By the time we see them—usually while doing boat maintenance—barnacles hardly seem alive. Even when they are more active, these animals are basically immobile. So you'd never suspect that barnacles are the species with the longest penis, relative to body size, in the world. That long filament in the photo above? That's a barnacle penis. Even more astounding: ThisCreature Cast video that shows barnacle penises in action, reaching out to investigate and (hopefully) mate with, nearby barnacles.
Naturally, where there are giant penises, there are experts who dedicate their lives to the study of giant penises. Southern Fried Science has a nice write up about the most recent discoveries published by J. Matthew Hoch, barnacle penis researcher.
Some of his interesting findings were that both wave action (yes! the motion in the ocean... this makes SO much more sense to me now). His study organism, the Atlantic acorn barnacle, Semibalanus balanoides, is known to have a penis with a exoskeleton with "accordion-like folds" that allows it to stretch to many times its relaxed length in order to find a mate.
Barnacles on wave exposed shores grow larger and their penises grow thicker/wider. They aren't necessarily longer than those that live in protected sites, nor do they have more folds allowing them to stretch greater distances. They just have thicker penises. This is likely a result of the water action. These barnacles have to have thicker penises for more support, making them less likely to break in the wave action and more likely to produce successful mating attempts.
Anyway, that's what Hoch is up to. The photo in this post was taken by another barnacle penis researcher (yes, there is more than one!) named Chris Neufeld who works out of the University of Alberta. He's found that stout-penised barnacles aren't the genetic result of generations of barnacles evolving to live in choppy water. Rather, the penises are pliable. No matter what kind of penis its parent had, a barnacle that finds itself living in rough water will develop a thick penis.