A Forest of Hydras

I've mentioned before, I'm a bit of a geek. I say "a bit of" because I'm something of an underachiever as a geek. I can program a bit, enough to sort numbers, work with text, and pull web pages, but I can't do anything particularly cool. I like Star Wars, but can't name the actor who played Uncle Owen or any of the characters from the Jedi Academy books. It's not really even geeky anymore to like the Lord of the Rings, since it became a big SFX movie.

Where I'm really a geek is life sciences. I'm the guy who, when the rest of the neighborhood is moaning over the tent-caterpillar infestation, will say, "Cool, kids, you've gotta come and see this!!! They're building a silk tent!!!" At 35, I'll be late for a meeting because I was watching two ant colonies fight on the sidewalk. My main interest in computer programming is from a desire to create artificial life simulations.

So I have a fish tank.

Well, it's not really a fish tank, because there are no fish in it. It's got copepods. Copepods are little crustaceans, not much bigger than a good-sized comma in standard newspaper print. They came with a few liters of pondwater I picked up back in December, and have been eating algae in the tank ever since. Their numbers range from several dozen to several hundred depending on... well, I haven't figured that out yet.

A few months ago, I found something else in the tank. Didn't know what at first... little things a bit bigger than sesame seed that crawled around on the glass, like flocks of tiny sheep, munching the algae growing there. These turned out to be ostracods (called, appropriately enough, "seed shrimp"), another kind of crustacean. This was interesting, because I'd had the tank going for months when they showed up. I can only guess they'd finally gotten numerous enough to notice. Since then, they've multiplied into the hundreds, and that glass of my tank is free of algae except for little spots that don't get much bigger than 2 to 3mm before they're grazed down.

In fact, having no predators in the tank, I've been worried about overpopulation, of both the copepods and the ostracods. Then last week I noticed a third kind of crustacean in there, tiny black dots a little smaller than poppy seeds that swim (unlike the ostracods, which mostly crawl) with a smooth motion (unlike the jerky copepods). I still haven't identified them.

Today, looking in on the little things, I noticed something new on the glass. A thin strand of bright green, not more than 2mm long. I looked closer, certain it couldn't be what I thought it was, and yep... it was. A tiny green hydra. Now, the presence of a hydra was fine news to me -- they're predators, rather like a freshwater anemone. I'd had some when I first gotten the water samples with the copepods, but I'd thought they hadn't survived the transfer to the tank; I'd been hoping to get to the little pond and get some more now that it's warm out.

And then as I looked I counted at least a dozen, some of them just tiny green specks, against the glass. I saw a few more on the sides. WTF? Where the had they come from? I haven't added anything but treated tap water since December, at the latest.

Now, the interesting thing about green hydras is the reason they're green; they have symbiotic algae in their bodies, which photosynthesize for them, so they aren't entirely dependent on predation. Finally, I looked down into the tank, and I noticed, in the right corner, a forest of hydras. Hundreds of them, in an area no more than 5 inches long by about a 1/3-inch wide.

Of course: the spot closest to the sunlight, which was also farthest from the front of the tank. Hidden at the top of the water, where he light refracted funny from most angles I'd look at it, and lost in the green of the free-floating algae in the water, I never saw them. They'd been there for months, eating larval copepods now and again, but probably mostly living off the sunlight. Many have tiny hydras budding off of them even now; no telling how many generations they're into. Only now, I guess, is the population high enough that they've had to spread out.

So this is pretty cool. I'm thinking I'm going to put off that trip to the pond for awhile and see what else turns up in the tank. So far it's had a marvelous property of balancing itself out.