'Sea Slug Grows New Penis After Sex'

The red-and-white sea slug gets rid of its penis after sex and then grows a new one in "extremely peculiar" behaviour, scientists have found.

The animal needs 24 hours between copulation to unroll an internally coiled and compressed juvenile penis to replace the used, external bit, they discovered.

It can even repeat this feat at least three times.

Named chromodoris reticulata in Latin, the human thumb-sized slug - technically a shell-less mollusc - inhabits warm waters in southeast Asia.

It is also an hermaphrodite, meaning it has both male and female sexual organs.

The animals perform dual sexual roles during copulation. They give sperm to a mating partner while simultaneously receiving sperm, which they store for later insemination.

Japanese biologists studied copulation between sea slugs that they had captured during scuba dives and held in a tank.

After each coupling, which lasted between dozens of seconds and a few minutes, every slug discarded its penis - a thread-like organ that it projects from its side into a partner's vagina.

The team also examined the microscopic structure and function of the male organs, observing an internal spiral structure they believe grows into a replacement penis.

"We propose that the tissue at the spiral part of the penis is compressed and undifferentiated, gradually differentiating into the 'next penis'.

"It may need approximately a day for the spiral structure to be ready for copulation," the team wrote.

They also wrote in the Royal Society journal Biology Letters : "No other animal is known to repeatedly copulate using such 'disposable penes'."

In another revelation about the sea slug's sex life, the scientists found its penis was covered with spines - and suggested these may be used to remove the sperm of previous partners being held in store by their mate.

The spines are backward-pointing, making it difficult to withdraw the penis after copulation. This may explain the organ's disposable nature.

"Chromodoris reticulata may compensate for the short-term cost of decreased reproductive opportunities caused by the loss of a penis with the reproductive advantage gained by sperm displacement," wrote the study's authors.

Various animals are known to discard parts of their body, such as the gecko which sheds its tail.