Male Anglerfish Fuse Into Their Mates and Become Sperm Sacs
He bites on, his body dissolves into hers, and his bloodstream merges with her circulation. He never lets go.
In the ceratioid family of deep-sea anglerfish, the male is roughly a fortieth the length of the female. He has no functional gut. He has working olfactory organs and outsized eyes for one reason: finding her in pitch black.
When he does, he latches on with hooked teeth, often on her belly or flank. Then his lips and skin start to fuse with hers. The two circulatory systems connect. Within weeks, his eyes recede, his fins shrink, his organs atrophy, and most of what remains is a pair of testes plumbed directly into her bloodstream. She feeds him; he provides sperm on demand for the rest of her life.
Theodore Pietsch, who has spent his career on these fish, has documented females carrying as many as eight attached males at once. The arrangement neatly solves the problem of finding a partner in the bathypelagic zone, where biomass is sparse and a single sex-specific search would otherwise be evolutionary suicide.
The immunology is the harder puzzle. Tissue fusion across two individuals is, in any other vertebrate, an immediate rejection event. In 2020, Jeremy Swann and colleagues sequenced the immune genes of four anglerfish species and found something startling: the families that practice sexual parasitism have lost or broken core machinery for adaptive immunity, including genes considered essential in every other jawed vertebrate. They cannot tell self from non-self the way we do. The fusion works because the rejection apparatus is gone.
The trade-off — give up the immune system, gain a guaranteed mate — is not one most lineages would survive. In the deep dark, it pays.
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