Some anglerfishes, like those of the Ceratioid group, employ an unusual mating method. Because individuals are presumably locally rare and encounters doubly so, finding a mate is problematic. When scientists first started capturing ceratioid anglerfish, they noticed that all of the specimens were females. These individuals were a few inches in size and almost all of them had what appeared to be parasites attached to them. It turned out that these "parasites" were highly reduced male ceratioids.
At birth, male ceratioids are already equipped with extremely well developed olfactory organs that detect scents in the water. The male ceratoid lives solely to find and mate with a female. They are significantly smaller than a female angler fish, and may have trouble finding food in the deep sea. Furthermore, the growth of the alimentary canals of some males become stunted, preventing them from feeding. These features necessitate his quickly finding a female anglerfish to prevent death. The sensitive olfactory 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 skin, and releases an enzyme that digests the skin of his mouth and her body, fusing the pair down to the blood-vessel level. The male then slowly atrophies, first losing his digestive organs, then his brain, heart, and eyes, and ends as nothing more than a pair of gonads, which releases sperm in response to hormones in the female's bloodstream indicating egg release. This extreme sexual dimorphism ensures that, when the female is ready to spawn, she has a mate immediately available.
The spawn of the anglerfish of the genus Lophius consists of a thin sheet of transparent gelatinous material 2 or 3 feet wide and 25 to 30 feet long. The eggs in this sheet are in a single layer, each in its own cavity. The spawn is free in the sea. The larvae are free-swimming and have the pelvic fins elongated into filaments. Such an egg sheet is rare in fish.