The female anglerfish’s shiny lure may be as irresistible to her lovers as it is to her prey

The female anglerfish’s iconic forehead appendage is called a lure for a reason.

Scientists have long known the creatures use the dangly, often bioluminescent accessory to draw in unwitting prey. 

Now, a new study posits the lure may have an additional function — a beacon for lonely little males looking for love in the darkest, coldest depths of the sea.

It’s fitting for a fish that biologist Alex Maile says is somehow both dazzling and terrifying in equal measures.

“They’re like beauty and the beast,” Maile, the study’s lead author, told As It Happens host Nil Köksal. “And I think that’s OK.”

The findings, published last month in the journal Ichthyology and Herpetology, are based on a DNA analysis of more than 100 preserved species of anglerfish from museum collections, illustrating how certain types of lures may have evolved to improve reproductive success. 

Marine biologist Milton Love, who wasn’t involved in the research, says the authors’ theory of a dual-purpose lure challenges conventional wisdom.

“I don’t know if it’s true or not. I’m not even sure how one would prove it,” said Love, a scientist at the University of California’s Marine Science Institute in Santa Barbara, Calif.

“But it’s great that people keep thinking of fresh ways to look at what we would have thought would be an established concept.”

Plenty of anglerfish in the sea

The most popular depiction of an anglerfish is probably the predator from Finding Nemo, whose bright lure illuminates its pointy-toothed grin. 

But anglerfish are, in fact, quite diverse, with hundreds of species living in different oceans and habitats.

The females have lures, an evolutionary modification of the dorsal fin spine. They’re like fishing rods, and the bulbs on the end serve as bait.

Some are long, while others are stubby. Some emit chemical compounds. Some glow with bioluminescent bacteria. Some don’t do anything at all. 

Maile, a PhD candidate at the University of Kansas, found himself wondering why they would all evolve so differently? 

“The variation in just the lure itself is essentially what compelled me to pursue this type of research,” he said.

So he, and his colleague, Matthew Davis from St. Cloud State University in Minnesota, mapped out the different types of lures onto an evolutionary tree, dated when likely divergences occurred using fossil evidence, then used computer modelling to connect those designs to different habitats and behaviours.

It’s not exactly up close and personal. But deepsea anglerfish, in particular, are hard to study, living too far down for scuba divers to reach, and often dying when removed from their frigid, high-pressure environment. 

“A lot of these species have never been seen alive,” Maile said. 

Once they started glowing, it was party time

The researchers found the first anglerfish lure developed in a common ancestor about 72 million years ago. Then, somewhere between 23 and 34 million years ago, bioluminescent lures appeared among the deepsea species.

After that, the researchers say, there was an explosion in diversity among anglerfish, suggesting that natural selection was at play, and bioluminescence helped the males find females in the dark waters. 

Maile says the males need all help he can get. The thumb-sized fish can be hundreds of times smaller than females, and they don’t have any lures of their own.

“But they have giant eyes and giant noses, indicative [that] they’re picking up some sort of bioluminescent cues or some sort of chemical pheromones,” he said.  

“The males, at a certain point in their life, actually stop eating. So once they become sexually mature, they’re kind of on a timeframe to look for a female.” 

Not the only sexy light in the ocean

Maile says glowing is not an unheard of mating strategy on the ocean’s depths. Other deepsea fish, including lanternfish and dragonfish, use bioluminescence to attract mates. 

In the latter case, one study found the males may have evolved to have large eyes, not unlike male anglerfish, to better spot the glowing females.

But for some anglerfish, once the male does find a female, it doesn’t end prettily.

It latches onto her by biting into her flesh. The lovers’ tissues and circulatory systems then fuse, and they become one. 

The male stays that way for the rest of his life, providing his sperm again and again, until he drops off like a wart, sometimes leaving a few stray teeth embedded in his host. 

While it seems like a grisly end for the male, Love says it’s a matter of perspective. After all, it’s their whole “goal in life.”

“So, you know, to me, that would end well.”