Bizarre Sex Helped Anglerfish Diversify and Dominate the Deep Sea, Study Suggests

The ocean’s lightless zone begins some 1,000 feet below the surface, and in some places, it extends more than seven miles deep. Dark, vast and empty, it’s a difficult place to find a mate.

But anglerfish—known for their agape mouths, fishing pole-like lures that extend from their foreheads and horror-movie blank stares—seem to have found a unique solution: hold their mates close, and never let them go.

More specifically, some anglerfish engage in sexual parasitism. Certain males—which have smaller bodies measuring just inches long—attach themselves with their teeth to the bellies of females, which are larger and can grow up to three feet long. After mating, the males remain affixed there permanently. Other anglerfish engage in obligate parasitism, where the male’s head dissolves into the female and the two fish’s circulatory systems fuse, the male becoming a permanent sperm-producing organ.

Eventually, the male loses all his organs, save for his testes, and the female can carry multiple males at once. This unusual mating method has likely helped anglerfish become one of the deep sea’s most diverse vertebrates, with some 300 species in all.

In a new study published last week in the journal Current Biology, researchers studied anglerfish genomes to better understand the evolutionary mechanisms behind this odd behavior.

“Sexual parasitism is thought to be advantageous to inhabiting the deep sea, which is Earth’s largest and most homogonous habitat,” Chase D. Brownstein, an evolutionary biologist at Yale University and a co-lead author of the study, says in a statement. “Once individuals find a mate in that vast expanse, obligate sexual parasitism allows them to permanently latch, which seems to be a critical aid to the evolution of deep-sea anglerfish.”

First footage of deep-sea anglerfish pair

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Some 55 million years ago, the ancestors of anglerfish looked vastly different from the creatures we know today—they were bottom-dwellers that “walked” on their fins along the ocean floor. But the fish transitioned to swimming and multiplying in the so-called “midnight” zone during a period of warm global temperatures, the study found, which occurred between 35 and 50 million years ago.

“It happened in the blink of an evolutionary eye,” Brownstein tells the New York Times’ William J. Broad. “It was like whales going back into the ocean. It was amazing.”

The team found that sexual parasitism and obligate parasitism arose as anglerfish shifted to living in the deep ocean, but they could not determine which strategy evolved first.

It wasn’t sexual parasitism alone that caused anglerfish to rapidly evolve, but a confluence of traits that made the advantageous mating method possible. For one, the animals had to develop the extreme body size difference between males and females. They also underwent changes to their immune systems so that females do not dispel male anglerfish after they attach.

“Instead of basically being the gas in the engine [of diversity], sexual parasitism was kind of like prepping the gas pedal,” Brownstein told New Scientist’s Christie Taylor in February, when the study was posted online as a preprint that had not yet been peer-reviewed.

The team says their research into anglerfish immune systems may one day inform advances in human health and medicine.

“Better understanding how deep-sea anglerfishes lost adaptive immunity could one day contribute to advances in medical procedures, such as organ transplants and skin grafting, where suppressing immunity is crucially important,” Thomas Near, an evolutionary biologist at Yale University and a co-author of the study, says in the statement. “It’s an interesting area for future medical research.”

The study follows and largely corroborates research posted late last year to the preprint server bioRxiv, in which scientists constructed an anglerfish evolutionary timeline.

“I think over the course of the next six months, anglerfish enthusiasts are going to be eating good with these two papers,” Kory Evans, a biologist at Rice University not involved with either study, told New Scientist.

In May, beachcombers discovered a female deep-sea anglerfish washed up on the sand in Oregon. Known as a Pacific footballfish, the sighting marked only the 31st time an individual of that species has been found. Others have appeared on Southern California beaches in recent years.

Christian Thorsberg | READ MORE

Christian Thorsberg is an environmental writer and photographer from Chicago. His work, which often centers on freshwater issues, climate change and subsistence, has appeared in Circle of Blue, Sierra magazine, Discover magazine and Alaska Sporting Journal.