A study reveals that anglerfish thrive in the deep seas through 'sexual parasitism,' in which males fuse with females



Anglerfish, which live in the bathyal layer of the ocean, known as the 'midnight zone,' where almost no sunlight reaches, are the most diverse group of deep-sea vertebrates, with an unusual hunting method that involves luring prey with a bioluminescent lure. A genetic analysis of the evolutionary history of anglerfish has revealed that sexual parasitism, a strange ecology in which a small male attaches himself to the female's body and becomes one with her, has played a major role in the success of anglerfish.

Reproductive innovation enabled radiation in the deep sea during an ecological crisis | bioRxiv

Anglerfish entered the midnight zone 55 million years ago and thrived by becoming sexual parasites | Live Science

Weird anglerfish mating strategy may have helped them evolve | New Scientist

In a study published in January 2024 on the preprint server bioRxiv, a research team led by Yale University evolutionary biologist Chase Brownstein analyzed the genomes of more than 160 species of deep-sea anglerfish to create an evolutionary family tree of anglerfish.

As a result, it was found that the ancestors of anglerfish, which used to walk on the ocean bottom using their pectoral fins, advanced into the deep sea's midnight zone approximately 55 million years ago, and then evolved diversified in just 5 million years.

At that time, volcanic activity released huge amounts of methane into the atmosphere, warming the Earth and causing tropical ocean temperatures to reach 36 degrees Celsius. This event is thought to have wiped out many deep-sea species, creating an ecological

niche in which the anglerfish's ancestors could thrive.

'What we discovered is that just as the land-walking ancestors of whales returned to the sea, the bottom-walking anglerfish went from the ocean floor back into the water,' Brownstein said.

The midnight zone of the ocean has no habitats for settlement, such as reefs, caves, or seaweed. Creatures that live in these areas have a hard time finding a mate, but the ancestors of the anglerfish overcame the darkness of the deep sea, where they rarely encounter mates, by acquiring a new reproductive strategy.

One of the weapons anglerfish use to find mates is their excellent sense of smell: Male anglerfish have nostrils so large that Brownstein describes them as 'very sci-fi,' and they use them to detect the pheromones released by females.

When a male anglerfish encounters a female, he will bite into the female's body, which can be up to 100 times his own, until he is ready to mate. In some species, the male's body will fuse with the female's, forming a 'sexual parasitism' in which the male shares his blood vessels and circulatory system with the female.

by Robbie N. Cada

Even before they moved into the deep sea, the ancestors of the anglerfish had a dwarf male morphology, with males being extremely smaller than females, and a weak immune system that allowed males to avoid being attacked by females.It is believed that the anglerfish has acquired the means to turn a single encounter into an everlasting bond by turning this immune defect into an advantage.

'I see this as an example of what's known as exoadaptation , where a trait that didn't have a clear adaptive role emerges in a new environment and acquires an adaptive role,' Brownstein said.

in Science,   Creature, Posted by log1l_ks