The Anglerfish and the Absolute Worst Sex on Earth

A Linophryne brevibarbata species of anglerfish. That attachment at the rear of her belly is not an appendage. It's a tiny male that has permanently fused to her. Had the pair not been caught, he would have lived the rest of his life like this, supplying sperm and pulling nourishment from her blood. Photo copyright London Natural History Museum

Teen movies are, at their core, veiled studies in evolutionary biology, with young men and women coming to sexual maturity and either giving into or resisting what is arguably an animal’s sole purpose on this planet — to find a mate. Some decide to wait until they’re married, others lack the desirable traits to even get that far, and still others succeed and consequently have to put off college for a while.

A female Linophryne polypogon.

But if the deep-sea anglerfish happened to have the cognitive and physical capabilities required to produce its own such films, there’d be decidedly fewer plot twists. Every single movie would go a little something like this: Boy meets girl, boy bites girl, boy’s mouth fuses to girl’s body, boy lives the rest of his life attached to girl sharing her blood and supplying her with sperm. Ah, a tale as old as time.

The over 300 extremely varied species of anglerfishes inhabit everything from shallow to super-deep waters, and are so named because they are fish that fish for fish using lures, which are actually highly modified spines of dorsal fins that have migrated to their snouts. But among the 160 deep-sea species, only some 25 engage in the aforementioned biting-fusing-mating, what is known as sexual parasitism. In this group, the diminutive male looks like an entirely different species, lacking the female’s enormous jaws and characteristic lure.

This is because he doesn’t need to hunt. He only exists to attach to a female, and according to evolutionary biologist Theodore W. Pietsch of the University of Washington, mates are so scarce down here that it might be that only 1 percent of males ever find a female. The rest starve to death as virgins — unfortunate guys in a sea that doesn’t have plenty of other fish.

Close-up of the head of a male Linophryne sp. showing its greatly enlarged nostril.

But it isn’t for lack of trying. The male has the biggest nostrils in proportion to its head of any animal on Earth, according to Pietsch. These sniffers are paired with extremely well-developed eyes, “so we think that it’s kind of a dual approach,” he said. “The female emits a species-specific smell, a pheromone, and the male searches out based on that, and then when the male gets close enough, the eyes can be used to distinguish the female of the correct species.”

And with two dozen other species of anglerfishes that engage in this manner of reproduction, the male had better be damn sure he chooses the right one. Luckily, the female puts on the red blue light — in the form of glowing bacteria living in her lure. Incredibly, some 90 percent of species in the deep utilize such bioluminescence.

“The bait out there is not only an organ of luminescence, but structurally it’s species-specific,” said Pietsch. “Every species of these 160 forms within this group, they have a pattern of filaments, and pigment patterns, and probably also light flash patterns, like fireflies. And they separate themselves out that way so that males can find females,” distinguishing “the tiny little differences between the structure of the bait.”

Once the male closes in, he bites onto the female, usually her belly, and their tissues fuse together to permanently join the pair in incredibly unholy matrimony. The male’s eyes and fins atrophy away, and here he will live out the rest of his life nourished by her blood, still breathing with his own gills and, importantly, still producing sperm.

“This establishes a hormonal connection,” said Pietsch, “so that probably the maturation of eggs and sperm is synchronized by the sharing of hormones. And once the eggs are mature and the male is ready, she extrudes the eggs” in a kind of gelatinous sheath that can be 30 feet long. This acts like a sponge, readily absorbing the water that the male has released his sperm into.

Keep in mind that this is happening several miles down, where there is little plankton for juvenile fish to eat. So the whole gelatinous mess is buoyant, slowly making its way to the surface, where the larvae hatch and feed, ideally growing big and then migrating down to the depths.

The females of these species can live 30 years, according to Pietsch, and over that time might collect several males, who provide sperm season after season after season (there is no “not now, honey, I have a headache” with anglerfishes). But other than the security of maintaining a constant source of sperm, why evolve such a complex ritual of reproduction in the first place?

“The idea is basically that it’s a deep-sea economy measure,” ichthyologist James Maclaine of London’s Natural History Museum wrote in an email to WIRED. “An anglerfish couple requires about half of the amount of food they would if the male was the same size as the female (and presumably living an unattached life). He is stripped down to the absolute bare essentials, she has to remain big due to the relative cost of making large eggs as opposed to tiny sperm.”

Where such a size difference between sexes, known as sexual dimorphism, gets really interesting is its manifestation in the world at large. The famed evolutionary biologist Stephen Jay Gould wrote about this in his essay “Big Fish, Little Fish” — which stars the anglerfishes, of course — arguing that in the majority of animal species, females are larger than males, because the latter often never need to fight for the former.

Read more at Wired Science