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Scientists are surprised by the discovery of the mating practices of a certain kind of nudibranch, a type of sea slug. It appears that the Chromodoris reticulata, found in the Pacific Ocean, has a disposable penis. After sex, it simply discards one and, 24 hours later, grows another one that is ready for action.

Nature reports that the nudibranch is a simultaneous hemaphrodite, which means that it has both male and female sex organs that it can use at the same time. As Bernard Picton, from the National Museums Northern Island, explained to the BBC,  "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."

However, the mating practices of C. reticulata are special. In order to observe them, a team of researchers from Nihon University, Osaka City University and the University of the Ryukyus, all in Japan, went on a scuba diving expedition in order to retrieve some of the sea slugs. Then the sea slugs were placed in pairs in different aquaria. Some of the sea slugs had been in isolation for the past 24 hours.

The researchers quickly noticed something unusual. The sea slugs were often incapable of mating for 24 hours. Before that period had elapsed, the sea slugs would simply take the female role or avoid mating in its entirety. Upon closer inspection, they realized that some of the sea slugs unplugged their penises from their partners and dragged them away; 20 minutes later, the penises were discarded.

According to Smithsonian magazine, the explanation lies in the animal's vas deferens, the internal coil that is used to propel sperm out of the body. After the penis is discarded, the vas deferens extends outward, forming a new penis. It is not clear what occurs when the coiled vas deferens runs out, or if that even occurs.

Researchers theorize that the disposable penis evolved because of competition. The tip of the sea slug's penis is covered with tiny barbs, removing the semen of the previous suitor who deposited sperm in their partner's vagina. That enhances the chance that the sea slug's sperm will be used for reproduction. However, if the slug holds onto the penis, it runs the risk of depositing a competitor's sperm into a partner - so it is simpler to just get rid of the penis altogether.

The study was published in the journal Biology Letters.