When it's mature, the male angler's digestive system degenerates, making him incapable of feeding independently. His sensitive olfactory organs help him sniff out a female anglerfish. When he finds one, he bites into her skin, and releases an enzyme that digests the skin of his mouth and her body, fusing the pair down to the blood-vessel level. He then atrophies into nothing more than a pair of gonads, which releases sperm in response to hormones indicating egg release. This extreme sexual dimorphism ensures that, when the female is ready to spawn, she has a mate immediately available.
[She knows who her baby's Daddy is.]