Because it's sea-cucumber porn. A solo scene. That's what this is. Straight up, hardcore sea-cucumber smut. For this is how sea-cucumbers reproduce, by raising their front ends and releasing sperm (or eggs) well clear of the ocean floor, where fertilization can take place. Plus there's just something inherently pornographic about a creature with a big'ol gonad (relative to the rest of its body) but no brain. And then there's the fact that it's basically a big, waving phallus, unloading a long string of sexual fluids.
Spotted on boingboing, under a pretty fantastic headline.