In the peculiar world of barnacle sex, shape matters just as much as size.
To cope with changing tides and a sedentary lifestyle, the gnarly crustaceans have evolved penises that are eight times the length of their bodies—the longest relative to body size of any animal.
(Related news: "'Probably the Oldest' Penis Found in Spider Fossil" [October 6, 2003].)
Now new research reveals that the creatures' lengthy members can also change shape to suit their environment.
Barnacles living in gentle waters have long, thin penises best equipped for maximum reach, the study found.
But those animals living in rough waters have shorter, stouter penises that are better able to withstand strong waves.
The findings suggest an animal's environment can sculpt its genitals as thoroughly as female choice or competition between males.
"It's just an example that says, Look, there's other things that can also be important for shaping the evolution of genitalia," said study co-author Christopher Neufeld of the University of Alberta, Canada.
Neufeld and colleague Richard Palmer describe the work online this month in the journal Proceedings of the Royal Society B.
Penis Pump
Despite being hermaphrodites that have both male and female genitalia, barnacles prefer to mate with other individuals whenever possible.
But as one of the few immobile species to use direct insemination, barnacles face some unique challenges.
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