If sea spiders had a creation myth, it would go something like this. An inebriated deity stumbles home after a hard day’s creating, finds a bunch of leftover legs, glues them together, and zaps them to life before passing out and forgetting to add anything else. The resulting creature—all leg and little else—scuttles away to conquer the oceans.
This is fiction, of course, but it’s only slightly more fanciful than the actual biology of sea spiders. These bizarre marine creatures have four to six pairs of spindly, jointed legs that convene at a torso that barely exists. “They have to do most of their business in their legs,” says Amy Moran from the University of Hawaii at Mānoa, who studies these animals. They have, for example, no lungs, gills, or respiratory organs of any kind. Instead, they rely on oxygen diffusing passively across the large surface area provided by their legs.
Their genitals are found on their legs, too. A female will grow eggs in her thighs—“it’s as if my arms were full of ping-pong balls,” says Moran—and release them through pores. A male, clambering over her, releases sperm from similar pores to fertilize the eggs, which he scoops up and carries around. Among these animals, the dads care for the young.
The legs are also where most of sea spiders’ digestion takes place. There’s so little distance between their mouths and anuses that their guts send long branches down each leg. Put your wrists together, spread your hands out, and splay your fingers—that’s the shape of a sea spider’s gut.