Ceratiidae
When the male finds a female, he bites into her skin, and releases an enzyme. The enzyme digests the skin of his mouth and her body, fusing the pair down to the blood-vessel level. The male then slowly wastes away, first losing his digestive organs, then his brain, heart, and eyes, and ends as nothing more than a pair of gonads. The gonads release sperm in response to hormones in the female's bloodstream. This extreme sexual dimorphism makes sure that, when the female is ready to spawn, she has a mate right there. Multiple males can be incorporated into a single female.