A new study illuminates the evolution of deep-sea anglerfish

Certain deep-sea anglerfish have developed a special kind of copulation that guarantees they will stay hooked to their mate for life once they find them in the vast open oceans.

The sexual parasitism of these anglerfishes, known as ceratioids, allows the little males to mate with their larger female counterparts. In certain species, the males bite the females after mating and then release them. In some, the male and female are fused. The male’s head dissolves into the female’s, and their circulatory systems merge in a process known as obligatory parasitism. He changes into a sexual organ that can produce sperm permanently.

A new study from Yale University sheds light on how sexual parasitism works in synergy with other traits associated with the fish to influence the diversification of anglerfishes.

The researchers believe that understanding the evolution of sexual parasitism may have implications for future medical advancements.

The scientists demonstrated how complex traits, like sexual parasitism, helped some anglerfish groups transition from swimming in the open, dark waters of the “midnight zone” to roaming shallow habitats like coral reefs.

Chase D. Brownstein, a graduate student in Yale’s Department of Ecology and Evolutionary Biology and the study’s co-lead author, said, “We found that a cascade of traits, including those required for sexual parasitism, allowed anglerfishes to invade the deep sea during a period of extreme global warming when the planet’s oceans where in ecological upheaval.”

For the investigation, researchers recreated the deep-sea species’ evolutionary history. They showed that between 50 and 35 million years ago, during the Paleocene-Eocene Thermal Maximum—a time of high global temperatures that caused extinction throughout the oceans—cerratioid anglerfishes underwent a rapid transition from being benthic walkers, which use modified fins to “walk” the ocean floor in the shallows, to deep-sea swimmers.

Because the different lineages diverged from one another so quickly, it was ultimately impossible for the researchers to draw a clear evolutionary tree for deep-sea anglerfishes. However, they discovered that the emergence of sexual parasitism accompanied anglerfishes’ descent into the deep water. However, they could not identify which of the two types of parasitism appeared first.

The simultaneous evolution of multiple features allowed for sexual parasitism. For instance, ceratioids had to undergo significant sexual dimorphism, resulting in huge females and little males. To prevent the bodies of the female hosts from rejecting the parasitic male, they also had to lose their adaptive immunity, which is a system of specialized immune cells and antibodies that fight and eradicate infections.

By reconstructing the evolutionary history of essential genes related to adaptive immunity, researchers discovered that several deep-sea anglerfish groups convergently lost their ability to defend against sexual parasitism. 

They also concluded that sexual parasitism, although developing as deep-sea anglerfishes entered the water, is not always the primary characteristic causing species diversity among ceratioids. However, it did make it possible for anglerfish to thrive in the midnight zone.

Brownstein said, “Sexual parasitism is thought to be advantageous to inhabiting the deep sea, which is Earth’s largest and most homogonous habitat. Once individuals find a mate in that vast expanse, obligate sexual parasitism allows them to latch permanently, which seems to be a critical aid to the evolution of deep-sea anglerfish.”

Senior author Thomas Near, professor of ecology and evolutionary biology in Yale’s Faculty of Arts and Sciences and Bingham Oceanographic Curator of Vertebrates at the Yale Peabody Museum, said, “The research has potential implications on human health.”

“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. It’s an interesting area for future medical research.”

Journal Reference:

Chase D. Brownstein, Katerina L. Zapfe, Spencer Lott, et al. Synergistic innovations enabled the radiation of anglerfishes in the deep open ocean. Current Biology. DOI: 10.1016/j.cub.2024.04.066

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