The penis in this barnacle (appendage at top) is barely the length of its tentacles — unlike the extra-long ones in some other species. The mollusc however disperses its sperm directly into seawater.

It can be hard to find a sexual partner when you are glued to a rock.

Barnacles famously get around this problem by having penises longer than their bodies, so that they can seek out relatively distant mates. But now it seems that some adopt another strategy, entrusting their precious bodily fluids to the currents.

Some of these crustaceans live alone, with no neighbours near enough to have sex with. In the case of the gooseneck-barnacle species Pollicipes polymerus, this presented a mystery: although some barnacles are thought to self-fertilize, scientists have never been able to witness reproduction of solitary P. polymerus, so these animals were thought to have to mate.

But Richard Palmer, a marine biologist at the University of Alberta in Edmonton, Canada, and his team have now shown that the barnacles are capable of capturing sperm released into the sea, which gives them another route to procreation.

“Barnacles are a highly studied animal,” says Palmer. “It’s exciting to us to find something so totally new in a group that is so extensively studied.”

Palmer and his team describe1 in Proceedings of the Royal Society B how they collected barnacles that were “well outside penis range” of any other individuals, but bore fertilized egg masses. By analysing the genetic make-up of these animals and their eggs, the researchers found that all the isolated barnacles' eggs contained DNA that could only have come from another animal. This indicated that they must have collected sperm from the water in a process sometimes called spermcast mating. Further genetic analysis showed that even barnacles located within mating distance of a fellow P. polymerus still collected sperm from the water.

Cast upon the waters

John Bishop, a marine ecologist at the Marine Biological Association of the UK in Plymouth and one of the scientists who coined the term spermcast mating, points out that this mechanism of fertilization is quite different from broadcast spawning, in which some aquatic animals release both eggs as well as sperm into the water2.

Spermcasting seems to be linked to animals that are fixed in place — or ‘sessile’, he says.

In the past, people often assumed that isolated sessile species “must sit there and self-[fertilize]” he notes, but now barnacles can be added to the list of spermcasters, which also includes sponges, corals and some molluscs. They are the first crustaceans known to employ this strategy.

“Most of the sessile groups have had the box ticked in spermcast mating,” says Bishop. “It goes with a sessile existence and more particularly with filter feeding.” (As filter-feeding animals, barnacles already posess the equipment to capture small particles in the water: see video below.)

And Pollicipes polymerus may not be the only species of barnacle that does it. Palmer and his student Marjan Barazandeh, the lead author of the latest paper, are now exploring whether two further species also trust their sperm to the waves.

A species of barnacle leaks sperm into the sea from the safety of its carapace.