Bizarre sea slug discovered with a disposable penis

The natural world is a strange, surprising place. Even jaded scientists were startled to discover a species of sea slug that is able to detach, re-grow and re-use its penis.

In a study published in the journal Biology Letters, Japanese researchers describe the sea slug Chromodoris reticulata and what they call its "disposable penis."

The slugs are a branch of animal known as nudibranchs, meaning they are "simultaneous hermaphrodites" and posses both male and female genitalia. Speaking with BBC News, Bernard Picton, a marine invertebrates specialist who was not involved in the study, described how exactly these strange creatures copulate:

"The genital apparatus is on the right hand side of the body. So two nudibranchs come together and one faces one way and one faces the other way, with the right hand side of their bodies touching. The penis from one fits into the female opening of the other one, and the penis from that one fits into the female opening of the first one, if you see what I mean."

To discover this unusual breeding habit, researchers collected C. reticulata while scuba diving off the coast of Okinawa, Japan. They placed the slugs in an experiment tank and observed 31 cases of copulation.

The penis itself is extended during mating but is usually coiled within the slug's body like a fire hose. Only part of the penis extends outside the slug's body, and that is the section that is discarded, similar to how a gecko can shed its tail.

After little more than a day, the slug is ready for another round, this time using a section of its penis that remained inside its body previously.

Ayami Sekizawa, the lead author of the study, told Discovery News, "The sea slug sheds one-third of the internal penis length after each copulation. The sea slug is able to grow the penis gradually to its original length."

Sekizawa added, "We have no idea about the evolutional conditions for this unique mating behavior."