Ostracod

Image: Daniel Stoupin

Sometimes, don't you just want to wrap yourself up in titanium-reinforced cotton wool and shut the whole wide world out for a little while? Maybe a long while? Let the Ostracod show you how it's done.

Ostracods are a class of at least 13,000 species of crustacean. The first thing you notice when you see one is that there's barely anything to see. That's not just because they're tiny, most of them being about 1 mm (0.04 in) long, it's also because they're entirely covered in a bivalve shell. There's just... nothing there!

Image: Natural History Museum, London
Peek-a-boo!

This shell is hinged on one side and can be open and shut at will. It's like a mussel, which is why Ostracods are also known as Mussel Shrimp. And when it's all closed up it looks like a tiny seed, hence Seed Shrimp. Of course, they're not actually a kind of shrimp, but that's nothing new for crustaceans; most things that get called "shrimp" aren't actual shrimp. Then again "shrimp" comes from the Old Norse for "thin person" so who's to say?

Image: Keyser, Dietmar
Under the shell. Head on right

Were you to open up the shell, you would find the little Ostracod snuggled up inside. They're mostly a head. A gigantic head with two pairs of large, bristly antennae used for swimming and an assortment of mouthparts. Behind it is the thorax with one or two pairs of appendages which may or may not be legs for walking. And usually that's about it - no abdomen to speak of!

Image: Brandão, Simone N.
Many marine species have spiky shells

Most Ostracods are so tiny that they needn't bother with a heart or gills. Many have only one, simple eye, while others have a pair of compound eyes to go along with it. In any case, with these scant resources Ostracods have become one of the world's most successful crustaceans! It's as if planet Earth went to the Ostracod beach and now finds grains of Ostracod sand all over the place.

Ostracods can be found in marine and freshwater environments all over the world. Most of them live right at the bottom, even burrowing into the sediment, while others prefer to swim and float around in the currents. A few are parasitic or otherwise live on echinoderms and other crustaceans and there are even some that have made it onto land! These intrepid species live in damp soil in Africa, Australia and New Zealand, carrying water with them in and on their shell.

Being so tiny, Ostracods can be carried far and wide by frogs and birds. And the craziest thing is that sometimes they can even survive getting eaten! That tiny shell is strong enough to have a chance of passing right through the digestive tract of a fish. If there's any time you want good, titanium-reinforced cotton wool, it's when you're in a digestive tract.

With such a diversity of habitat it's no wonder there is also a diversity of feeding habits. Some are herbivores, scratching off algae and such from whatever surface they're on. Others are scavengers in the sediment. The ones that float and swim around could be filter-feeders, capturing food with their long antennae.

There are also carnivores, the most terrifying of which may gang up in huge numbers to take down worms and fish far larger than themselves. They go for the old "eat it alive" method, like armour-plated piranha singling out the choicest, vulnerable parts of the body. Horror, basically. Or the Nanobot Apocalypse.

Image: eclectic echoes via Flickr
Ostracod on a plant. That's all

There's something else that's significantly bigger than an Ostracod. Males of some families can have sperm up to ten times longer than his entire body. As you might imagine, he doesn't have too many of them and he keeps them all coiled up in his gigantic gonads. So weird. I mean, mammals can sling gallons of sperm all over the place if they want to, it's a very different experience when each one is longer than you are.

Image: eclectic echoes via Flickr

Along with the pumps for getting the stuff out there, the sexual organs can take up half of the male's whole body! I wonder what people would look like if the more they thought about a body part, the bigger it got? I guess you'd put on weight every time if you thought about going on a diet. And when you get backache, your back would get huge. Horrible!

Image: Giuseppe Vago

Some Ostracods don't bother with sperm. They don't even bother with males. Many freshwater Ostracods reproduce by parthenogenesis, where the female basically clones herself. Like Water Fleas, these populations only produce males when they have a mind to and such methods allow them to live in temporary water bodies, leaving behind hardy, drought-resistant eggs when it all dries up.

Ostracod fossil

Also, the oldest known penis comes from an Ostracod. It's some 425 million years old. That's the other thing about that shell, it not only allows Ostracods to go all over the place, it also allows them to stay exactly where they are. In other words, they form really great fossils. There are even more fossilised species than there are living ones, dating back all the way to the Cambrian some 500 million years ago.

So Ostacods have it all. Safe and secure all over the world, they live fast, live long and they leave a good looking fossil. Where's my all-enclosing shell?