Black-eyed babies

The water in my aquarium was cloudy. It shouldn't have been; I'd just finished a complete tank clean-up. But all the residents seemed happy and busy, so I let it be.

Early this morning, just after the tank lights came on, I noticed movement along the wall nearest a light; little flecks of light, dancing in clouds of smaller flecks that moved like flocks of starlings, swirling, coming together, then washing away in the current, only to turn and spin into the flock again.

What were they? I fished some out with an eyedropper and got out the microscope.

Zoea! Crab babies! Cute, big-eyed, dancing babies!

This is one of the "big" flecks of light. Big eyes, long spines in front, long tail.

Side view

These were hard to photograph, since they are never still. But the tiny specks were even worse:

Three zoea, and a few of the tinies.

At first, I thought the littest specs were copepods, and the zoea were eating them, but looking closely, I realized that they moved differently, were a different shape, and were continuously waving a bunch of little legs. First molt of crab zoea? I don't know.

One dancing zoea, dozens of little critters. The dark spot in the centre is part of their body; the eyes are smaller, at the front. One near the tail of the zoea shows a clear profile.

There were thousands of these.

Three zoea, one showing off his headgear.

The female black-clawed crab had been carrying eggs when I first saw her. When I changed the water a couple of days ago, they were gone. I think these may be her babies, freshly hatched.

And mostly, freshly eaten, too. Many marine invertebrates, like crabs have thousands of offspring at a time; the world they are born into is so dangerous, so full of hungry mouths, that most of them die in infancy. The thousands I saw this morning are gone; checking the tank half an hour ago, I found only three dancing zoea. The barnacles and anemones have probably finished off the rest.

More info:

 Apart from the Dromiacea, all crabs share a similar and distinctive larval form. The crab zoea has a slender, curved abdomen and a forked telson, but its most striking features are the long rostral and dorsal spines, sometimes augmented by further, lateral spines. (Wikipedia)

One of Haeckel's drawings. A typical crab zoea, for comparison. (via Wikipedia)

From SeaGrant, Alaska, here's a photo to show the size of these babies; these are red crab zoea.

Zoea and pencil.

There's a photo of a group of zoea, apparently through a microscope, at that link. And Keith Davey, in Australia, has a couple of wriggly gifs, showing the zoea and the megalopa stages of baby crabs.