What happens is the larvae that make it safely to the ocean floor without incident grow up to be females about eight to 15 centimeters long. The larvae that instead get attached to one of the adult females becomes a male.
This male worm is only 1 or 2 millimeters long, and it is immediately inhaled by the female. But it doesn't die. No, nature is not that kind. Instead, it lives the rest of its life trapped inside the female's ovary, in a chamber called the androecium, which literally means "the small man room."
This image was taken by undercover aid workers, who left in disgust.
Once there, it has no purpose but to continually spew sperm onto her eggs to inseminate them. Obviously it can't eat, because its mouth is busy 24 hours a day, so it has to absorb its nutrients from the female while she belittles him for being a parasite.
The Redback Spider Gets Eaten While He's Boning
Nobody expects spiders to have great bedroom manners. But the redback spider, a species of black widow native to Australia (where else?), is the only species where the male will deliberately offer himself as food to the female.
Being a dude spider is already kind of a rough deal, since male spiders tend to be much smaller than their female counterparts, and spider females have a notoriously difficult time distinguishing between their partners and their dinner. If you can imagine trying to have sex with an angry grizzly bear, that's what male spiders have to deal with every time they get laid.
"SWF: Likes it rough."
The actual act of sex between spiders is a little more complicated than our "roll on, gyrate, roll off" version. Male spiders have to wrangle females like rodeo cowboys long enough to insert their pedipalps into two slots in the female's abdomen. This process takes some time, and when your mate is 10 times bigger than you and actively trying to murder you through the entire act, it's in your benefit to have some kind of distraction.
"I forgot the safe word! WHAT IS THE SAFE WORD?!"
The redback spider alone is willing to make the ultimate sacrifice for poontang. He just goes right ahead and impales himself on her ravenous fangs, keeping her busy for just long enough, hopefully, to complete the deed before he dies in screaming agony. You may be thinking this is all pretty counterintuitive for reproduction's sake, but most female spiders have the ability to store sperm without it going bad, so they only have to mate once to keep making babies for the rest of their lives. It's like they have their own private IVF clinic, but with more murder.
For more horror from the animal kingdom, check out 6 Terrifying Ways Crows Are Way Smarter Than You Think. Or learn about The 6 Biggest Badasses Who Lived As The Opposite Sex.