Ever meet one of those couples that seems attached at the hip?

You know the angler fish. Glowing lure, big fangly teeth, kinda looks like an evil cantaloupe.

What you may not know is that all angler fish that fit this description are female. It was a bit of zoological mystery for a while; researchers couldn’t figure out why all the angler fish they ever caught were female, and why they seemed to be covered in some kind of bloodsucking parasite.

Meet the bloodsucking parasite:

It’s the male angler fish. If the female angler looks like a cantaloupe possessed by Satan, imagine the male as a pea possessed by a fetus. That’s the size difference. It is perhaps the most extreme example of sexual dimorphism in the animal kingdom, and definitely the most extreme example of sexual co-dependency.

When the female is ready to mate, she releases a potent pheromone. A tiny male, swimming blind in the absolute darkness, follows the scent to her. He bites her gonads, then releases an enzyme from his mouth that dissolves her flesh a little. The two begin to literally fuse; their skin and muscles knit together, and even their blood vessels connect, so that the female’s heart controls both of their circulatory systems. The male atrophies and dies… or is “absorbed,” in a sense, leaving the female with a testicle she can use to fertilize her eggs at her leisure.

That’s pretty much what the male is: a free-swimming testicle. Nature has devolved the male into a primitive, fetal fish. It has no other purpose in life than to be sewn on to a female as a new nutsack. (See that gooey white stuff inside him? Know what that is? Hint: He doesn’t have digestive organs.) Why? Well, on the empty expanse of the abyssal plain, finding a mate is, heh, a shot in the dark. If you’re lucky enough to find one, you have to bite down and make sure you don’t let go. Angler fish have taken the principle to hyperbolic extremes, making literal the biblical saying “man and wife are one flesh,” while perhaps ignoring the old adage, “there are plenty of other fish in the sea.”