Spider crabs build big pyramid in pre-migration act

Close to 1,000 spider crabs make a huge pyramid in the shallows of Port Phillip Bay, Victoria, Australia. Photo courtesy of Pink Tank Scuba
Close to 1,000 spider crabs make a huge pyramid in the shallows of Port Phillip Bay, Victoria, Australia. Photo courtesy of Pink Tank Scuba

PT Hirschfield and some friends were diving near the Blairgowrie Pier in Port Phillip Bay, Victoria, in Australia when they encountered a most unusual sight: close to 1,000 spider crabs crawling over each other to make a huge pyramid on the ocean floor.

It’s the time of year when all the spider crabs gather for a mass migration and simultaneous molt, so the divers were on the lookout for the congregating spider crabs and found this (one minute of video captures the essence):

The odd behavior comes as a pre-migration act, but not much is known about it.

“Unfortunately, not much official research is done on the spider crabs so it’s more guesswork based on observation than anything,” Hirschfield told GrindTV in an email. “I hope to spend a solid week underwater with them when the migration proper happens to continue studying them more closely, but the pre-migration stages remain somewhat of a mystery.”

Close to 1,000 spider crabs make a huge pyramid in the shallows of Port Phillip Bay, Victoria, Australia. Photo courtesy of Pink Tank Scuba
Close to 1,000 spider crabs make a huge pyramid in the shallows of Port Phillip Bay, Victoria, Australia. Photo courtesy of Pink Tank Scuba

Hirschfield added that the sight of a spider crab pyramid is “pretty unusual, at least in the shallows. I’m not aware of anyone having seen a pyramid this high, though who knows what happens at depth?

“The behavior I observed was really just a lot of climbing and a few scuffles as crabs got in each other’s faces…A few of the crabs in the pyramid were also adorning themselves with sponges becoming decorator crabs, which is a camouflage mechanism.”

All pre-migration behavior is said to be short-lived. For instance, Hirschfield recently saw a couple thousand spider crabs in the shallows at the start of a two-hour dive and they had all but vanished by the end of the dive.

So it is unlikely that the pyramid lasted very long, making the encounter that much more special.

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