Image caption All sea slugs have complicated mating behaviour, but Chromodoris reticulata has surprised scientists with its ability to lose and then re-grow its penis 
 
 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 study is published 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. 
 
 I haven't seen anything like this before Bernard Picton, National Museums Northern Ireland 
 
 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 
 
 Image caption The Japanese team of scientists observed the animals as they mated 
 
 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. 
 
 They do have very very complicated biology Bernard Picton, National Museums Northern Ireland 
 
 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." ||||| Sea animal has grow-again penis 
 
 PARIS — Scientists reported Wednesday on the bizarre sex life of a sea slug that discards its penis after copulation. Then grows a new one. 
 
 "No other animal is known to repeatedly copulate using such 'disposable penes'," Japanese biologists wrote in the Royal Society journal Biology Letters, describing the behaviour as "extremely peculiar". 
 
 Dubbed Chromodoris reticulata in Latin, the red-and-white slug -- technically a shell-less mollusc -- inhabits warms waters in Southeast Asia. 
 
 The critter needs 24 hours between couplings to unroll an internally coiled and compressed juvenile penis to replace the used, external bit, the scientists found. 
 
 It can repeat this feat at least three times. 
 
 The human thumb-sized slug is 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. 
 
 The team observed 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'," the team wrote. 
 
 "It may need approximately a day for the spiral structure to be ready for copulation." 
 
 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 authors. 
 
 Various animals are known to discard parts of their body, such as the gecko which sheds its tail. 
 
 Few, though, are willing to part with their penis, the team noted with clinical understatement. 
 
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– The Chromodoris reticulata has a peculiar way of mating: After the deed is done, the sea slug sheds its penis ... and then grows a new one within 24 hours and does the whole thing again. "I haven't seen anything like this before," says one expert. A team of Japanese researchers discovered the surprising behavior in the species, which lives in the Pacific Ocean, the BBC reports. Chromodoris reticulata is the first creature known to have what they're calling a "disposable penis." Other creatures discard their sex organ post-copulation, but they don't re-grow it. The researchers called the slug's behavior "extremely peculiar." A large part of the slug's penis is actually an internal one that's coiled inside its body; this is what regenerates the detached portion after sex. The studied slugs were seen to mate three times in a row, 24 hours apart each time; it's not clear if the penis could regenerate once again, perhaps months later. The evolutionary purpose? Scientists aren't quite sure, but they say the purpose of the first sex act may be to remove any sperm left by a previous partner. The disposable penis is far from the slugs' only sexual quirk: The penises are equipped with spines that could help with the removal of prior sperm, AFP reports. The creatures are also "simultaneous hermaphrodites," which means they have male and female sexual organs that can be used simultaneously.