How Do Anglerfish Mate? Tiny Males That Fuse to Females

Why finding a mate is so hard in the deep sea

Deep-sea anglerfish live in the open ocean, mostly between about 200 and 3,000 m (660–9,800 ft) deep. It is pitch dark, food is scarce, and the fish are spread thinly through an enormous volume of water. A female may go a long time without meeting another member of her species.

Over millions of years, anglerfish evolved an unusual solution. The two sexes became extremely different in size, a pattern called sexual dimorphism. Females stayed relatively large, which lets them make more eggs. Males became dwarfs whose main job is to find a female. In Krøyer’s deep-sea angler, a female can be more than 60 times longer than a free-living male and, by one estimate, around half a million times heavier.

How male anglerfish find females

Males of most deep-sea anglerfish do not have a lure. Instead, they have senses built for searching. In some families males have huge nostrils that pick up chemical signals called pheromones released by females; in others they have large, sensitive eyes that may spot the female’s glowing lure or other light organs. Because each species’ lure has its own shape, these lights may help males find the right partner.

A young male lives on energy stored in his liver while he searches. When he finds a female, he grabs her with special hooked “teeth” on the tip of his snout and jaws, formed from bones that in females make up the fishing rod. What happens next depends on the species.

Three ways anglerfish mate

Scientists recognize three main patterns of mating in deep-sea anglerfish:

Permanent fusion seems to have evolved several times within the deep-sea anglerfish, perhaps up to seven times, which shows how strongly the deep sea favors it.

Sexual parasitism: fusing for life

In species with obligatory parasitism, the male bites into the female’s skin, often on her belly or sides. Over time, the tissues around his mouth fuse with hers, and their blood vessels join so that nutrients flow from her blood into his body. The male keeps his own heart and gills and breathes on his own, but he no longer needs to feed.

As fusion progresses, the male’s eyes and nostrils shrink and his body becomes little more than a sperm-producing attachment. In some species, neither partner becomes sexually mature until they are joined. Afterward the pair functions almost like a single animal with both sexes, so whenever the female is ready to spawn, a mate is already in place. Scientists call this sexual parasitism, because the male lives off the female like a parasite, but it benefits both partners.

A female may carry more than one male. Up to eight males have been found on a single female triplewart seadevil. The most extreme size difference is seen in Photocorynus spiniceps, whose mature attached males are only 6.2–7.3 mm (about 0.25 in) long.

How does the female’s body accept the male?

Normally, any vertebrate’s immune system attacks tissue from another individual, which is why organ transplants in humans need careful matching and medicines. So how can two anglerfish fuse without rejection?

A 2020 study led by researchers at the Max Planck Institute of Immunobiology and Epigenetics in Freiburg, Germany, published in Science, examined the genes of anglerfish species. It found that species which fuse have lost or disabled key parts of the adaptive immune system, the branch that makes antibodies and recognizes foreign tissue. In the species with the most permanent fusion, genes needed for working T cells and antibodies appear to be missing altogether. The fish seem to rely on their other, innate defenses to fight infections, a discovery that surprised immunologists.

The humpback anglerfish: a temporary partner

The humpback anglerfish belongs to the black seadevils, a family that does not fuse. Males grow to only about 1.5–2.9 cm (0.6–1.1 in), while females reach about 18 cm (7 in). When a male finds a female, he holds on with his hooks only while they spawn and then lets go to search for other mates. Pairs found attached together showed no sign of tissue fusion, and both sexes become mature on their own.

Eggs, larvae and the next generation

Like other anglerfish, female deep-sea anglers release their eggs inside a floating, jelly-like mass called an egg raft or veil. The attached male releases sperm at the same moment, and the raft soaks up the water and sperm, helping the eggs get fertilized. The raft then drifts upward toward the sunlit surface layer.

The larvae hatch and grow in the upper water, usually less than 200 m (660 ft) deep, where tiny food is plentiful. Even at this early stage, female larvae can be told apart by the first sign of a lure on the snout. As they mature, they change into their adult form and sink into the deep sea, where the search for a partner begins again. The answer to why females have that lure is in why do anglerfish have a light?, and to see how small these fish really are, read how big are anglerfish?