Till Death Do Us Part (Literally)
Male anglerfish permanently fuse to females, their bodies merging until the male is nothing but a pair of testes attached to the female's body.
In the pitch-black depths of the ocean, finding a mate is challenging. Deep-sea anglerfish have evolved a solution so extreme it seems like science fiction: males permanently fuse to females in an act of sexual parasitism.
When a tiny male anglerfish (often 10-40 times smaller than the female) finds a female, he bites into her body and never lets go. His mouth fuses with her skin, their blood vessels connect, and over time, the male's body degenerates. His eyes shrink, his internal organs dissolve, until he's essentially reduced to a pair of testes dangling from the female's body.
A single female can carry multiple males attached to her body — living sperm banks she can use whenever she's ready to reproduce.
This arrangement benefits both parties in an environment where encounters are rare. The female gains a permanent source of sperm, while the male gains a constant supply of nutrients through their shared bloodstream.
Remarkably, the male's immune system doesn't reject the foreign tissue of the female (or vice versa). Scientists discovered that anglerfish have evolved modified or missing immune genes that would normally prevent such fusion.