Thanks to Brilliant for supporting this episode of SciShow.

Go to Brilliant.org/SciShow to check out their course: Knowledge and Uncertainty. [♩INTRO]. While us humans tend to enjoy a good cuddle, we also generally like to have at least a little bit of personal space.

But not anglerfishes! In some species, the males literally become one with their partners. And to survive this fusion, they've gotten rid of something that we think is kind of essential: the part of their immune system that recognizes and remembers pathogens!

Anglerfishes have terrifyingly oversized mouths, huge teeth, and fishing poles attached to their foreheads, complete with bioluminescent lures. So, they're pretty unique creatures. But what's really out there is how some anglerfish species mate.

When the male finds the girl of his dreams which, for the record, is the first one he can find he gives himself over to her. Entirely. These males are a lot smaller than females.

We're talking as little as 160th his partner's size and roughly half a million times lighter. And these tiny males have one purpose in life: to pass on their genes. But finding a mate is complicated in the dark depths of the ocean where they live.

So a super-mini male has to sniff out his love interest by tracking a species-specific pheromone that she emits. And when he finds her, he bites her. In the most romantic way possible, of course.

But then, his mouth dissolves, and his tissues fuse with hers. The pair will eventually share everything, including a circulatory system. And they'll remain that way forever.

She provides the nutrients, and he fertilizes the eggs, ‘til death do they… well, die. Because even then they don't separate. This “sexual parasitism” evolved for practical reasons: because, if you do find that elusive soulmate in a vast, empty darkness, you don't want anything getting between you.

In fact, this system works so well that it evolved multiple times in anglerfishes. Now, if this happened with people, we'd reject our codependent suitors out of hand. our bodies would oust or destroy the foreign cells. But as far as we can tell, females in these fusing species can't do that, because they've lost the soldiers of their adaptive immune system: the part that attacks pathogens in a specific way and remembers them in case they show up again.

In most vertebrates, specialized white blood cells called T cells and B cells are responsible for launching an adaptive immune response. They wield super customizable proteins which allow them to spot and remember invaders. But to customize those weapons, the cells need to cut up and mash together sections of their DNA.

And typically, this process is initiated by recombination-activating genes, or “RAGs.” But anglerfish species that fully fuse have lost key parts of those genes. So, their immune cells can't customize immune proteins, and therefore, can't spot or remember foreign cells. Even ones that temporarily attach have ditched some of the molecules involved in making those customizable proteins ready to go.

But what's really weird is that the fishes don't seem to be adversely affected by any of this. In humans, the absence of an adaptive immune system would be considered “severe combined immunodeficiency,” a condition that's fatal without treatment. Scientists don't really know how anglerfishes avoid such lethal consequences.

It could be that they've just got really strong innate immune systems, which probably wouldn't recognize the cells of another fish as an invader. Or maybe they have another kind of adaptive immunity that we just haven't identified yet. But if we can figure this conundrum out, we could potentially help immunocompromised humans safely navigate their world, too though hopefully, without needing to permanently attach them to their partners.

Thank you for watching this episode of SciShow, which was sponsored by Brilliant. If you're finding yourself at home a bit more than before, with or without a partner, and are looking for something fun and educational to do, you might want to check out the courses that Brilliant has to offer. They span a wide variety of topics in science, math, and computer science.

Like, their new Knowledge and Uncertainty course teaches tools that can help you deal with the uncertainties of life which could be helpful because there's a fair bit of that going around these days. But one thing you can be certain about is that the first 200 people to sign up at Brilliant.org/SciShow will get 20% off an annual Premium subscription! So head on over there if you want to learn more. [♩OUTRO].