Obelia and the Very Messy Limpet

When we go to the beach, I pick up a bag of veggies for the animals in my aquarium. The crabs love green sea lettuce; I often see them grab a floating leaf, tear it into chunks and gobble it down. They're fond of a side salad of red Turkish towel, too. The big brown anemone seems to do better when there is a bit of rotting kelp nearby. Hermit crabs like the red stuff; thin red algae and Turkish towel. Some hang out on the sea lettuce, picking at the surface.

Snails and limpets feed on fresh eelgrass, but the hermits like it best a bit aged, covered with weedy growths. They browse knee-deep in the "pasture", whether eating the weeds or searching for bits of detritus, I don't know.

Eelgrass blade, with fuzz and a couple of limpets.

The last batch of fuzzy eelgrass brought along a small limpet, as fuzzy as the grass itself.

Limpet, bubbles, and a city on a hill.

The limpet is about 1/2 cm. lengthwise, less than 1/4 inch. I looked at it through a magnifying glass, to see if the tiny spiral worms were alive, and found a thriving community, more diverse than I had expected. I've been trying to get photos ever since, difficult since I needed it to stay underwater so that the waving, feeding critters would be out. There was so much activity that, even in still water, the eelgrass blade floated to and fro.

Side view.

In the peak of the limpet shell, a new eelgrass plant is growing. Under its shade, there's a circular patch of bryozoans. The red blob above this, and red lines in other spots, are feeding tubeworms. At the rim, two tiny (less than a millimetre long) mussels are hanging on.

Empty worm cases (Spirorbis) on the top of the shell.

The feeding tentacles of many tubeworms, in the crevices between stalks and spiral worm shells, are only visible under a microscope; among them tiny crawling and swimming things wander about.

A mussel on the eelgrass. Maybe a millimetre long. And on its tip (look closely) there's another shell, possibly a limpet.

It would be much easier to examine this limpet shell without all the weeds cluttering up the view. Or so I thought, until I came to my senses and paid attention to them, too.

One branch, covered with buds.

More than buds.

The "weeds" are animals: hydroids, one of the Obelia* species. They are cnidarians, related to the jellyfish; the adults form stalked colonies, but the young start out as medusas, miniature jellies. The "buds" are polyps, some feeding, some reproductive. In the photo above, just above the centre, you can see one open polyp, feeding. The tentacles are tipped with stinging capsules that explode when a prey animal touches them, incapacitating it so that the tentacles can bring it down to the mouth.

When a polyp is disturbed, it shuts down quickly. Most of the ones I saw were closed. I guess I was scaring them.

A closed polyp, with the tentacles pulled inside a thin case.

Life cycle of an Obelia. Image from Kent Simmons, U of Winnipeg.

The reproductive polyp has no tentacles. Inside it, baby medusas form and are set free as they mature.

Empty reproductive polyp.

I saw one tiny stalk with the open tentacles at the top, hiding at the base of the eelgrass blade, either a new branch on the old colony or a young polyp, recently settled. It was too small, too transparent for my camera.

*Obelia. Sounds like a woman's name. Would you name a kid Obelia?