Thursday, May 30, 2019

Aquarium update

Surprising happenings in my aquarium! Up until a few months ago, almost every time I changed the water in the tank, I removed everything so that I could scrub the walls without scratching them or upsetting the residents. The hermit crabs and crabs went in one dishpan, the snails in a yogurt container, the barnacles and shells with anemones attached in another, seaweeds and eelgrass in a bucket, and so on. While they were there, I fed the hermits, checked over everything for problems, weeded out dying seaweed.

And, putting everything back, I counted. At last count, there were 40 hermits, most of them tiny. And 4 crabs. A varying number of snails; up to 70, down to about 40 (the big anemones eat them and I go to the beach for more.)

Orange-striped green anemones in old barnacle shells.

After the debacle with poisoned water this winter, I'd not been able to find a reliable source of water, so instead of bringing in new every couple of weeks, I've been cleaning and changing the filters a couple of times a week, adding fresh, filtered water as needed to bring down the salinity. I've scrubbed the walls and weeded, but without emptying the tank; it all takes time. So I haven't been counting my resident population.

Pink-tipped green anemone with her baby (clone) in an old moon snail shell.

The water situation improved. I found a source of safe water, and cleaned out the tank. And counted the hermits and shore crabs. I expected to have lost a few over the winter; nothing lives forever. But when I counted (three times, to be sure) there were 43 hermits, and still the 4 crabs, full-grown now. The tiny hermits were bigger now; I didn't see any pinhead-sized ones. But they were all there. Amazing!

A couple of the crabs were carrying young. They took good care of their brood, but of course, it was all a waste, wasn't it?

Out in the wide ocean, the baby crabs (zoea and then megalops) are microscopic free-swimmers that become part of the plankton, the base of the food chain that feeds invertebrates, fish, birds, and ultimately us. Life is precarious out there; everything is looking for a tasty bit of wriggly meat. Ma crab has released thousands of zoea; only a very few will survive.

In my tank, predators are few. The oysters, the anemones, the worms; that's about it. Except for one big, starving, insatiable predator: the pump and its filters. It sucks in seaweed fragments, incautious amphipods, copepods, and of course, all the crab and hermit crab babies. I never expect any native-born youngsters in the aquarium; the filters eat them.

Oyster lips and orange-striped green anemones

So I never expected this. I was watching a young hermit, when I saw a crab-like movement underneath an old shell. Crab-like, but too small. I had to get out my hand lens to look at it. And it was a crab! A Baby Crab! Tiny, tiny, really tiny; a new pre-adult, apparently grown from scratch in my tank!

And now, checking carefully under stones and shells, I found two more, one about 4 mm. across, the other a whole cm. across.

The second smallest. Very shy, running away from my and my camera. His "carpet" is a shard of broken abalone shell.

Over the winter, I've brought home a couple of handfuls of eelgrass and rockweed, a blade or two of red algae: it hasn't been a good winter (or spring) for fresh beach veggies. I always check this over very carefully, looking to see what live things have hitchhiked. There were a few tiny periwinkle snails, no hermits, an amphipod or two, and no crabs. And a damaged jellyfish last week, which the crabs ate. That's it.

So maybe, just maybe, those extra three hermits are also home-grown, not three pinhead hermits I'd missed before.

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