Why Sea Slugs Dispose of Their Own Penises

It’s cute and brightly colored, but this species of sea slug has a macabre sex life. Photograph by Jon Aguirre, My Shot


When it comes to kinky sex, nature has quite the imagination. Some animals devour their suitors after doing the deed, while others attach themselves, in a parasitic fashion, to their mates in order to reproduce.

But a new study finds that the ostentatious sea slug, or nudibranch, may take the cake—one species of this marine invertebrate cuts its own penis off after mating and regrows a new reproductive organ within 24 hours, whereupon it’s ready to mate again. (See sea slug pictures.)

“I have been working on the anatomy of nudibranchs for 20 years and I have never seen anything like that,” said Angel Valdes, a sea slug expert at California State Polytechnic University in Pomona, who was not involved in the study.

Valdes added in an email that sea slugs are known to sever other parts of their body in a process called autotomy. Some species will shed protrusions called cerata while others will drop the frilly “skirt” that runs around their body in order to distract a predator while the slug makes its escape. (Read about nudibranch defenses in National Geographic magazine.)

Ayami Sekizawa, of Osaka City University in Japan and lead author on the study, wrote in an email that while other animals, such as some species of octopuses, also break off their reproductive organs after mating, they can’t grow them back as far as researchers know.

Removable Penises

Most nudibranchs are simultaneous hermaphrodites, meaning individuals have both male and female reproductive organs—and they can deliver and accept sperm at the same time.

They can also store sperm from several mates and choose which ones to use in fertilizing their eggs by digesting the sperm they don’t want. (Also see pictures: “Fiery Sea Slug Discovered, Lays Lacy Egg Case.”)

When Sekizawa and her colleagues studied the mating habits of Goniobranchus reticulata (known as Chromodoris reticulata until last year), collected from shallow coral reefs near Sesoko Island (map) in the East China Sea, they noted that individuals would only sever their penis after disengaging from their partner.

And when researchers examined the discarded penises, they found sperm entangled in the backward-facing spines that cover the organ.

Sekizawa speculated that by removing their penis from their mate, the sea slugs were increasing the chances that their partner would use their sperm to fertilize its eggs, rather than a competitor’s sperm.

“If the sea slug left the penis in the mating partner’s female organ, it could not remove sperm of preceding mates,” wrote Sekizawa, whose study appeared February 13 in the journal Biology Letters. The researchers would need to conduct DNA tests in order to confirm this though.

Three Times a Charm

Sekizawa and her colleagues also found that intact individuals who hadn’t severed their male organ had a spiral structure in the middle of the inner duct of the penis. Discarded penises were missing the spiral structure.

They speculated that the spiral provided enough length for the sea slugs to grow and sever their penis at least three times, based on the fact that one G. reticulata in the study discarded and regrew its penis three times. (See “Barnacles Can Change Penis Size and Shape.“)

Valdes noted that other sea slug species have similar reproductive structures and couldn’t understand why this behavior hadn’t been observed before. But he’s excited to see whether other nudibranchs share this macabre habit.


Meet the Author
Jane J. Lee is a news writer and editor at National Geographic.