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A sea slug that is able to detach, re-grow and then re-use its penis has surprised scientists.

Japanese researchers observed the bizarre mating behaviour in a species called Chromodoris reticulata, which is found in the Pacific Ocean.

They believe this is the first creature known that can repeatedly copulate with what they describe as a "disposable penis".

The in the Royal Society's journal Biology Letters.

Male and female

The sex life of the sea slug is complicated even before detachable organs come into play.

Almost all of these creatures, which are also known as nudibranchs, are thought to be "simultaneous hermaphrodites". This means they have both male and female sexual organs and can use them both at the same time.

Bernard Picton, curator of marine invertebrates at the National Museums Northern Ireland, explained: "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.

"They are both donating sperm to the other one."

But the finding that one species has added another layer of complexity to copulation surprised the sea slug expert.

"I haven't seen anything like this before," he said.

Sexual healing

The Japanese team observed sea slugs that they had collected from shallow coral reefs around Japan. They saw the animals mate 31 times.

The act took between a few seconds and a few minutes, after which the creatures would push away and shed their penises, leaving them on the floor of the tank.

However, the researchers were surprised to discover that just 24-hours later, the sea slugs had regenerated their male organs and were able to mate again.

Closer examination of the animals' anatomy revealed that the sea slugs had a large part of their penis coiled up in a spiral inside their bodies, which they would then use to replenish their missing part.

The scientists also noted that the penis was equipped with spines.

At most, the animals were able to copulate three times in succession, with each bout separated by about 24 hours.

It was not clear once the internal penis was used up after these attempts whether the animal's sex life - at least the male part of it - was effectively over or - after a few weeks or months - the organ could regrow.

No great loss

Sea slugs are not the only animals who abandon their penis.

Orb weaving spiders are known to lose their male organs after sex, as does a sea creature called the periwinkle and land slugs belonging to the genus Ariolimax.

However the researchers believe that Chromodoris reticulata is the first creature known that can re-grow its appendage - and its disposable penis gives it a sexual advantage.

The Japanese team says that in the first act of copulation the penis may be used to remove any sperm left by any competitors that its partner has mated with.

With the first penis and the rival sperm then abandoned, the second penis can be used to inject the sea slug with another dose of its own sperm, ensuring that their genes are the ones that are passed on.

Mr Picton said: "They do have very, very complicated biology - and a lot have awfully complicated things in terms of reproduction."