Anglerfish are marine fish that attract prey by dangling a fleshy, bait-like appendage (the esca) in front of their heads. The appendage, which resembles a fishing pole, is attached to the end of the dorsal fin's foremost spine (the illicium), which is separated from the rest of the fin.
Anglerfish belong to the order Lophiiformes, which includes three suborders, 15 families, and about 215 species. The order Lophiiformes is in the class Osteichthyes, the bony fishes, which, in turn, is in the subphylum Vertebrata, phylum Chordata.
Anglerfish are distributed throughout the world and include both free-swimming (pelagic) species and seabed-dwelling (benthic) species. Most species of anglerfish live on the ocean floor and have bizarre body forms. They have small gill openings located at or behind the base of the pectoral fins, which are limb-like in structure; some species also have limb-like pelvic fins. The swim bladder of anglerfish is physoclistic, that is, it has no connection to the digestive tract.
The common names of many anglerfish are particularly colorful: for example, the flattened goosefish, the warty angler, and the spherical frogfish. The flattened goosefish inhabits the muddy bottom of the continental slopes of the Atlantic, Indian, and western Pacific Oceans and remains motionless as it angles for prey with its esca. The balloon-shaped warty angler is found in South Australia and New South Wales, and the spherical frogfish, which propels itself in hopping motions with pectoral fins modified into special "jumping" limbs, is found in tropical and subtropical seas.
The diet of anglerfish consists mostly of other fish, although some species consume marine invertebrates. For example, batfish, anglerfish with free flaps of skin, also eat marine snails, clams, crustacea, and worms; deep-sea anglers of the family Melanocetidae consume copepods and arrow worms in addition to fish.
Deep-sea anglerfish, which belong to the suborder Ceratioidei, live at depths between 4,920 and 8,200 ft (1,500 and 2,500 m). The deep-sea anglerfish lack pelvic fins, are free-swimming (pelagic) and do not remain still on the bottom, as do other anglerfish. Deep-sea anglerfish are distributed widely from subarctic to subantarctic waters, but are absent from the Mediterranean Sea.
Anglerfish range in length from about 4 in to 2 ft (10–60 cm). The males of four families of the suborder Ceratioidei show a dramatic sexual dimorphism, in that males are much smaller than the females. The male Ceratias holboelli, for example, grows to a maximum length of only 2.5 in (6 cm) while the females can reach up to 4 ft (1.2 m) in length.
Soon after birth, parasitic male deep-sea anglers use their pincerlike mouths to affix themselves to a female. The tissue of their mouths fuses completely with the tissues of the female, so that their blood supplies mingle. The female is the hunter, attracting prey with a luminous esca, while the male receives nourishment via the shared blood supply. The male, assured of a life without the need to hunt for food, ceases growth, except for development of reproductive organs. In turn, the female is assured a lifetime supply of sperm from the male to fertilize her eggs.
Some anglerfish are used by humans as food; for example, the European goosefish, Lophius piscatorius, also called monkfish, is highly prized; and frogfish are sometimes sold for home aquariums and are occasionally eaten.