SC Connecticut News

Bufoceratias wedli, a deep-sea anglerfish species, reproduces through a version of sexual parasitism in which the male temporarily attaches to the much larger female, according to a new study by Yale researchers.

As the planet’s most expansive ecosystem, the deep sea can be a challenging place to find a mate. However, scientists say some deep-sea anglerfishes have evolved a unique method of reproduction that ensures once they find a partner in the vast open waters, they remain latched for life.

These anglerfishes, called ceratioids, reproduce through sexual parasitism. In this process, tiny males attach to their much larger female counterparts to mate. In some species, males bite the females and then release once mating is complete. In others, the male permanently fuses to the female. In obligate parasitism, the male’s head dissolves into the female and their circulatory systems merge; he transforms into a permanent sperm-producing organ.

In a study published May 23 in Current Biology, Yale researchers examined how sexual parasitism works in synergy with other traits associated with these fish to influence their diversification. Anglerfishes are found throughout oceans and are named after the fishing rod-like appendage females use to lure prey.

“People tend to have single-trait explanations for why a group of animals can thrive in a given ecosystem,” said Chase D. Brownstein, co-lead author and graduate student at Yale’s Department of Ecology and Evolutionary Biology. “But in most living things, it seems that several distinctive innovations work synergistically to exploit new habitats.”

Using genetic data from anglerfish genomes, researchers showed how complex features like sexual parasitism helped some groups transition from shallow habitats such as coral reefs to swimming in dark waters where sunlight cannot penetrate — known as the "midnight zone."

For their study, researchers reconstructed the evolutionary history of deep-sea species. They demonstrated that ceratioid anglerfishes transitioned from benthic walkers using modified fins on ocean floors in shallows to deep-sea swimmers around 50-35 million years ago during the Paleocene-Eocene Thermal Maximum — a period marked by high global temperatures causing widespread extinction.

Ultimately unable to infer a clear evolutionary tree due to rapid divergence among lineages leaving relationships unresolvable among them — they did find origins coinciding with transitions into deeper seas though couldn’t determine which form occurred first between temporary attachment or obligate parasitism.

Multiple traits evolved simultaneously enabling sexual parasitism: extreme sexual dimorphism (large females/miniature males), shedding adaptive immunity (specialized immune cells attacking/eliminating pathogens) so female hosts’ bodies don’t reject parasitic males were necessary developments according findings reconstructing key gene histories involved adaptive immunity revealing convergent degeneration enabling practice though not necessarily driving species diversification but aiding success within midnight zones per Brownstein's comments.

“Sexual parasitism is thought advantageous inhabiting Earth’s largest/most homogeneous habitat,” said Brownstein adding "Once individuals find mates within vast expanses obligate practices allow permanent latching critical aid evolution deep-sea varieties.”

Senior author Thomas Near professor ecology/evolutionary biology Faculty Arts/Sciences Bingham Oceanographic Curator Vertebrates Yale Peabody Museum noted potential implications human health saying understanding loss adaptive immunity could inform medical advances such organ transplants/skin grafts requiring suppressed immunity being crucially important suggesting interesting future research areas.

The study was co-authored by Katerina L Zapfe/Alex Dornburg University North Carolina Charlotte Spencer Lott Yale Richard Harrington US Department Natural Resources Marine Resources Division Ava Ghezelayagh University Chicago.

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