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I bought a big bag of them (around $10) from one of those oceanside shell shops in Galveston, Texas. They were called Goose Barnacles.
I've seen big clumps of them for sale in various LFSs, also ones made out of resin.
Some octopuses will hide in them - my bimac would not.
Hm, I looked a little around and often the same pair of arms are refered to as "the first two" or "front two"
So basically the octopus has a "front" and "back".
this nicely shows in this picture here for example:
Note that the octopus faces left although he is about to move towards the camera sideways, one eye leading the body. So the opposide of the mantle has to be the front part Although I think most pepole who see this picture would say that the octopus faces right. It just looks more natural to most people, but when you consider the position of the beak, it all makes sense that he actually faces left
I think your octo was too big to be an O. rubescens.
They're hard to distinguish when they are smaller, and those whose photos I posted were still quite small. O. rubescens has a body of 10cm as an adult, arms to around 30 cm each, while, as you know, a GPO gets much larger.
The distinguishing features of O.rubescens are ring-like white markings. There are two pairs of these, one on the body and the other at the base of the arms in front of the eyes. These can be hard to see on younger octos.
I suspect that you had a smaller GPO - not all of them get so large.
And just a personal comment - a lot of people seem to get bitten by O. rubescens - I first encountered this in comments of divers. And sure enough, when I was visiting the NRCC, one of the little red octos bit Jonathan!