Even Charles Darwin marvelled at the length of the barnacle's penis. In some species, it's up to eight times the body length.

Not all barnacles are so well-endowed, however, raising the question of how animals that can't reach their neighbours get it on.

Now, new research suggests that at least one short-penised barnacle, the Pacific gooseneck barnacle, oozes sperm directly into the water for mates to capture, an approach that researchers previously considered too haphazard for the sedentary crustaceans.

Like the sperm of other arthropods, barnacle sperm can't swim. That's a powerful selective pressure to evolve foolproof methods of delivering sperm directly to eggs, said Joseph Pawlik, a marine biologist at the University of North Carolina, Wilmington, who was not involved in the study.

Barnacles face another challenge because they're stuck in place.

Researchers have thought that the animals' long penises, which wave around until they find a neighbour and eject sperm into an egg-bearing cavity, were their only means of solving the problem, aside from a few rare cases of self-fertilisation.

After fertilisation, the eggs hatch into larvae that feed, grow, and roam until they find a good place to settle down, preferably near other barnacles.

Some barnacle species, however, such as the blue-gray Pacific gooseneck barnacle (Pollicipes polymerus) commonly found in tide pools on the Pacific coast of North America, have penises hardly as long as their own bodies, up to 19mm. That's barely long enough to reach their immediate neighbours, much less stretch to distant paramours, said Marjan Barazandeh, a PhD student at the University of Alberta in Canada, and lead author of the new study.

Although biologists assumed that the hermaphroditic P. polymerus made up for its short penis by self-fertilising when it couldn't reach a mate, Barazandeh wasn't convinced.

Little data supported the idea, she said, and she found that isolated barnacles did not produce offspring.

Many other fixed marine organisms, such as sponges, reproduce by releasing their sperm into the water, trusting that their hermaphroditic mates will capture it.

Although the idea was unheard of among crustaceans, Barazandeh observed barnacles leaking sperm at low tide and wondered if the same phenomenon, called spermcasting, might account for the unexplained pregnancies.

To test the idea, Barazandeh collected 37 individual barnacles with fertilised egg masses that were multiple body lengths apart from one another — far out of reach of wandering penises.

Then she used a modern genetic tool: paternity testing. Nearly all of the egg masses contained DNA from another barnacle, demonstrating that the sperm had indeed travelled through the water column, she and colleagues report online in the Proceedings of the Royal Society B.

"This is the first evidence of releasing and capturing of sperm in any crustacean that we know of."

The idea that "sloppy sperm could just be floating around," will likely make marine biologists cringe, Pawlik said.

Previous work on gene flow in barnacle populations has always assumed that the crustaceans can only have sex with their immediate neighbours or self-fertilise.

If other barnacles also use ocean currents to cross-fertilise, scientists may have to rework their population models, he said.

The results also have wider implications for reproductive biology in crustaceans, adds Kevin Eckelbarger, a marine biologist at the Darling Marine Center at the University of Maine in Walpole.

Although "we are all taught that crustaceans copulate," the paper "points out other ways nature can solve a biological problem," he said.

"Why depend totally on penis length to reach a neighbouring barnacle?" he asked.

"Why not spermcast as well? What's to lose?"

- SCIENCE NOW