For Barnacles, Sex Is a Real Stretch

Barnacles can radically change the size and shape of their penises to fight the waves and have sex.

Here is the challenge for the tiny hard bodies: Barnacles want to mate but are permanently bound to whatever rock or hull they once latched onto. Given that, they have evolved the longest penises of any creature for their size — up to eight times their body length — to seek out and have sex with their neighbors. (Most barnacles are hermaphrodites that alternate between male and female sexes over time.)

But large penises can be a problem, what with waves crashing down on the surfaces where these crustaceans often dwell. A too-long penis could flop around uselessly in such turbulence, drastically cutting down a barnacle's chances for procreation.

"The benefit of a longer penis is obvious for the barnacles — it helps them reach more barnacles — but the tradeoff is that it could wave around wildly on shores exposed to waves," explained researcher Christopher Neufeld, an evolutionary marine biologist at University of Alberta in Edmonton.

Barnacles catch food by sticking their feathery legs into the water, and they can alter the size and shape of their legs. "Over beers, we thought maybe barnacles could change the size and shape of their penises as well," Neufeld told LiveScience.

To see how barnacle penises handled rough water, Neufeld and his colleague Richard Palmer compared barnacles (Balanus glandula) from gentle, protected harbors and rougher, exposed coasts. To see how long the organs could grow, the researchers artificially inflated the barnacle penises with tubes and hypodermic syringes.

Barnacles that lived in rougher waters had penises that were shorter, stouter and more than twice as massive as those of barnacles in gentler waters. Barnacles transplanted away from their original shore could readily produce the right kind of anatomy for the water they found themselves in.

Neufeld speculated this adaptability results from the fact that these creatures could end up stuck on surfaces with various ocean conditions.

"There's the chance that the environment barnacle larvae end up settling is vastly different from the one experienced by their parents — their parents might have lived in a protected harbor, but the larvae might end up drifting to a wave-exposed shore," he explained. "So they deal with this variation by being able to change their legs and penises."

Neufeld and Palmer will detail their findings online Feb. 6 in the journal Proceedings of the Royal Society B.

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