Yale researchers have discovered how sexual parasitism among deep-sea anglerfish, in which males are permanently attached to females, facilitates their adaptation from shallow habitats to the deep-sea “midnight zone.” Combining genetic analysis and evolutionary biology, this research suggests that immunosuppression methods can be used in medicine.

The deep sea, the largest ecosystem on the planet, can be a difficult place to find mates. But scientists have discovered that some deep-sea fish have evolved a remarkable reproductive strategy that allows them to stay together for life once they find a mate in the vast waters.

These fish, called ceratioids, reproduce through sexual parasitism, in which small males attach themselves to much larger females to mate. In some species, males bite females and release them after the mating process is completed. In some, the male will mate with the female forever. In a process called obligate parasitism, the male’s head becomes entangled with the female’s and their circulatory systems merge. It develops into a permanent sex organ that produces sperm.

Evolutionary advantages explored

In a new study published May 23 in the journal Current Biology, Yale researchers examined how sexual parasitism works in synergy with other fish-related traits to influence the diversity of anglerfish, an animal found throughout the ocean whose name is inspired by the rod. It resembles the appendage that females use to attract prey.

Understanding the evolution of sexual parasitism has implications that researchers believe could one day influence the development of medicine.

Using genetic data from fish genomes, researchers showed how complex traits such as sexual parasitism help some groups of fish switch from wandering in shallow habitats such as coral reefs to swimming in the dark, open waters of the “midnight zone,” a deep-sea ecosystem. Where sunlight cannot penetrate.

“People tend to have clear explanations for why a group of animals might thrive in a particular ecosystem, but in most creatures, many different innovations seem to work synergistically to adapt to new habitats,” says graduate student Chase D. Brownstein. department of Ecology and Evolutionary Biology at Yale University and one of the lead authors of the study. “We found that a set of traits, including those required for sexual parasitism, allowed anglerfish to invade the deep sea during a period of extreme global warming when the planet’s oceans experienced environmental shocks.”

Genetic findings and their implications for medicine

For the research, scientists reconstructed the evolutionary history of deep-sea species. They show that the rapid transition of ceratioid anglerfish from demersal walkers to deep-sea swimmers, which used modified fins to “walk” on the ocean floor in shallow water, occurred during the Paleocene-Eocene Thermal Maximum, 50-35 million years ago. A period of high global temperatures causing ocean extinctions.

Ultimately, researchers were unable to construct a clear evolutionary tree of deep-sea anglerfish because different lineages diverged from each other so quickly that relationships between lineages remained tenacious, Braunstein said. However, Braunstein found that the origins of sexual parasitism coincided with the deep-sea migration of anglerfish, but they were unable to determine which of two forms of parasitism (temporary attachment or obligate parasitism) emerged first.

Many features evolved simultaneously to make sexual parasitism possible. For example, ceratioids had to evolve extreme sexual dimorphism with large females and tiny males. They also had to lose adaptive immunity (a system of specialized immune cells and antibodies that attack and destroy pathogens) to prevent the female’s body from rejecting the parasite male.

By reconstructing the evolutionary history of key genes involved in adaptive immunity, the researchers learned that multiple groups of deep-sea anglerfish convergently degenerated their adaptive immunity, enabling sexual parasitism. Although sexual parasitism evolved as deep-sea anglerfish moved deeper into the sea, scientists concluded that this was not a key trait driving species diversity among ceratioids. But Braunstein said this allows anglerfish to thrive in the midnight zone.