Eyeless, limbless and wormlike, the Ommatokoita is a crustacean, related to lobsters and shrimp, though we're pretty sure it doesn't taste as good with warm butter. Most of its cousins actually work for a living, scavenging the ocean floor, but Ommatokoita is the lazy bum of the crustacean world, as it craves only one horrible, horrible thing.
Ommatokoita feeds off the eyeballs of the Greenland shark, a shark so badass it will eat a goddamned polar bear. The parasite burrows itself into the shark's eye and then just anchors itself there, drinking delicious eye-jelly. It sits there, eating eyeball, for the rest of its life. Just more proof that the ocean is literally made of horror.
Luckily for the Greenland shark, it doesn't actually need eyes to hunt -- it's just that badass. Extra lucky, because almost all Greenland sharks have hell-shrimp in their eyes. Some scientists have postulated that the parasites may attract prey to the shark like symbiotic fishing lures. That's right -- the shark finds itself in a situation where bugs are eating its eyeballs forever and manages to benefit from it.
"Excellent. Sea monsters are eating my eye ... all goes according to plan."