When scientists first studied these creatures, they were surprised to find that all of the fish that they captured were female. Further, these female fish had male gonads (testicles) attached to them.
Male angler fish are born with a highly developed olfactory function (sense of smell) and mouth parts, but no digestive system. Their only task in life is to sniff out a female and bite onto her. An enzyme then devours both his mouth and her body, where bitten, until they meld into each other. The male slowly atrophies until all that is left are his gonads. This effectively makes the female hermaphrodite (both male and female). The male gonads release sperm in response to an enzyme signal whenever an egg is produced. And so this happy courtship cycle begins again. Romantic or what?!