We had a good, hard, pelting rainstorm a few days ago, all day long. Across the street in the vacant lot, that means we'll find big puddles, slowly draining through the hard-packed clay soil. They're temporary ponds, lasting only a few weeks unless it keeps on raining, but while they last, they're busy. Crows congregate around the larger ones, bathing and arguing. A few times, we've found mallards resting there. Insects drop in for a drink; beetles and snails rest in the cool mud.
At the far end of the lot, I stopped to inspect a new pond, a few inches deep.
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| About a quarter of the puddle. |
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| The top layer of clay had dried and cracked; the cracks remain, even underwater. |
But what were those tiny specks in the water? When I bent down to look, I could see that they were swimming about. Hundreds upon hundreds, thousands of them, as small as dust, and about the same colour as the yellowish mud. I took a photo with the flash, to see if that would make them more visible.
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| They're all over, but hard to see against the muddy bottom. |
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| Zooming in, no flash. There are a few springtails on the stick, about 1 mm. long, slightly bigger than the swimmers. |
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| The same photo, cropped. Now the critters are recognizable as ostracods. |
Ostracods are small crustaceans, typically around 1 millimetre (0.04 in) in size, but varying from 0.2 millimetres (0.008 in) to 30 mm (1.2 in) in the case of Gigantocypris. Their bodies are flattened from side to side and protected by a bivalve-like, chitinous or calcareous valve or "shell". (Wikipedia)
Under a good lens, an ostracod looks sort of like a swimming clam. If the light is right, a hint of the body can be seen through the shell; imagine an amphipod, all 14 legs vibrating constantly, inside a clamshell, with a few legs or antennae occasionally peeping out.
They live in both fresh and salt water environments, from the poles to the tropics. I have often found them in sand or seaweed from the intertidal zone. Some 65,000 species have been identified; there are probably many more to be discovered.
These ones, the ones in my puddle, have selected a particularly difficult environment. The water is clear, there is plenty of vegetation to serve as food, but if the sun shines, their home will disappear in short order. Some species of ostracod will live for up to a year; these guys won't have the chance. A few weeks, if they're lucky.
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| At one end of the pond, there is a mass of fibers. Old moss? Leftover trash, well rotted? I couldn't tell. But the ostracods were less active around them, easier to photograph. Click to see the photo full size; you can see the clam shape, and on one, up in the right hand corner, a tentacle peeping out. |
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| An ostracod at home in a bowl. The one eye is plainly visible. Some ostracods have two. |
How do these animals manage to live under these conditions? First, they don't waste time. They spend up to 80% of their lifespan laying eggs. These eggs are resistant to dessication; they can "sleep" in dry soil for years, until the rains come again. Some adults and young are also able to go into stasis when the pond dries up or freezes.
And also, they reproduce in vast numbers. Most of the ones in this pool won't survive the dry times ahead, but enough will to produce the next generation.
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| Also in the puddle were a number of these swimming bugs, very fast, zipping around the bottom, occasionally popping up to the surface for a second or two. I think they may be the Acilius diving beetle. |
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| And a water strider, one of two. |
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| Examining the photos, looking for a clear shot of an ostracod, I discovered this snail on the bottom. It is a species I have not seen here before. |
The white oval to the left of the snail looks like an abandoned clam shell. (Look at the photo full size.) This may be a molted valve. Like other crustaceans, the growing ostracod has to abandon the hard outer shell, in the case of freshwater ostracods, 8 times.
There's an excellent
YouTube video here, showing the live ostracod kicking inside her shell. Watch all the way to the end, when she suddenly starts zipping around.
And
this is mind-boggling:
Ostracods possess the largest sperm in the animal kingdom in both relative and absolute terms. Ostracod sperm can be up to ten times the length of the male's body! Some male ostracodes need a special organ (Zenker's organ) to aid in sperm transport.
However, about a third of freshwater ostracod species don't worry about that;
they are parthenogenic, and need no males to reproduce.
