Anglerfish

In 2005, near Monterey, California, at 1474 metres depth, an ROV filmed a female ceratioid anglerfish of the genus Oneirodes for 24 minutes. When approached, the fish retreated rapidly, but in 74% of the video footage, it drifted passively, oriented at any angle. When advancing, it swam intermittently at a speed of 0.24 body lengths per second, beating its pectoral fins in-phase. The lethargic behaviour of this ambush predator is suited to the energy-poor environment of the deep sea.[15]

Another in situ observation of three different whipnose anglerfish showed unusual inverted swimming behavior. Fish were observed floating inverted completely motionless with the illicium hanging down stiffly in a slight arch in front of the fish. The illicium was hanging over small visible burrows. It was suggested this is an effort to entice prey and an example of low-energy opportunistic foraging and predation. When the ROV approached the fish, they exhibited burst swimming, still inverted.[16]

The jaw and stomach of the anglerfish can extend to allow it to consume prey up to twice its size. Because of the small amount of food available in the anglerfish's environment this adaptation allows the anglerfish to store food when there is an abundance.[17]

Skeleton of the angler fish Lophius piscatorius: The first spine of the dorsal fin of the anglerfish acts as a fishing rod with a lure.

The name "anglerfish" derives from the species' characteristic method of predation. Anglerfish typically have at least one long filament sprouting from the middle of their heads, termed the illicium. The illicium is the detached and modified first three spines of the anterior dorsal fin. In most anglerfish species, the longest filament is the first. This first spine protrudes above the fish's eyes and terminates in an irregular growth of flesh (the esca), and can move in all directions. 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.[18] Some deep-sea anglerfish of the bathypelagic zone also emit light from their esca to attract prey.[19]

Because anglerfish are opportunistic foragers, they show a range of preferred prey with fish at the extremes of the size spectrum, whilst showing increased selectivity for certain prey. One study examining the stomach contents of threadfin anglerfish off the Pacific coast of Central America found these fish primarily ate two categories of benthic prey: crustaceans and teleost fish. The most frequent prey were pandalid shrimp. 52% of the stomachs examined were empty, supporting the observations that anglerfish are low energy consumers.[20]

Linophrynidae: Haplophryne mollis female anglerfish with males attached

Antennariidae: striated frogfish, Antennarius striatus

Some anglerfish, like those of the Ceratiidae, or sea devils employ an unusual mating method.[21] Because individuals are locally rare, encounters are also very rare. Therefore, finding a mate is problematic. When scientists first started capturing ceratioid anglerfish, they noticed that all of the specimens were female. These individuals were a few centimetres in size and almost all of them had what appeared to be parasites attached to them. It turned out that these "parasites" were highly reduced male ceratioids. This indicates some taxa of anglerfish use a polyandrous mating system.

Certain ceratioids rely on parabiotic reproduction. Free-living males and unparasitized females in these species never have fully developed gonads. Thus, males never mature without attaching to a female, and die if they cannot find one.[2] At birth, male ceratioids are already equipped with extremely well-developed olfactory organs[22] that detect scents in the water. Males of some species also develop large, highly specialized eyes that may aid in identifying mates in dark environments. The male ceratioid lives solely to find and mate with a female. They are significantly smaller than a female anglerfish, and may have trouble finding food in the deep sea. Furthermore, growth of the alimentary canals of some males becomes stunted, preventing them from feeding. Some taxa have jaws that are never suitable or effective for prey capture.[22] These features mean the male must quickly find a female anglerfish to prevent death. The sensitive olfactory organs help the male to detect the pheromones that signal the proximity of a female anglerfish.

The methods anglerfish use to locate mates vary. Some species have minute eyes that are unfit for identifying females, while others have underdeveloped nostrils, making them unlikely to effectively find females by scent.[2] When a male finds a female, he bites into her skin, and releases an enzyme that digests the skin of his mouth and her body, fusing the pair down to the blood-vessel level.[22] The male becomes dependent on the female host for survival by receiving nutrients via their shared circulatory system, and provides sperm to the female in return. After fusing, males increase in volume and become much larger relative to free-living males of the species. They live and remain reproductively functional as long as the female lives, and can take part in multiple spawnings.[2] This extreme sexual dimorphism ensures that when the female is ready to spawn, she has a mate immediately available.[23] Multiple males can be incorporated into a single individual female with up to eight males in some species, though some taxa appear to have a "one male per female" rule.[2]

Symbiosis is not the only method of reproduction in anglerfish. In fact, many families, including the Melanocetidae, Himantolophidae, Diceratiidae, and Gigantactinidae, show no evidence of male symbiosis.[24] Females in some of these species contain large, developed ovaries and free-living males have large testes, suggesting these sexually mature individuals may spawn during a temporary sexual attachment that does not involve fusion of tissue. Males in these species also have well-toothed jaws that are far more effective in hunting than those seen in symbiotic species.[24]

Sexual symbiosis may be an optional strategy in some species of anglerfishes.[2] In the Oneirodidae, females carrying symbiotic males have been reported in Leptacanthichthys and Bertella—and others that were not still developed fully functional gonads.[2] One theory suggests the males attach to females regardless of their own reproductive development if the female is not sexually mature, but when both male and female are mature, they spawn then separate.[2]

External video

Angler Fish – YouTube

Weird Killer of the Deep – YouTube

The anglerfish: The original approach to deep-sea fishing – YouTube

3D scans reveal deep-sea anglerfish's huge final meal – YouTube

One explanation for the evolution of sexual symbiosis is that the relatively low density of females in deep-sea environments leaves little opportunity for mate choice among anglerfish. Females remain large to accommodate fecundity, as is evidenced by their large ovaries and eggs. Males would be expected to shrink to reduce metabolic costs in resource-poor environments and would develop highly specialized female-finding abilities. If a male manages to find a female, then symbiotic attachment is ultimately more likely to improve lifetime fitness relative to free living, particularly when the prospect of finding future mates is poor. An additional advantage to symbiosis is that the male’s sperm can be used in multiple fertilizations, as he always remains available to the female for mating. Higher densities of male-female encounters might correlate with species that demonstrate facultative symbiosis or simply use a more traditional temporary contact mating.[25]

The spawn of the anglerfish of the genus Lophius consists of a thin sheet of transparent gelatinous material 25 cm (10 in) wide and greater than 10 m (33 ft) long.[26] The eggs in this sheet are in a single layer, each in its own cavity. The spawn is free in the sea. The larvae are free-swimming and have the pelvic fins elongated into filaments.[13] Such an egg sheet is rare among fish.