Anglerfish is a terrifying sea creature that lives over 1,000 meters (3,200 feet) deep in the sea and looks no less than a sea monster straight out of a Hollywood movie. However, what’s even more terrifying is the way this creature reproduces.

Ceratioids, a type of anglerfish, exhibit sexual parasitism, a rare reproductive behavior where the much smaller male fish permanently merge their bodies with the larger female fish. Once attached, the male acts like a parasite, relying entirely on the female for nutrition. In return, it ensures that when the female releases eggs, it always has a ready source of sperm for reproduction.

More than 20 species of anglerfish show sexual parasitism (in some cases, the attachment is temporary while in others it is permanent) and scientists have known about it for some time. They believe this reproductive strategy may have also played an important role in the evolution of the fish. 

However, what they never fully understood is how the deep sea creature developed such behavior in the first place. Finally, a new study from researchers at Yale University provides some valuable insights into the origin of sexual parasitism in anglerfish.

“Sexual parasitism is thought to be advantageous to inhabiting the deep sea, which is Earth’s largest and most homogenous habitat. 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,” Chase D. Brownstein, one of the lead study authors and a researcher at Yale, said.

Evolution of sexual parasitism in anglerfish

Millions of years ago, anglerfish didn’t swim but walked the ocean floor using modified fins, but during the Paleocene-Eocene Thermal Maximum (~56 million years ago) our planet witnessed intense global warming, forcing many animals to go extinct. 

This is when anglerfish migrated from shallow ocean waters to the deep sea — and turned from benthic walkers to deep-sea swimmers. According to the study, this was also when the fish developed sexual parasitism.

When a male anglerfish encounters a female, he bites onto her body and releases an enzyme that dissolves the skin of his mouth and her body, fusing the two together. Over time, the male’s body essentially becomes a part of the female, with his organs shrinking and his body degenerating. Eventually, he is reduced to a pair of gonads that the female uses to fertilize her eggs whenever she is ready to reproduce. This allows her to have a readily available supply of sperm, eliminating the need for her to find a mate in the vast and dark ocean again.

This extreme form of sexual parasitism ensures the survival of the species in an environment where encounters between potential mates are rare. The male benefits by ensuring his genes are passed on, albeit at the cost of his autonomy and physical form.

The study authors conducted a genomic study of the deep sea fish and reconstructed its evolutionary history revealing several traits that possibly contributed to the development of sexual parasitism in anglerfish.

“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 were in ecological upheaval,” Brownstein said.

These traits included sexual dimorphism (difference in physical appearance of males and females) that led to large females and tiny males, and degeneration of adaptive immunity (body’s defense mechanism against foreign cells and pathogens).

While the extreme difference in body size allowed males to easily latch to the females and fuse their bodies with them, losing adaptive immunity made female anglerfish’s immune system unresponsive to the male parasite. This is why female fish antibodies don’t attack male cells when the male fish merges its body into the female. 

“We determined that the degeneration of the genomic basis of adaptive immunity and extreme size differences between male and female anglerfishes coincide with the origin of ceratioids and the evolution of sexual parasitism,” the study authors note.

However, further research is required to confirm this theory and identify the genetic changes that led to these evolutionary changes.

Significance of sexual parasitism and its origin

Anglerfish live in the deep sea and they adopted sexual parasitism millions of years ago. Why should we even care about their reproductive strategy? Well, we should because one of the traits that enabled this creature to develop sexual parasitism could help us overcome many challenges associated with organ transplants.

For instance, a major risk during organ transplant operations is the rejection of the transplanted organ by the patient’s immune system. However, we could overcome this challenge if we figured out a way to make humans temporarily shed their adaptive immunity.

“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,” said Thomas Near, senior study author and a professor at Yale.

The study is published in the journal Current Biology.