem to be compatible (tank raised hatchlings have proven to be very compatible). One member was successful with keeping O. bimaculoides hatchlings together but recent hatchlings kept in a bare tank by a student in MA has shown that this pairing is definitely not always successful. CaptFish and I have had multiple O. briareus hatchlings in larger tanks and only one has survived to the juvenile stage in each of three tank environments.
All the attempts we have recorded of keeping mixed species (even with dividers in the tanks) together have proven unsuccessful.