Anglerfish Drop Their Immune Defenses to Find Love

Male anglerfish are major clingers. To avoid mistaking mates as foreign tissue, the deep sea couples lost part of their immune system in evolution

An anglerfish female—the iconic, toothy deep-sea fish with a bioluminescent rod on her forehead—and her male mate, which is six-times smaller and more finger-like in shape
The comparatively massive female anglerfish (Melanocetus johnsonii) with her tiny mate permanently fused to her belly. Edith A. Widder / Max Planck Institute of Immunobiology and Epigetics

Love can leave us defenseless, but for some species of deep sea anglerfish letting their guard down for new romance is in their genes. New research finds that evolution has actually eliminated an integral part of the ghoulish fish’s immune system to make sure that when they find a mate, nothing stands between them and complete union, reports Katherine J. Wu for the New York Times.

That’s because certain species of anglerfish have adopted what might seem like an extreme approach to the vast, lightless dating pool of the deep. When a male finds a female, which can be up to 60 times his size, he clamps onto her underside with tiny translucent fangs. The comparatively minuscule male’s love nip then turns into a permanent attachment: his mouth, and eventually even his blood vessels, fuse to the female to provide her eggs with on demand fertilization. (Talk about clingy.)

In biological terms, the male becomes a sexual parasite incapable of surviving without his beloved—his internal organs, with the exception of his testes, shriveled and useless. In a final twist, there are even a few species known to collect multiple supplicant males, accumulating as many as eight of the glorified sperm sacks.

The rub for immunologists is that this kind of body melding shouldn’t be possible for the same reasons humans can’t just go swapping organs willy nilly. An ancient part of the vertebrate immune system called adaptive immunity is programmed to seek and destroy any foreign substance that gets into the body, from viruses to bacteria, reports Erin Garcia de Jesus for Science News.

“When you look at [these fish], you scratch your head and think, ‘How is that possible?’” Thomas Boehm, an immunologist at the Max Planck Institute of Immunobiology and Epigenetics in Germany, tells Science News. The adaptive immune system’s aggressive response to invaders is why organ transplants must be assiduously matched for compatibility, “but these creatures seem to be doing it without knowing what’s going on.”

To figure out how the fish's immune system accommodates their frightfully intimate couplings, researchers sequenced the genomes of 13 of the 168 known anglerfish species, which are named for a bioluminescent lure that dangles in front of their faces on a rod-like appendage. The study included a range of species, four which attach to their mates temporarily, three that don’t latch on at all, and six that permanently attach to their mates. The species that blend blood and tissue were split evenly between those that keep one male around and those that retain a small roster of reproductive material.

The study found that in two of the species which display this last and most extreme form of sexual parasitism, certain genes closely associated with the adaptive immune response were missing, the authors report this week in the journal Science.

“It’s quite shocking,” Elizabeth Murchison, a geneticist at the University of Cambridge who wasn’t involved in the study, tells Katarina Zimmer of the Scientist. “I suppose we shouldn’t have too many preconceptions about what is and isn’t possible in nature. Evolution produces all sorts of wacky outcomes, and this is one of them.”

Specifically, the anglerfish that permanently fuse multiple males to a single female had lost the ability to produce T cells and antibodies, two types of immune cells that are fundamental to the body’s ability to identify and fight off interlopers, according to the Times.

“If I had to diagnose [those two fish] … I would say, ‘OK, this is red alert, we really have to do something because this is severe combined immunodeficiency. Fatal prognosis,’” Boehm tells Science News. In humans, severe combined immunodeficiency is a genetic disorder that so weakens the immune system that it usually proves fatal within the first year of the person’s life, per Science News.

The angler species that keep their tissue-mingling sex lives one on one exhibited similar but less severe genetic alterations, while those that engage in fleeting attachments appeared to retain the ability to produce T cells and a limited antibody response.

The findings raise new questions about how these anglerfish that have compromised their immune systems to hang onto their mates manage to stay healthy.

“Clearly, these animals are doing fine,” Zuri Sullivan, an immunologist at Yale University who wasn’t involved in the study, tells the Times. Maybe other parts of the immune system have ramped up their activities to compensate for the loss of antibodies and T cells, Sullivan posits.

Probing these questions will require more samples from these elusive denizens of the deep sea, a process that took years in the case of the 31 specimens used in the present study.

“We are not quite sure what lessons the anglerfish will teach us,” Boehm tells the Times. “But we know they have done something really incredible.”

Get the latest stories in your inbox every weekday.