Anglerfishes are fish that are members of the teleost order Lophiiformes. They are bony fish named for their characteristic mode of predation, in which a fleshy growth from the fish’s head (the esca or illicium) acts as a lure.

Anglerfish are also notable for extreme sexual dimorphism seen in the suborder Ceratioidei, and sexual parasitism of male anglerfish. In these species, males may be several orders of magnitude smaller than females.

Anglerfish occur worldwide. Some are pelagic, while others are benthic; some live in the deep sea (e.g., Ceratiidae) while others on the continental shelf (e.g., the frogfishes Antennariidae and the monkfish/goosefish Lophiidae). Pelagic forms are most laterally compressed, whereas the benthic forms are often extremely dorsoventrally compressed (depressed), often with large upward-pointing mouths.

A mitochondrial genome phylogenetic study suggested the anglerfishes diversified in a short period of the early to mid Cretaceous, between 130 and 100 million years ago.

Ranging in color from dark gray to dark brown, these carnivores have huge heads that bare enormous, crescent-shaped mouths full of long, fang-like teeth angled inward for efficient prey grabbing. Their length can vary from 8.9 cm (3.5 in) to over 1 m (3 ft) with weights up to 45 kg (100 lb).

Most adult female ceratioid anglerfish have a luminescent organ called the esca at the tip of a modified dorsal ray (the illicium, or “fishing rod”). The organ has been hypothesized to serve the obvious purpose of luring prey in dark, deep-sea environments, but also serves to call males’ attention to the females to facilitate mating. The source of luminescence is symbiotic bacteria that dwell in and around the esca. In some species, the bacteria recruited to the esca are incapable of luminescence independent of the anglerfish, suggesting they have developed a symbiotic relationship and the bacteria are unable to synthesize all of the chemicals necessary for luminescence. They depend on the fish to make up the difference. Electron microscopy of these bacteria in some species reveals they are Gram-negative rods that lack capsules, spores, or flagella. They have double-layered cell walls and mesosomes.

In most species, a wide mouth extends all around the anterior circumference of the head, and bands of inwardly inclined teeth line both jaws. The teeth can be depressed so as to offer no impediment to an object gliding towards the stomach, but prevent its escape from the mouth. The anglerfish is able to distend both its jaw and its stomach, since its bones are thin and flexible, to enormous size, allowing it to swallow prey up to twice as large as its entire body.

Many anglerfish species are deep-sea dwellers, which poses a challenge to ecologists who hope to study and observe the fish. Anglerfish morphology reflects the value of energy conservation for these organisms which often live in extremely prey-scarce environments. Some researchers suggest this is why many ceratioids minimize their energy use by remaining lethargic and using a lie-and-wait hunting strategy. Anglerfish are particularly well suited to conserve energy because they are able to hunt and forage while remaining lethargic, devoting just 2% of energy intake to swimming.

In one rare ROV observation of an in-situ anglerfish, researchers observed several rapid swimming and avoidance behaviors. In 74% of the video footage, the fish was observed passively drifting. Occasionally, it would also exhibit rapid burst swimming. While drifting, the fish weakly beat its pectoral fins in a behavior known as sculling. The sculling behavior observed is suggested as necessary to keep the fish in a neutral position in the water and to counteract any displacing currents. The bursts of fast swimming typically last less than five seconds. The swimming behavior in this video is similar to that seen in other in-situ footage of a ceratioid anglerfish.

Another in-situ observation of three different whipnose anglerfish showed unusual upside-down swimming behavior. Fish were observed floating upside-down completely motionless with the illicium hanging down stiffly in a slight arch in front of the fish. Notably, the illicium was hanging over small visible burrows. The observers suggested this is one effort to entice prey and is another example of low-energy opportunistic foraging and predation. When the ROV approached the fish, they exhibited burst swimming, still upside-down.