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The humpback anglerfish or common black devil, Melanocetus johnsonii, is a deepsea anglerfish in the family Melanocetidae, found in tropical to temperate parts of all oceans at depths to 2,000 m (6,600 ft). Its length is up to 2.9 cm (1.1 in) for males and up to 18 cm (7.1 in) for females.
Male humpback anglerfish are much smaller than the females, being no more than 3 cm (just over an inch) long, whereas the female typically reaches 18 cm (7.1 in) in length. Humpback anglerfish remain free-swimming into adulthood, which is not the case with other deepsea anglerfish: with these, males swim freely when young, but before reaching adulthood, the male fixes itself permanently to the rear of the female's body, living thereafter as a parasite of the female. The male's internal organs now atrophy as the fish shares the female's blood and becomes simply a sperm provider as required.