The only hard body part of an octopus is it's beak. Thus, an octopus can fit through any rocky opening that will allow it's beak to pass, and they usually reside deep in rocky crevices with amazingly small openings. We found a young juvenile living in a beer bottle recently. As explained yesterday, this octopus has grown to become the apex predator of this reef, and has chosen a spacious home with a very large opening. Kitsap County, WA