Why a 6 mm Male Anglerfish Fuses Into the Female, and Why She Lost Her Immune System

A mature male of the species Photocorynus spiniceps measures 6.2 to 7.3 millimetres long, smaller than any other mature fish known to biology, and he will spend his adult life permanently fused to the flank of a female roughly seven to eight times his length.

A reproductive strategy without a parallel

Deep-sea anglerfish belong to the suborder Ceratioidei, a clade of bony fish that lives almost entirely below the sunlit ocean. The English ichthyologist Charles Tate Regan formally established the group in 1912, originally as the "division Ceratiformes." The suborder now contains roughly 170 named species spread across 11 recognized families, among them Ceratiidae, Himantolophidae, Melanocetidae, Linophrynidae and Centrophrynidae.

What sets these fish apart from every other vertebrate group is not the dark they live in or the lure on the female's head. It is what the male does when he finds her. In the great majority of ceratioid species the male is dramatically smaller than the female, and reproduction proceeds through what the literature calls sexual parasitism, a natural parabiosis in which two genetically distinct animals merge into a single organism. The Wikipedia entry on the anglerfish puts it plainly: "Sexual parasitism is the attachment of male to the much larger female, sometimes fusing together as an example of natural parabiosis."

The size disparity is the most extreme in vertebrates. In Krøyer's deep-sea anglerfish, Ceratias holboelli, mature females reach a standard length of 77 centimetres, with a maximum total length of 120 centimetres, while mature males reach only 14.5 centimetres and weigh roughly an order of magnitude less than the smallest females. In Photocorynus spiniceps, the gap is wider still. Mature males measure between 6.2 and 7.3 millimetres, females reach up to 50.5 millimetres, and Wikipedia describes the males simply as "smaller than any other mature fish."

The mechanics of fusion

The mechanism is precise. A free-swimming male locates a female and bites onto her body with a denticular apparatus, a set of modified tooth-like denticles in place of conventional jaws. His tissues then physically fuse to hers. The two circulatory systems join, nutrients flow from female to male through the merged blood vessels, and the male retains only his functional gills for oxygen and his testes for reproduction. The Wikipedia article on Ceratias holboelli describes the endpoint cleanly: the male "attaches itself to the female's body as a sexual parasite, eventually becoming incorporated into the female's tissues and blood vessels."

Once attached, the male's sensory organs progressively degenerate. Eyes and nostrils atrophy. Heart, gills and fin rays remain. The body increases in volume on the female's bloodstream but is otherwise reduced to a permanent sperm-producing appendage on her flank. In the triplewart seadevil, Cryptopsaras couesii, free-swimming pelagic males stay small, with the largest reaching about 14.3 millimetres, but once a male attaches and begins receiving the female's bloodstream nutrients he grows, with parasitic males measured between 35 and 56 millimetres collected from a single host.

A single female can carry more than one. One documented Cryptopsaras couesii female of 316 millimetres was found carrying 8 attached males at once. In Linophryne the attached males almost always fuse to the female's ventral midline; in Cryptopsaras couesii they attach almost anywhere on the body, including on the warts that give the species its English name. After spawning the female releases eggs in a gelatinous mass, an "egg raft" or "veil" containing thousands of canals, that floats up from depth into the sunlit epipelagic zone where the larvae hatch.

Why the deep ocean demands it

The reason for all of this is statistical. Anglerfish larvae drift in shallow water, usually less than 30 metres and rarely below 200 metres, but most adults live between 200 and 3,000 metres deep, and some Thaumatichthys specimens have been collected as deep as 6,000 metres. Down in that lightless layer, individuals are thinly distributed and anglerfish are weak swimmers. Crossing paths with another member of your own species is a low-probability event.

The numbers bear that out even in obligately parasitic species. Out of 600 metamorphosed Cryptopsaras couesii females examined, only 6.2 percent carried any attached males. The species ranges from 75 to 4,000 metres in depth, with most specimens between 500 and 1,250 metres, and the American ichthyologist Theodore Gill recorded the type locality off New York at a depth of 3,083 metres in 1883. Even when the biology has fully committed to fusion, the encounter itself remains rare.

That rarity is the engine. The Wikipedia article on Ceratioidei cites Regan's 1926 statement on the adaptive logic: "When the female is ready to spawn, she has a mate immediately available." Sexual parasitism evolved because the alternative, finding a mate twice in one lifetime, was too rare to count on.

The lure, and the genes that made fusion possible

The female's lure is anatomically a first dorsal-fin ray, composed of a pterygiophore that anchors it, an illicium that forms the rod and an esca at the bulbous tip. The esca of most ceratioid species houses symbiotic bioluminescent bacteria. The light is controlled by the fish, and the emitted colors include pink, purple, white, yellow, orange, yellowish-green, blue and blue-green, with blue-green dominating because that wavelength penetrates seawater best.

A 2019 eLife study by Lydia J. Baker and colleagues examined the genetics of that relationship and found that the bacterial symbionts inside the esca are acquired from the environment and have genetically reduced genomes; they have irreversibly lost the genes needed for independent free-living survival outside the host. The lure's light is the product of an organism that can no longer exist alone.

The fused male sits in the same category. By every rule of vertebrate biology, the female's adaptive immune system should recognize his tissue as foreign and reject it. It does not. A 2020 study published in Science by Jeremy Swann and colleagues, DOI 10.1126/science.aaz9445, showed that ceratioid anglerfish have lost or pseudogenized core genes of the adaptive immune system, including those required for B-cell and T-cell function. The immune tolerance that allows male and female tissues to fuse without rejection has a genetic cause, and the cause is loss.

What the paradox now means

The story behind these animals was assembled slowly. Henrik Krøyer described Ceratias holboelli in 1845. Regan named the suborder in 1912 and, in 1925, published the description of the most extreme size dimorphism in the group from a single male specimen collected by the Danish research vessel Dana in the Gulf of Panama, at 7°15'N, 78°54'W, at roughly 1,250 metres depth. Erik Bertelsen's 1951 monograph on ceratioid ontogeny and distribution, and Theodore W. Pietsch's comprehensive studies in 2005 and 2009, remain the foundational references.

The Swann paper closed the loop between the morphology those workers cataloged and the cell biology that makes it possible. One consequence is that the deep-sea anglerfish has become a model for human organ-transplant tolerance research, where the immune system normally rejects foreign tissue and clinicians spend a career trying to talk it out of doing so. A fish that solved the same problem by genetic deletion is now read alongside that literature.

It is worth holding the paradox in plain view. Photocorynus spiniceps females have been described as "effectively a hermaphrodite" because they carry the male's gonads inside their own outline. The lineage reached that state by surrendering the adaptive immune system that every other vertebrate uses to fight infection. The trade was paid, generation after generation, because in the deep pelagic the chance of ever meeting a partner is too rare to waste.

Sources

• Wikipedia: Ceratioidei
• Wikipedia: Anglerfish
• Wikipedia: Sexual parasitism
• Wikipedia: Ceratias holboelli
• Wikipedia: Photocorynus spiniceps
• Wikipedia: Triplewart seadevil
• Wikipedia: Bioluminescence