The band King Missile’s 1992 single “Detachable Penis” might seem like a tripped-out nightmare straight from the male psyche, but it’s all too real for one strange sea creature. The sea slug Chromodoris reticulata has a disposable penis that it sheds after sexual intercourse, according to a new study in the journal Biology Letters.

Researchers in Japan collected the red-and-white sea slug in shallow coral reefs off the coast of Okinawa. They stuck pairs of sea slugs in a small tank and watched the critters mate, documenting what they rather aptly called “bizarre mating behavior.”

C. reticulata is a hermaphrodite: Each animal possesses both male and female reproductive organs, and can use both at the same time. So in 31 trysts, the researchers watched the sea slugs line up head-to-toe (so to speak) and jab their long, thin penises into their partners’ vaginas, while simultaneously receiving their partners’ sperm. The trysts lasted anywhere from tens of seconds to several minutes. A few minutes after it ended, the sea slugs each dropped their penises.

Part of the penis is stored in a spiral inside the body, and once the sea slugs discarded the used part of their members, the undifferentiated tissue in the spiral would develop into a usable organ again. About 24 hours after copulating, they’d be ready for another round.


The sea slugs appeared to have enough penis for at least three chances to inseminate a partner, the authors noted.

When the researchers examined the detached penises, they found that the surface was covered in backwards-facing barbs, which seemed to have sperm clumps caught in them.

The researchers think that the spines might serve to remove a competitor’s sperm from the partner’s female reproductive organs. Since a sea slug’s female organs are able to store sperm from multiple partners at a time, this could be a way of clearing out the competition, the authors note.

It would take genetic analysis on the removed sperm to confirm this, the scientists say. In any case, the barbs would could act like a grappling hook, catching inside the vagina, making the penis difficult to remove after sex.


“This difficulty must be the essential reason for the particular penis [amputation],” the authors note.

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