Monday, December 07, 2009

Underwater tug-of-war

"Waste not, want not," they always said. I took it to heart. While the nudibranches, Melibe leonina, were slowly dying in a bowl of clean sea water on my table, I noticed a few cerata, fin-like structures on the back of the sea slug, lying loose.
"In some nudibranchs, cerata are used as decoy tactics. Typically, these cerata are not armed with nematocysts, but when attacked, the nudibranch will autotomise or drop one or more of its cerata. The dropped cerata produce a sticky secretion and wriggle energetically for some time after being cast off, presumably causing a distraction and allowing the nudibranch to escape." Wikipedia

These weren't wriggling by the time I found them, but they did look interesting, almost like tiny slugs or flatworms themselves. I wondered if the crabs would eat them. I spooned up a couple and dropped them into the aquarium. A hermit crab snatched the first one from the water as it came down; a few seconds later, one of the crabs had dragged the second into its hole under a rock. Neither crab was planning to share.

I collected the rest of the cerata and dropped them in, one at a time. An anemone nabbed one, and closed in on it, so I nudged another in the direction of a larger anemone. Snatch, fold, gulp! It was gone in a few minutes and the column was bulging.

The barnacles didn't like them. One touched the net of the large black thatched acorn barnacle, and it closed down tightly and stayed that way all night.

The next day, I dropped in a dead nudibranch, entire, which started a sand-wrestling, claw-waving free-for-all. One small crab hid under the rock and pulled the whole nudibranch down after himself, but that wasn't to be permitted. Imagine a cat half-way standing on her head to reach that finger wriggling 'way down under the sofa cushions; put a shell on her, and you've got a hungry hermit after a hidden snack.

Later on, when I looked in, the remains of the nudibranch was spread out on the sand, with a circle of scavengers all working together on it. The next morning, it was gone entirely.

The other three Melibes died. I put them in the freezer. A couple of days ago, I floated a Melibe ice cube in the tank, and watched, camera in hand.

Hermit Rex grabbed it on its way down, again. But a second later, the Blue-clawed hermit scrambled over the rocks and clamped onto the other side. Both pulled back, stretching the flesh to its limit. Rex is bigger; he had the advantage, but he was trying to eat and tug at the same time. Blue got a good grip and yanked. Almost got it, but ... The tug-of-war lasted quite a while, while the other crabs and hermits lined up to watch and wait their chance.

They look like they're sharing here, but Rex is just trying for the advantage of height. The fringed edges of the hood are still entire. Blue is pulling on it; Rex has the thicker body.

Eventually, one lost his grip, and Snowflake, the white crab, dashed in and ripped a chunk off. The other hermits descended on the tattered remains. By morning, again, not a morsel was left.

Big Green, holding a tiny portion of leftovers.

Even the worms were involved. They are too timid to enter the fray, but sneak around the edges, looking for crumbs, retreating in a rush if a crab leg touches them. This one stretched out almost his full length, about 6 inches.

The snails are algae eaters; they weren't interested.

Periwinkle. Isn't he pretty?


  1. I have no idea why I like your posts so much - but I do.

    So weird but so fascinating

  2. Mark, the universe is like that; terribly weird and always fascinating. May as well fit in.



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