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Male Anglerfish Ditched Immune System for Sex

Close up of female anglerfish
Edith A Widder


The bizarre world of anglerfish sex has just started to make a little more sense. Researchers have uncovered how the males of the species make it possible to merge their bodies with their female lovers during a process known as “sexual parasitism.”

Two Become One

In the deep dark depths of the ocean, dating can be a real challenge. That’s especially true if you’re a tiny male anglerfish looking for a mate. While female anglerfish can be 60 times bigger than the males, and weigh up to 500,000 times as much, they’re relatively rare and hard to spot in the vast blackness of the sea where they live at depths of between 980 and 16,400 feet. So once a male does indeed find a female—who sometimes helps him along by lighting up a dongle attached to her snout—he doesn’t let go.  

In 25 out of 160 anglerfish species, the males bite into the females’ bellies, where they begin the process of fusing with them. The two fish come to share the same blood supply, and the males’ eyes and fins eventually atrophy as he becomes nothing more than an external sperm-production organ. 

‘How Is It Possible?’

The thing is though, while the whole fusing-two-bodies-into-one thing might sound great in a romance flick, scientifically, it shouldn’t be able to happen.

“How is it possible that genetically similar organisms—in this case, members of the same species—accept each other so readily when tissue rejection is the usual and expected result of any such union?” said co-author Ted Pietsch, professor emeritus at the University of Washington School of Aquatic and Fishery Sciences. “Just witness the difficulties surrounding organ transplantation in humans. We now see great potential down the road for a better understanding of the problem.”

That potential comes from a study Pietsch did with his colleagues at the Max Plank Institute of Immunobiology and Epigenetics in Germany. They started by looking at molecules called major histocompatibility (MHC) antigens, which lie on the surfaces of cells and alert the immune system when an invader like a virus or bacteria is present. These are the molecules that typically act up when foreign tissue, like an organ, is introduced.

But in the case of the male anglerfish, the researchers found that their genes that encode for MHC molecules were missing. On top of that, they discovered that antibodies, as well as killer T cells, which are the infection-busting soldiers in the immune system, were either severely blunted or missing altogether in the fish. 

Losing Immunity

This all adds up to an immune system that would lead to the death of most animals, but one that doesn’t have a problem with tissue rejection, and is therefore perfect for the male anglerfish’s sexual parasitism. 

In lieu of a typical immune system, the researchers postulate that male anglerfish have developed different types of defenses that keep them healthy until they find a mate. However, more work is needed to determine precisely what those defenses are.

The researchers are hopeful that continued work could lead to strategies to boost human immune systems one day, especially in immunocompromised patients.

The report has been published in the journal, Science.