I assume you are up late studying now days so you will have an opportunity to play with it when you take a study break . I am guessing it is the same species as Puddles and Beldar. Both learned to play squish and were very personable. The bold red and white is very striking if you get to see it but my two only displayed when unhappy so after a time in the tank their color was just kind of reddish brown. My two had a green speckled look (in addition to the bold white spots) that gives them a briareus look but I don't recall them ever being white. The speckles were not that evident to the eye but show up in the photos. They would be one of my favorite species (whatever it is) if it were for the hours that they keep.
Are the back arms considerably smaller than the front 4? Both of mine seemed to have little use for the back ones excep to prop against when on the sand. The front four were not only longer but much thicker.
I saw a lot of similarity to a young O. briareus with mine but not a merc. The arms were definitely diagnostic so I am guessing she is not the same species as the two I had. Did you ask them for some hint of where it was captured? Since it was Diver's Den (Maya was DD but Puddles LA) and not the standard Live Aquaria, they should be able to tell you.
Like Crissy has mentioned about Adopus, the Macropus complex has a wide variety of species (look through my favorite book - if you have not already - and you will see a few really odd ones) attached to the group. Have you seen the green fluorescent dots like on O.briareus yet? Several of my pictures of Puddles would make you think O.briareus but he definitely was not.
There is the possibility that this one is so young that the arm ratio has not yet reached adult proportions but I kind of doubt it since my O.briareus and O.mercatoris hatchlings both were proportionate when they were very small. However, this is a pelagic born species and they start out not looking much like the adults (unlike the two large egged ones I have raised. I have not had the opportunity to see a small egg hatchling yet so I am going by photos but Monty is likely a small egg species and the proportions on his arrival have not changed much).
Well things seem to be progressing nicely. She, still guessing, denned up exactly where D would have predicted, in one of my large purple barnacles. She is using a snail shell as a door that she keeps sealed up during the day but open at night.
hummm, the ONLY octo I have seen that will choose a den with only one opening is a merc ... but there are lots of them I have not even seen in pictures, let alone gotten close up and friendly with However, you may have a very expensive mercatoris if denning can be considered diagnostic.
Not much to report. I haven't seen her for quite a bit. I know she is in her hole cause her snail door moves every night. I have not observed any feeding yet and its got me a little nervous. I think I'm just going to buy a bunch of snails hermits and ghost shrimp, which is what she was eating at LA, and fill the aquarium.