Anglerfish

Anglerfish are defined by gills that open behind the pectoral fins (as opposed to other fish whose pectorals lay behind the gill opening), depressible teeth that can hinge back, joints of the epiotic bone, the form of the pectoral fin radials, and the luring apparatus (see subsection).

The body form of anglerfish vary by habitat and mode of life. Frogfishes are usually short-bodied and use their limb-like pectoral fins to crawl or perch among reef organisms, usually relying on camouflage. Batfishes and sea toads are usually flattened or globose bottom-dwellers that support themselves on modified fins. In contrast, the pelagic deep-sea anglerfishes have reduced locomotory structures and are usually weak swimmers.

Anglerfish can wiggle the esca to make it resemble a prey animal, which lures the anglerfish's prey close enough for the anglerfish to devour them whole. Some deep-sea anglerfish of the bathypelagic zone also emit light from their esca to attract prey. In bottom-dwelling species, predation often occurs as luring leading to surprise attacks. Frogfishes resemble sponges, rock, algae, or other benthic organisms and lurk until prep approaches, while batfishes and monkfishes wait on or near the seabed. When prey is within range, the fish opens its large mouth rapidly and pulls it in by suction.

The jaw and stomach of the anglerfish can extend to allow it to consume prey up to twice its size. Because of the limited amount of food available in the anglerfish's environment, this adaptation allows the anglerfish to store food when there is an abundance.

All anglerfish are weak swimmers, including the pelagic deep-sea anglerfish. Demersal species often "walk" on the bottom upon their pectoral and pelvic fins. The pelvic fins were lost in the deep-sea anglers.

The deep-sea anglers often drift without actively swimming; In situ observation of female Oneirodes and whipnose anglerfish (from ROVs) recorded that they often passively float in place or in a current, but they were sometimes observed to attempt to flee from the ROV, beating its pectoral fins in-phase while undulating its tail fin. The lethargic behavior of these ambush predators is suited to the energy-poor environment of the deep sea.

The deep-sea anglerfish employ an unusual mating method: because individuals are locally rare, encounters between two of the same species are also very rare, and finding a mate can be problematic; this has led to the development of sexual parasitism in anglerfish, where the males latch onto their mates using their mouths, which may not be suitable or effective for prey capture. When scientists first started capturing ceratioid anglerfish, they noticed that all of the specimens were female, and on some of these they had what appeared to be parasites attached to them, which turned out to be highly dimorphic male ceratioids. This is one of the few instances of naturally occurring parabiosis. In some species of anglerfish, fusion between male and female when reproducing is possible due to the lack of immune system keys that allow antibodies to mature and create receptors for T-cells.

In the History of Animals, Aristotle described the "Fishing-Frog" (one of the local Lophius species, like L. piscatorius or L. budegassa) as an example of a marine species well adapted to their environment, those equipped with "ingenious devices" that it uses to capture prey, alongside the Torpedo. He noted that fishing-frogs that have lost their lure appeared to be thinner than those still intact.