How Do Anglerfish Mate?

Genetic analyses shed light on a fish’s infamously strange mating ritual. Scientists do not know much about the mating process in the anglerfish, and they have never seen one in male form. Born in a dark, deep-sea world, the males of some species of anglerfish exist purely to snuff out mates.

Anglerfish mate when a male fish discovers a female and bites into her flesh. He remains there for a short while before fusing into the female’s body and becoming a parasite. During this time, the male integrates into the female’s body and relinquishes control over its organs. The female extracts sperm from it as needed.

Male-female anglerfish relationships are one example of the sexual parasitism prevalent across twenty species of animals. Anglerfish are extreme examples of sexual dimorphism, where the female anglerfish outshines the males by any measure.

Unlike other anglerfish, members of the suborder Ceratioidei exhibit extreme sexual dimorphism — i.e., the females are significantly larger than the males. Some anglerfish are remarkable for their extreme sexual dimorphism and a sexual symbiosis between the smaller males and much larger females, seen in the deep-sea suborder Ceratiidae.

How Anglerfish Copulate in the Wild

When an anglerfish copulates, the male fuses his tissues with the tissues of the much larger female. Their strange mating ritual is that a male anglerfish will nip at the underbelly of a female fish, permanently merging into her tissues. The tiny male anglerfish fused his tissues to that of a larger female in the population, which allowed them both to share not just sperm but also blood and skin. The video is never-before-seen footage of the people, with a tiny male anglerfish fused into a woman’s body.

Over time, the free-living male anglerfish epidermal tissues fused to those of a female with specificity, and their circulatory systems were united. However, the precise mechanism by which fusion was mediated is not known. To sustain the process, female anglerfish evolved a new kind of immune system that does not consider the male anglerfish to be foreign tissue, allowing them to accommodate up to eight partners at once.

How the males and females fuse and escape being rejected by one another immune systems–like an organ transplant with an errant match–has been a puzzle. Now, genetic tests of museum specimens and preserved loggerheads show species that mate does not possess genes that are essential to forming critical parts of the immune system.

Mating Varies Across Anglerfish Species

In some species of anglerfish, male-female pairing during reproduction is possible because the immune system keys are missing, which allows antibodies to mature and make T-cell receptors. Compared with anglerfish that do not mate, species that do mate are missing genes that aid the production of novel antibodies, which become better at binding to perceived threats during future encounters.

In anglerfish species — Photocorynus spices and Haplophryne Mollis — in which several males may fuse with one female, antibodies may not be formed at all, according to one report. Two deep-sea anglerfish species, including Photocorynus spiniceps (a preserved female is shown with a tiny male attached to her back), have no genes involved in developing antibodies or immune cells that recognize invaders and kill foreign or infected cells.

Free-living males and unparasitized females of certain anglerfish species have never developed complete gonads. Some species of anglerfish, such as the Cryptopsaras, may harbor as many as eight male parasites. In some species of anglerfish, such as the Cryptopsaras, the females may even essentially become hosts to several males — sometimes carrying as many as eight parasitic partners. Scientists have known about this strange mating behavior since they have found dead males attached to dead females.

Morbid Mating Discoveries of the Anglerfish

Scientists already knew anglerfish copulated this way; dead male anglerfish were found attached to dead females. In 1922, Icelandic biologist Bjarni Saemundsson found a female anglerfish with two smaller fish attached by the mouths of the fish on its stomach. Two smaller fish are bound by the mouths of the fish on its stomach.

Even more remarkable, it had a male anglerfish attached to its abdomen, marking the first time the strange mating habits of deep-sea anglerfish had been captured on film. In what looks more like a blinding light show than a mating ritual, the fist-sized female Fanfin Anglerfish hovers over the deep sea, a tiny male Anglerfish attached to its lower abdomen.

Footage of that fish shows the female fanfic devil. In the video, which shows anglerfish copulating, the anglerfish’s dozen or so hair-like filaments appear to be emitted, as well as the fish fins. In this video, the male Fanfin Seadevil anglerfish latches onto a female, biting into her stomach. Sixty times smaller than females in this species, this is also the first time a male Fanfin Seadevil anglerfish is seen.

The Difficult Existence of Male Anglerfish

Like all species of anglerfish, this one has been mating its entire life. Male anglerfish, who are just a tiny fraction the size of the females they mate with, use their big, sensitive eyes and nostrils to help them home in on a chemical released by females. The genetic processes underlying female anglerfish have escaped scientists since the first discovered pair, trapped dead together in a net in Icelandic waters, was caught a century ago.

Male ceratoid fish are noticeably smaller than female anglerfish, which can struggle to find food in deep water. One explanation for the evolution of sexual symbiosis is that comparatively small numbers of females in the deep-sea environment left few opportunities for mating selection between the anglerfish.

In anglerfish that are parasitic, reproductively dependent, and reproductively dependent (Creatiidae, Linophrynidae, and Neoceratiidae families), breeding is possible only after the fusion of males with females. This bizarre mating ritual is essential to the deep-sea ctenidia since they seldom encounter one another at depth, and it guarantees that little anglerfish will ever be created.

The lopsided power structure is not just your average host-parasite relationship, scientists recently revealed, because this process of sexual merging has caused female anglerfishes to evolve a unique immune system that takes foreign tissue from male anglerfishes and makes it her own.

Nicholas Finn

I've been the captain of a fishing boat for over 20 years, and I created Pirateering to share my knowledge of and interest in seafaring.

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