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.
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.
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.
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.
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