Buzz
When you picture an anglerfish, you likely imagine a creature like the one above: a giant mouth, sharp teeth, and a glowing lure hanging from its head. In short, an ideal nightmare subject.
In the 19th century, when scientists first began discovering and classifying anglerfish from a specific branch of the anglerfish family—the Ceratioidei suborder—that’s the image they had as well. However, they were only seeing part of the story. All the specimens they worked with were females, and they had no idea where the males were or what they looked like.
Researchers occasionally came across other fish that seemed to share a similar body structure, but they lacked the terrifying jaws and lure characteristic of ceratioids and were far smaller—sometimes just 6 or 7 millimeters long—and were categorized into different taxonomic groups.
It wasn’t until the 1920s—nearly a century after the first ceratioid was documented—that the picture began to clear up. In 1922, Icelandic biologist Bjarni Saemundsson discovered a female ceratioid with two smaller fish attached to her abdomen by their snouts. He initially thought it was a mother with her offspring but was puzzled by the unusual arrangement.
“I cannot fathom how or when the larvae, or young, become attached to the mother. I cannot believe the male attaches the egg to the female,” he wrote. “This remains a mystery for future researchers to solve.”
While Saemundsson left the mystery unsolved, Charles Tate Regan, working at the British Museum of Natural History in 1924, took over the investigation. Regan also found a smaller fish attached to a female ceratioid. After dissecting it, he discovered it wasn’t a different species or the female’s offspring—it was her mate.
Parasitic Love
The “missing” males had always been there, just overlooked and misidentified. Regan and other scientists, like Norwegian zoologist Albert Eide Parr, soon understood why male ceratioids looked so different. They don’t need lures or large jaws because they don’t hunt; they rely on the females. Regan wrote that the ceratioid male is “simply an appendage of the female, entirely dependent on her for sustenance.” In essence, a parasite.
When ceratioid males search for a mate, they follow a species-specific pheromone trail to a female, who often helps guide them further by flashing her bioluminescent lure. Once the male finds a suitable partner, he bites into her belly and attaches himself, where his body eventually fuses with hers. Their skin and blood vessels merge, allowing the male to draw nutrients from his mate’s blood. The two fish effectively become one.
With his body permanently attached, the male no longer needs to worry about basic fish functions like sight, swimming, or eating. His unnecessary body parts—eyes, fins, and some internal organs—atrophy, degenerate, and fade away, leaving him little more than a lump of flesh clinging to the female, feeding off her and supplying sperm whenever she’s ready to spawn.
Not all anglerfish exhibit such extreme sexual dimorphism or parasitic mating. In other suborders, there are males that live freely, capable of hunting and only attaching to the females temporarily for reproduction before moving on. However, for deep-sea ceratioids, who might rarely encounter each other in the dark depths, this bizarre mating strategy is an essential adaptation to keep mates close and ensure the survival of their species. For us, it’s both fascinating and unsettling, a stark reminder of how the natural world can be as strange as any fictional tale.
Naturalist William Beebe captured it perfectly in 1938, writing, “But to be driven by impelling odor headlong upon a mate so gigantic, in such immense and forbidding darkness, and willfully eat a hole in her soft side, to feel the gradually increasing transfusion of her blood through one’s veins, to lose everything that marked one as other than a worm, to become a brainless, senseless thing that was a fish—this is sheer fiction, beyond all belief unless we have seen the proof of it.”
A version of this article was originally published in 2014 and has been updated for 2023.
