The sensitive organs help the male to detect the pheromones that signal the proximity of a female anglerfish. When he finds a female, he bites into her flank, and releases an enzyme which digests the skin of his mouth and her body, fusing the pair down to the blood vessel level. The male then atrophies into nothing more than a pair of gonads that release sperm in response to hormones in the female's bloodstream indicating egg release.
This is an extreme example of sexual dimorphism. However, it ensures that when the female is ready to spawn, she has a mate immediately available.
(Photo: David Paul/Mark Norman, ACF)