Losing your penis after sex would be most men's worst nightmare. Fortunately for one particular sea slug, it can grow its detachable penis back.
Researchers in Japan have found that the nudibranch Chromodoris reticulata loses its penis after copulation only to regrow the appendage so it can mate again 24-hours later.
Their findings have been published in the journal Biological Letters.
Nudibranches are simultaneous hermaphrodites, meaning they have both male and female organs and fertilise others while they themselves are being fertilised.
The researchers, led by Ayami Sekizawa, a biologist at Osaka City University, collected 31 pairs of Chromodoris reticulate from coral reefs off Okinawa, Japan.
The pairs were placed in tanks and observed as they mated. Mates simultaneously penetrate each other with their penises, which are covered in backwards-facing spines.
Sex can last tens of seconds to a few minutes. At most, the animals could have intercourse three times in quick succession.
Once the act was complete, the Chromodoris reticulate would shed their penises, which dropped to the floor of the tank.
Within 24-hours the sea slug was ready to go again.
The researchers discovered the creatures' penis is compressed and spiralled internally. They hypothesised that the spiral portion of the penis forms more cells that will eventually grow into the next penis.
It is not known whether the creature's ability to inseminate others is finished once the coiled penis is used up, or whether the organ can regrow in the future.
Other creatures, such as orb weaver spiders, the periwinkle and some land slugs, lose their penises after sex, however the Chromodoris reticulata is the only one known to be able to regrow the organ.
The scientists suggest the detachable penis may be used to remove sperm from a mating partner that may been left by a rival. The researchers said DNA testing is needed to determine whether sperm found on the spines of the penis is that of a rival, or whether it is excess sperm pushed out by the recipient.
"Chromodoris reticulata may compensate for the short-term cost of decreased reproductive opportunities caused by the loss of a penis with the reproductive advantage gained by sperm displacement under severe sperm competition," the team conclude.