You'd think that acorn barnacles would have a tough time mating, being stuck to a rock and all. Fortunately, they evolved the "longest penis relative to their body size of any animal." The Brown University evolutionary biologists behind Creature Cast explain it in a new video shot by postdoc Stefan Siebert. Professor Casey Dunn assures us that the clip, and accompanying post, is SFW. Unless, perhaps, you're a barnacle. "Mating when you are stuck to a rock"