i didn't, until today. jesus.
basically, the huge, ugly typical angler fish with the bioluminescing lures are all female, and the males are tiny with stunted organ systems. the two mate by the male biting into her, and using enzymes to dissolve his mouth/her skin, until he's fused with her blood vessels. he then continues to degenerate, losing eyes, brains, digestive organs etc, until he's noting but testicles.
the angler fish technically becomes a hermaphrodite, with parasitic fish balls on her, so she can have kids whenever. and because the sperm was from a genetically distinct animal, all the normal genetic recombination can occur to produce distinct progeny.
all this because angler fish live few and far between, and the males are basically born to do this, and are exceptionally equipped to seek out females.
really strange. pretty interesting though