Anglerfish are the members of the order Lophiiformes. They are bony fish named for their characteristic mode of predation, wherein a fleshy growth from the fish’s head (the esca or illicium) acts as a lure; this is considered analogous to angling.
Some anglerfish are pelagic (live in the open water), while others are benthic (bottom-dwelling). Some live in the deep sea (e.g. Ceratiidae) and others on the continental shelf (e.g. the frogfishes Antennariidae and the monkfish/goosefish Lophiidae). They occur worldwide. Pelagic forms are most laterally (sideways) compressed whereas the benthic forms are often extremely dorsoventrally compressed (depressed) often with large upward pointing mouths.
A mitochondrial genome phylogenetic study suggested that anglerfishes diversified in a short period of the early to mid Cretaceous, between 130 and 100 million years ago.
The fish are named for their characteristic method of predation. Anglerfish typically have at least one long filament sprouting from the middle of the head; these are the detached and modified three first spines of the anterior dorsal fin. In most anglerfish species, the longest filament is the first (the illicium). This first spine protrudes above the fish’s eyes, and terminates in an irregular growth of flesh (the esca) at the tip of the spine. The spine is movable in all directions, and the esca can be wiggled so as to resemble a prey animal, and thus to act as bait to lure other predators close enough for the anglerfish to devour them whole. The jaws are triggered in automatic reflex by contact with the tentacle.
Some deep-sea anglerfishes of the bathypelagic zone emit light from their escas to attract prey. This bioluminescence is a result of symbiosis with bacteria. The bacteria may enter the esca from the seawater through small pores; however, this is speculative and the mechanism by which ceratioids harness these bacteria is unknown. In the confines of the esca, they can multiply until their density is such that their collective glow is very bright.
In most species, a wide mouth extends all around the anterior circumference of the head, and both jaws are armed with bands of long pointed teeth, which are inclined inwards, and 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 (its bones are thin and flexible) to enormous size, allowing it to swallow prey up to twice as large as its entire body.
Some benthic (bottom-dwelling) forms have arm-like pectoral fins which the fish use to walk along the ocean floor. The pectoral and ventral fins are so articulated as to perform the functions of feet, enabling the fish to move, or rather to walk, on the bottom of the sea, where it generally hides itself in the sand or amongst seaweed. All around its head and also along the body, the skin bears fringed appendages resembling short fronds of seaweed, a structure which, combined with the extraordinary faculty of assimilating the colour of the body to its surroundings, camouflage the fish in areas abundant with prey.