Six-legged tigers

A couple of days ago, I went over to the vacant lot again, to check on the ant nest (of which, more later). It's been raining off and on, so the puddles are still there, but they've shrunk to about half the size they were a week before. The first one is barely an inch deep now, in the deepest part.

I was crouched over this puddle, trying fruitlessly to get a photo of a fast-swimming black bug, when I noticed what looked like a small fish. At least, it was long and streamlined, and moved like a fish. I abandoned my frustrating bug and went fishing with a small bug-collecting bottle.

The "fish" was also fast, but it attempted to hide in the loose mud at the bottom, and I was able to scoop it out, mud and all. In the bottle, it looked more like a miniature lizard:

Mini lizard with 6 transparent legs? 1/2 inch, jaws to tip of feathery tail.

When I released this one back into the puddle, I saw another, double the size. Too big for my bottle; I chased him all over the puddle with the camera instead.

6 hairy legs, a two-pronged tail, and a pair of sharp-looking hooks in front.

Later, when I looked them up, I learned that they are water tigers, the larvae of the Predatious Diving beetle, which may have been the black bug I was chasing at first. I'll have to go back again and actually catch one to be sure.

These beetles, adults and larvae, can be found in almost any body of water. The larvae hatch from eggs laid inside the stem of underwater plants. Or, in this case, in the stems of plants that at least for a while were underwater. Some live as larvae for up to a year, but in temporary ponds and puddles, they may mature in a few months. When they are ready, they crawl up onto land, make a burrow in the mud, and pupate there. After a week or more, they hatch and return to the water as adults.

Both adults and larvae are air breathers, although they live in water. The adults are scuba divers; they bring their air down with them, holding bubbles under their wings. The larvae breathe through the tip of the abdomen, holding it up just above the surface, but can go a while between breaths.

Two small water tigers. One is breathing; his abdomen tip and the two prongs of his tail are out of the water.

Tigers. The name gives a hint as to their habits; they're voracious predators, able to stalk and kill other insects, small worms, tadpoles, and even fish. And then, there are the defenseless smaller residents of the puddles; they'll take them, too; easy pickings.

Returning to the story: the big water tiger I was following stopped for a meal. The puddle is still as full of ostracods as ever; you can see some resting in the sediment in all three photos above. But here and there they formed clumps or heaps. And the ostracods in these heaps were dancing about like bubbles in a fast-boiling pot. Or like the mating snails on Crescent Beach.

A small clump of happy ostracods.

The water tiger meandered here and there around the puddle, not in any hurry, stopping occasionally to look around. Eventually, he found one of the ostracod gatherings, and dived in headfirst. He dug around for a minute, selecting or trapping one, then turned and sped off with an ostracod in his jaws, to eat it elsewhere. I chased him, trying for a photo, but now he was in a great hurry, in no mood to share his meal with any other water tiger, nor with the shadow above him. I soon lost him.

Selecting the juiciest, fattest ostracod. Tail on the surface, head in the grocery bin.

More info:

• Nature View by Bill Gibbons
• AgriLife Extension
• Photo of larva, BugGuide
• Adult, BugGuide

Here today, gone tomorrow

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.

About a quarter of the puddle.

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.

They're all over, but hard to see against the muddy bottom.

Zooming in, no flash. There are a few springtails on the stick, about 1 mm. long, slightly bigger than the swimmers.

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.

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.

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.

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.

And a water strider, one of two.

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.