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. "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". The human thumb-sized slug is an hermaphrodite, meaning it has both male and female sexual organs. They give sperm to a mating partner while simultaneously receiving sperm, which they store for later insemination. 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. 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 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.