A species of sea slug discards its penis after sex - and then just grows a new one to use the following day.
Like other nudibranchs, Chromodoris reticulata is a simultaneous hermaphrodite. During copulation both partners perform a male and female role, with each partner donating and receiving sperm.
Afterwards the two sea slugs separate and crawl away, effectively dragging their penises behind them - then after around 20 minutes their used organs simply fall off. Even more amazingly, they are able to grow a new one and copulate again within 24 hours.
Researchers from Japan made the eye-opening discovery after studying the breeding behaviour of a group of the sea slugs collected off the coast of Okinawa, in an experiment tank.
Further investigation by the team revealed a tightly coiled spiral structure inside those slugs that hadn't recently copulated - and this was found to be lacking in those individuals that had dropped their penises.
Dr Ayami Sekizawa of Osaka City University told Nature.com: "We think this spiral is effectively a penis-in-waiting that will uncoil rapidly after penis disposal and grow into a new penis."
The organ of C. reticulata is interesting for other reasons. It features spines, possibly to hook out the sperm of rival males that had previously mated with the same breeding partner. These spines also point backwards, meaning it's difficult for the slug to withdraw its penis after mating, and the team say this is a possible reason why the organ is shed afterwards.
The research was published in Biology Letters.
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