Leftover legs

A barnacle lives his entire adult life standing on his head, glued to something hopefully solid. He has one eye, which he never gets to use***, locked down there inside his castle. His two tentacles are barely there, and glued down, besides.  He never leaves home.

Barnacle community. Photo from last April.

He* does have sensitive hairs on his legs, and he waves them about in the water, so he's not completely out of the world. He has to eat, somehow.

He's a crustacean, like the crabs and my hermits and shrimp. And crustaceans molt. They discard the suit they've grown out of, and grow a whole new one. Crabs and shrimp back out of their old carapace, pincers, legs, eyestalks and all. Hermits, living in a borrowed shell, have to leave the shell first, then back out of the old skin.

But the barnacle; does he molt, down there in his dungeon? How does he manage?

Every so often, I see a transparent barnacle body floating in the aquarium. Today, one got stuck on the wall, where I could get a photo.

Shed barnacle molt

The barnacle can't back out; he's glued in place. So instead, he has pushed his outer casing off, as we would take off a pair of pants. Inside a one-person tent. Sitting on the tent floor. And once he had it off, he tossed it out the door and let the current wash it away.

And then he swells up a bit, grows a size, and somehow - explanations vary - pushes or erodes the inside of his walls to make his tent** fit until the next molt.

A crab leaves behind a recognizable crab, carapace, legs and all. A hermit molts and leaves his head and thorax and all those legs and tentacles and flags, but the abdomen is always just a shred. The barnacle's leftovers are a bit harder to identify,

This may help. Image from University of Southern California. 

The long, feathery appendages in the second photo are the legs, or cirri, which is all we usually see, waving about, fanning food bits towards the mouthparts; those smaller spiky things. The rest is body, but it doesn't conform to the usual head, thorax, abdomen; everything is simplified and rearranged. And then shredded, as the barnacle yanked it all off, splitting it open at the head end, where he is nailed to the floor.

* "He" is really s/he; a hermaphrodite. But "he" is easier to say.

** Castle, dungeon, tent, suit, trousers, nails: whatever metaphor works at the time.

*** Update: except as a sensor for his garage door opener.

The University of Puget Sound museum has a good barnacle page.