The Swimming Penis
Male argonaut octopuses have a detachable, sperm-filled arm that swims to the female on its own — scientists originally thought it was a parasitic worm.
The argonaut (paper nautilus) has one of the strangest reproductive strategies in the ocean: a detachable, autonomous penis.
Male argonauts are tiny — only about an inch long, compared to females who can reach 10 inches and create beautiful paper-thin shells. To mate, the male doesn't approach the female directly. Instead, he detaches a specialized sperm-filled arm called a hectocotylus, which then swims on its own to the female.
The first scientists to observe these swimming arms didn't realize what they were. In 1829, naturalist Georges Cuvier described them as a new species of parasitic worm. It wasn't until later that scientists realized these "worms" were actually octopus penises.
After the male releases his hectocotylus, he can regenerate a new one — though it takes time. Some males die shortly after mating; others survive to mate again. The female stores the arm in her mantle cavity until she's ready to fertilize her eggs.
This detachable arm strategy solves a dangerous problem: female argonauts sometimes eat males. By sending just an arm, the male can fertilize eggs from a safe distance.