Their penises have to be long enough to find and inseminate distant neighbors, but not so long that they flop around uselessly in strong waves.
Previous work by Palmer's lab had shown that barnacles can change the shape and size of the feathery leg-like appendages they use to catch food depending on wave conditions.
To find out if barnacles also have shape-shifting penises, Neufeld and Palmer compared the penises of white acorn barnacles collected from rough waters against those taken from in protected harbors.
But barnacle penises are retractable and are not always easy to observe.
"It's hard to get barnacles to extend their penises on demand in the lab," Neufeld said.
So the team artificially inflated the barnacles' genitalia with seawater using a custom-made penis pump built out of tubes and hypodermic syringes.
The researchers also transplanted barnacles living in gentle waters to rough waters and vice versa, to make sure the penis variations they observed were a result of the environment and not due to genetic differences.
The results showed that barnacles could indeed change the width and length of their penises to suit the wave conditions of their new homes.
Heather Proctor, a biologist at the University of Alberta who was not involved in the study, said she was astonished by how pliable barnacles penises are.
"That runs rather counter to what we know about other arthropod copulatory apparatuses," Proctor said.
The body size of land arthropods such as insects and spiders might vary a lot within a species, but their genitalia tend to remain more or less constant.
"These terrestrial arthropods are trying really hard to keep their genitalia exactly the same size," Proctor said.
"So it's really neat to see that this aquatic arthropod is so flexible in the morphology of its penis."
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