Compelled to mate, yet firmly attached to the rock, barnacles have evolved the longest penis of any animal for their size - up to 8 times their body length - so they can find and fertilize distant neighbours.
Graduate student Christopher Neufeld and Dr. Richard Palmer from the Department of Biological Sciences at the University of Alberta have shown that barnacles appear to have acquired the capacity to change the size and shape of their penises to closely match local wave conditions.
When wave action is light, a longer (thinner) penis can reach more mates, but at times of higher wave action, a shorter (stouter) penis is more manoeuvrable in flow and therefore can reach more mates.
The research, published in Proceedings of the Royal Society B, suggests that sexual selection - competition with other males, female choice, sexual conflict between males and females - is not required to explain variation in genital form.
In barnacles, this variation appears to be driven largely by the hydrodynamic conditions experienced under breaking waves.
And a silly music video. The related material is around 1:55. Ignore the first minute or so.
I've also read studies that detail how barnacles determine where they plant themselves: before cementing themselves head first onto a surface, each barnacle senses chemical signals from other members of its particular species, so it knows it's close to potential mating partners and so that it's not too far above or below the tide that its species prefers. And for those of you that didn't know, barnacles were an eight-year detour for Charles Darwin. He completely revised the Cirripedia, spending 8 long years classifying new species and revising old, making major discoveries in the field. The time he spent on barnacles was partly in response to a comment from Joseph Hooker that only those who intimately know species, either having described or revised many taxa, were qualified to discuss processes of evolution. And in fact his work on barnacles gave him the necessary academic respect to release his theory of natural selection with the support of many scientists.