This story appears in the December 2016 issue of National Geographic magazine.

Keeping your mate extraordinarily close—as in permanently fused to your body—has its advantages.

A mile or more down in the lightless ocean, deep-sea anglerfish search for partners. The 162 species of this Ceratioid suborder form odd couples: The males are dwarfed, the females many times larger (some three feet long). Yet they’re uniquely equipped to find each other.

The male’s outsize nostrils pick up the female’s waterborne pheromones. His well-developed eyes search for a spot of light: the bioluminescent lure on a stalk adorning the female’s brow. Ted Pietsch, a University of Washington ichthyologist, says the lures’ different shapes, pigment patterns, and flash patterns tell a male when he’s found a female of his species to hook up with.

“Hook up” is putting it mildly. Rather than risk separation from his mate in the vast dark, the male clamps his teeth onto some part of her and stays put. “Eventually the skin of male and female grows together,” Pietsch says; vessels join “so her blood flows through his body.” Fins and other disused body parts wither away until the male is only what the female needs him to be: a sperm factory.

This sexual parasitism bears fruit. When the female’s eggs are ready, she signals the male. As he releases sperm, she releases a gelatinous egg mass that expands in water, absorbing the sperm.