# Black seadevil

Black seadevils are small, deep-sea lophiiform fish of the family Melanocetidae. The five known species are all within the genus Melanocetus. They are found in tropical to temperate waters of the Atlantic, Indian, and Pacific Oceans, as well as one other species endemic to the Ross Sea.
One of several anglerfish families, black seadevils are named for their intimidating appearance and typically pitch black skin. The humpback anglerfish (Melanocetus johnsonii) was featured on a 1995 cover of Time magazine and became a flagship species for deep sea fauna.

## Taxonomy

The black seadevil family, Melanocetidae, was first proposed as a subfamily in 1878 by the American biologist Theodore Gill. The only genus in the family is Melanocetus, originally proposed as monospecific in 1864 by the German-born British herpetologist and ichthyologist Albert Günther when he described the humpback anglerfish (M. johnsoni). The type locality of M. johnsoni is off Madeira. The 5th edition of Fishes of the World classifies the family Melanocetidae within the sexually dimorphous suborder Ceratioidei of the anglerfish order Lophiiformes.

## Etymology

Both the black seadevil family name, Melanocetidae, and the genus name, Melanocetus, are a combination of melanos meaning "black" and cetus, which means a "large sea creature", typically used to refer to whales. Günther did not explain this choice of name but did note the uniform black colour, including the inside of the mouth of M. johnsoni.

## Species

The black seadevil family and its single genus, Melanocetus, has six valid species classified within it. Only two have been assigned common names:
* Melanocetus eustalus Pietsch & Van Duzer, 1980
* Melanocetus johnsonii Günther, 1864 (Humpback anglerfish)
* Melanocetus murrayi Günther, 1887 (Murray's abyssal anglerfish)
* Melanocetus niger Regan, 1925
* Melanocetus polyactis Regan, 1925
* Melanocetus rossi Balushkin & Fedorov, 1981

## Physical description

Black seadevils are characterized by a gelatinous, mostly scaleless, globose body, a large head, complemented by large, sharp, glassy, fang-like teeth lining the jaws of a cavernous, oblique mouth. These teeth are depressible. Some species have a scattering of epidermal spinules on the body, and the scales (when present) are conical, hollow, and translucent. Like other anglerfishes, black seadevils possess an illicium and esca; the former being a modified dorsal spine—the "fishing rod"—and the latter being the bulbous, bioluminescent "fishing lure". The esca is simple in black seadevils (with either a conical terminus or anterior and posterior ridges in some species), and both it and the illicium are free of denticles.
Black seadevil bioluminescence is produced by symbiotic bacteria; these bacteria are thought to enter the fish's esca via an external duct. In at least two species, the esca is not luminous until this duct develops, suggesting the bacteria originate from the surrounding seawater. The bacteria belong to the family Vibrionaceae have yet to be cultured in vitro, although they are apparently different in each anglerfish species.
The eyes of black seadevils are small; the pupil is larger than the lens, leaving an aphakic space. Common among deepsea anglerfish is the strong sexual dimorphism in melanocetids: while females may reach a length of 18 cm (7 in) or more, males remain under 3 cm (1 in). Male seadevils lack both lures and teeth. Females have large, highly distensible stomachs which give the ventral region a flabby appearance.
Pelvic fins are absent in both sexes. All fins are rounded with slightly incised membranes; the pectoral fins are small. The single dorsal fin is positioned far back from the head, larger than and above the retrorse anal fin. In life, black seadevils are a dark brown to black. The skin is extremely soft and easily abraded during collection or even by simple handling.

## Life history

The Melanocetidae appear to buck the trend in deepsea anglers, in that the males—despite not feeding as an adult and thus being little more than couriers of sperm—are free-living rather than parasitic. Brief attachment to the female does probably occur, however, as evidenced by an observation of mistaken identity: a male humpback anglerfish was found attached to the lip of a female horned lantern fish (Centrophryne spinulosa) of an unrelated (though also nonparasitic) family of anglerfish, Centrophrynidae. Little else is known of their reproduction: they are presumed to not guard their offspring, instead releasing buoyant eggs into the water which become part of the zooplankton.
While adults have been trawled from as deep as 3,000 m (9,900 ft), larvae appear to remain in the upper 100 m (330 ft) of the water column and gradually descend with maturity. Males likely outnumber—and mature well before—females by a wide margin.
The females use their bioluminescent "fishing poles" to lure both conspecifics and prey, which include crustaceans and small fish such as lanternfish and bristlemouths; the seadevils' highly distensible stomachs also allow them to swallow prey larger than themselves. In contrast with males, females are poor swimmers and spend most of their time motionless, waiting for something to approach their lures. Predators of black seadevils are not well known, but include lancetfish.
Black seadevils have only been filmed twice: once by Bruce Robison of Monterey Bay Aquarium Research Institute, in the Monterey Canyon at 600 m (1900 ft) via a remote-operated diving vehicle in 2014, and again in 2025, by a team of researchers onboard the vessel Glaucus, who recorded a humpback anglerfish making its way to the ocean's surface.