Zoologger: First animal with ovaries on the outside

Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Species: Allapasus aurantiacus
Habitat: On and around the seabed off the coast of California

If there's one way we can be sure that life on Earth really is the result of evolution, and not the guiding hand of a cosmic engineer, it's the hideous design flaws. The examples are too numerous to list, but let's just consider one: human males have their testicles on the outside.

It seems they work better that way, because sperm production works best slightly below human body temperature. But it isn't half inconvenient – as any male who has ever been kicked in the goolies will tell you.

Spare a thought, then, for the newly-discovered acorn worm Allapasus aurantiacus. The females are the first animals known that have their ovaries on the outside. But according to their discoverers, they are the first of many.

Deep-sea worms

Acorn worms are quite different to the more familiar annelid worms, as they are close-ish relatives of backboned animals. They live on the sea bed, often burrowing into the sediment.

No one had noticed A. aurantiacus until June 2002, when Karen Osborn of the Smithsonian Institution in Washington, DC spotted one via a remotely-operated vehicle deep into the Monterey Submarine Canyon off California, around 3000 metres down. Intrigued, she had it brought to the surface.

Once Osborn got a closer look she realised it was an acorn worm. Unusually large eggs, each almost 2 millimetres across, were pouring out of it. The new species belonged to a family of acorn worms called Torquaratoridae, which all live in the deep sea – unlike many acorn worms, which prefer the shallows.

Ovaries on display

Each worm has two "wings" – flaps of skin on the main body along much of its length. In an unprecedented arrangement, the ovaries are attached to the inner surfaces of these wings.

"Usually you want to protect these things, and keep them near and dear," Osborn says. Even human testicles have several layers of skin protecting them. But the eggs of A. aurantiacus are only protected by a single layer of cells. That might make it easier for sperm to reach them, Osborn says.

She has since found a few males, whose genitals are in the same place on their skin. It's not clear how they fertilise the females' eggs. One possibility is that the males release sperm into the water, whereupon the females take it in through their gills and squirt it over the ovaries – which are ideally placed by the gill outlets.

Floating free

The worm uses its wings as sticky pads to attach itself to the sea floor. "They secrete a ton of mucus, and that probably helps them adhere," Osborn says. "Mucus is a big part of their lives."

Mucus may also be the key to the worm's ability to float above the sea bed – something that only the deep-sea acorn worms do. Osborn thinks they secrete a balloon of mucus around themselves, which catches currents that then carry the worm away.

But first they have to get off the sea bed, and to do that they excrete the contents of their guts. This material acts as ballast, so getting rid of it means they drift upwards.

First of many

Osborn and her colleagues have since found over a dozen acorn worms in the same family. They all have external ovaries and the distinctive wings. One species has hermaphrodite forms, another first for acorn worms.

Worms aren't known for their parenting skills but in a further surprise, at least one species uses the wings to shelter its offspring. Osborn found a single female, of a species closely related to A. aurantiacus, that was sheltering well-developed eggs and a few larvae under its wings.

She suggests that the acorn worms' strange lifestyles are adaptations to life on the sea floor, where food and mates are scarce. In a place like that, it makes sense to move around in search of new feeding grounds, to make use of any and all sperm that comes your way, and to keep your young close until they're ready to take care of themselves.