mutualism b/w octos and hermit crabs?

Jul 7, 2006
hi...i'm a new member! :grin:
right now i'm in South Caicos (Caribbean) taking a marine ecology class. for the past few weeks i've been checking up on a caribbean reef octopus that lives near the school's dock. i had only seen him inside his den up until last night when i finally saw him half out of it. unfortunately, my dive light scared him back inside, but as i watched, i realized he wasn't the only guy in the neighborhood. at least 20 hermit crabs were crawling in and around his den, at one point even completely blocking the entrance for a good 20 minutes. i have never seen so many crabs at once before, much less around a predator's den!
i assume they were there scavenging empty shells from the octo's latest meal (of which there are always many scattered about), but still i was surprised--i have never heard of anything like it!
can anyone shed some light on this?
I have noticed this as well with the giant Pacific octopus, and in fact there is a whole host of organisms that will be living in a GPO's midden, including hermit crabs, umbrella crab (cryptolithodes), and many snails. There is a paper on this relationship in GPO dens that includes many other associates such as sculpins and starfish:

Hartwick EB, Thorarinsson G (1978) Den associates of the giant Pacific octopus, Octopus dofleini (Wulker). Ophelia 17(1):163-166

I hope this helps out a little bit.

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