Dancing jester

Nothing is ever as simple as it seems. Especially underwater. Everything is layered; things grow on shells, on seaweed, on rocks; other plants and animals grow on those, and are home, again, to another community of something else. And everything eats. Hydroids eat plankton; nudibranchs and hermit crabs eat hydroids; crabs eat hermits when they can catch them; gulls eat crabs. So do we, if they're big enough.

On one of the blades of the skinny eelgrass I brought home the other day, I noticed a fuzzy barnacle. And the fuzz, looking at it with a hand lens, was not the expected diatoms or algae.

Side view of the blade of eelgrass. The barnacle is about 3 mm high.

These look like some type of hydroid; there's a stalk, a polyp surrounded by tentacles, these ones with knobs on the tips. But I can't find them in any of the books or web pages I've searched, probably because they are too small to attract attention.

Our common Obelia hydroids are branched.

Small Obelia, on another blade of the same plant. The eelgrass blade is 4 mm. wide.

But the new ones are clubs with at the most two polyps at the top. And they're noticeably orange, rather than semi-transparent like the Obelias.

But wait a minute! Looking at that first photo again, what is that thing that looks like a little man in a jester's hat? That's no hydroid!

I watched for a while, and the jester began to dance.

See him there, on the centre left side of the eelgrass?

It's a Caprellid, a skeleton shrimp. I never expected to see one so small. But nothing else dances like they do.

Standing upright again, waving antennae and big pincers (the jesters hat).

I found three more skeleton shrimp, all about the same size, along the blade of eelgrass. Standing upright, with the pincers raised, they mimic the hydroids, possibly as protection from predators that will avoid the stinging tentacles of a hydroid, and otherwise would find a caprellid quite tasty.

Most species are predators that sit and wait like a praying mantis, with their gnathopods ready to snatch any smaller invertebrates which come along. They accentuate their adaptive form and colouration by assuming an angular pose, resembling that of the fronds among which they live. They remain motionless for long periods of time while waiting to ambush their prey, often protozoa or small worms. (From Wikipedia, Caprellidae)

Wikipedia adds that they are eaten, in turn, by anemones, nudibranchs, and fish. I think I would add hermit crabs to the list. The colonies of hydroids and skeleton shrimp lasted only a night, with a dozen hermits busy cleaning the eelgrass. All that was left in the morning was the barnacle.

....

And there's more! Empty doughnuts tomorrow.