Friday, February 28, 2014

Party's over

The tide was low again; not the summer-time half-way to the far side of the bay, but maybe about a third of the way. It was a pleasant walk to the edge of the water, splashing through a few ankle-deep streams, but the sand on the bars was barely damp.

Out on the last sandbar before the tide turned again, many hooded nudibranchs were dying on the slopes, a sad sight, but also a reminder of ongoing life.

Melibe leonina, the lion's hood sea slug, about 4 inches long.

Melibe doesn't have many enemies; she exudes a fruity perfume (smells like watermelon) that seems off-putting to most predators in the eelgrass and kelp beds where she lives. Except the kelp crabs, of course; nothing discourages a crab. So, in general, she dies of other causes. Exhaustion, for one.

When spring approaches, they gather in large groups to mate. They're hermaphrodites, so each one inseminates another, and at the same time has her eggs fertilized. Then they all have the job of laying eggs, in long, coiled ribbons, up to 30,000 eggs to a ribbon, which is attached to a blade of eelgrass.

That duty done, the tired nudibranchs let themselves float with the tide until they come to rest on the sand to die.

Most of the ones we found were still living, but extremely lethargic. I brought a few home and let them rest in a bowl of clean sea water. They moved around, sluggishly; one actually tried to capture some amphipods I gave them, but by morning, they had all died.

Look at the photo again: the hood is about half the size of the rest of the body. The sea slug spreads this out like a net, then closes it again on its prey, anything from a copepod to a small fish.

I found a short video on YouTube that shows them feeding. They remind me of the carnivorous Venus flytrap.

The squiggly lines in Melibe's body are branches of her digestive tract. It extends even into the paddles along her back, the cerata. These cerata are easily detached when the critter is stressed; most of the ones I sort of "rescued" had shed half of them by the time they arrived here. The rest fell off overnight.

If the stress is temporary, for example if the slug is fleeing a crab, the lost cerata grow back, the same way crabs' legs and pincers regenerate. The freed cerata may serve as a distraction for the predator, while the nudibranch swims away uneaten.

My hermits and crabs love the taste of these. Evidently they're not bothered by a tiny bit of fruity aroma; they grab the cerata quickly, then fight over them. I froze most of them for later treats.

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