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.
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."
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