Inside the extremely bizarre sex lives of anglerfish
The fish, who live in pitch-black darkness, likely use their bioluminescent lures to find mates. Males then attach themselves to the females—and never let go.
Over 3,000 feet below the surface, off the coast of Greenland, a curious fish waits. Perched in pitch-black darkness, it dangles a bright blue orb atop its head. The tantalizing glow will slowly draw other unsuspecting fish closer until they’ve unwittingly navigated directly into the jaws of the deep sea’s most fearsome-looking predator—the anglerfish.
Since 1878, researchers had assumed the anglerfish’s bioluminescent lure had evolved to capture prey: It hangs just in front of the fish’s mouth, enabling it to entice smaller deep-sea fish without illuminating its body and revealing its deadly game. But a new study in the journal Ichthyology and Herpetology offers a clue, buried in the anglerfish’s evolutionary history, that the lures may have evolved for something else—sex.
“Papers like this go to show us that there's so much complexity in the lure and how it evolved—way more than you need to draw in prey,” says Tracey Sutton, a marine biologist at Nova Southeastern University who was not involved with the research. The new study is “almost certain to become a real classic in the field.”
Mates for life
Deep-sea anglerfish have a peculiar and mysterious mating ritual. Males latch onto the larger females like tiny tadpole vampires. Then they never let go.
“They bite on and they're completely set for life,” says Alex Maile, an ichthyologist at the University of Kansas and lead author of the study. The males then lose vital functions in their body, like eyesight and immune system responses, as they fuse into the anatomy of the female. Their testes grow enlarged, their body elongates, and they become a sausage-like appendage to the female. “They essentially just get used to hav[ing] sex all the time.” One female pulled from a trawler had six males attached, he adds.
But watching a female anglerfish attract a male in their natural habitat is difficult. “The odds that you're going to watch an anglerfish in a dating scene or trying to attract a male are at least bigger than a trillion to one,” says Maile. To see the fish, you either need deep-sea autonomous rovers or trawlers that can scoop them up with nets from great depths.
“It's very expensive to do collecting trips,” says Matt Davis, an ichthyologist at St. Cloud State University and co-author of the study. “You could spend $100,000 a day taking a boat out into the Pacific and maybe only catch one of these fishes in a 24-hour cycle.”
“They’re basically living in outer space,” adds Sutton. That scarcity has left many questions unanswered about their natural history, like: How do the males even find the females, and do the lures come into play?
In the new research, Maile and Davis focused on 118 museum specimens and used a large collection of DNA data from anglerfish across shallow water and deep-sea habitats, with the hopes of obtaining an understanding of how their lures evolved. The results determined that the anglerfish’s bioluminescent lure led to an explosion of anglerfish diversity, hinting at the glowing lure’s purpose beyond mere hunting: attracting the opposite sex.
Glowing history
Anglerfish evolved over 72 million years ago in shallow waters, then over the following tens of millions of years migrated into the rest of Earth’s vast ocean. The earliest anglerfishes had simple mechanical lures that bobbed up and down or mimicked typical prey items like worms and finned fish. Later, some evolved to have chemical lures, which secrete a trail of chemicals into nearby currents to entice prey like clams and other invertebrates.
Their final innovation was bioluminescent orb, which evolved around 34 million years ago. The glowing lures are kind of like a petri dish, collecting bioluminescent bacteria from the water as the anglerfish age, says Maile. They can even shutter the orb with a small flap of skin to create a visual signal that echoes through the deep. Males are significantly smaller than females and usually don’t have lures at all, whereas females have a panoply of shapes and sizes. “It can go from just being one straight rod with a ball on the end of it,” says Sutton, to an elaborately decorated lure with multiple frilly pieces of flesh.
Yet despite their bizarre and striking appearance, relatively little research had explored where the lures came from and how they evolved until the current study. The results suggest that once anglerfish began migrating into the deep-sea ocean and developing their bioluminescent lures, they began to rapidly diversify and evolve. Their lures were like lighthouses that formed unique populations of deep-sea anglerfish.
“The groups are probably using light in the lures for communication,” says Davis—this emits a radiating glow in the deep like a siren call to tiny males. Early on, this light likely enabled the anglerfish to find each other, allowing them to reproduce, form genetic barriers, and ultimately birth new species. The bioluminescence led to an explosion of diversity, making the bioluminescent anglerfish the most diverse of the known anglerfish species. The study also determined that these deep-sea anglerfish evolved more rapidly than other anglerfish in shallow waters with different types of lures.
Sutton is excited about the new research. “All this complexity is tied directly to the evolution of the group, so [the anglerfish] must use it to recognize each other in some way, to synchronize themselves,” he says. “Finding a mate in the dark is one thing…[using the light to sync] hook-up behavior is completely different; it’s mind-boggling.”
