Why Some Anglerfish Males Become Part of the Female
In several groups of deep-sea anglerfish, the biggest challenge to reproduction is finding another fish at all. These animals live far apart in darkness, where visual signals are of little use and chance encounters are rare. A tiny male that finds a female may therefore gain more by staying with her than by swimming away to search for another mate.
He attaches by biting her skin. In species with permanent attachment, the contact triggers a gradual biological merger: the tissues at the bite site grow together, and the pair develop a shared blood supply. The male is no longer just clinging to the female’s exterior; nutrients and oxygen can reach him through their connected circulation.
A living source of sperm
As the male becomes dependent on the female, much of his body’s independent function is reduced. His reproductive organs remain useful, however, and can supply sperm when the female releases eggs. Some females carry more than one attached male, giving them access to sperm from several partners without needing to encounter another fish at the moment of spawning.
This arrangement is called sexual parasitism: one animal remains attached to another and depends on it while serving a reproductive role. It is not universal among anglerfish. In some species, males attach temporarily or the pair do not fuse in this way.
The strategy trades independence for reliability. A free-swimming male could keep searching, but in a vast habitat where another female may be difficult to find, permanent attachment makes a successful encounter count. When the female is ready to reproduce, the mate she found in the darkness is already there.
