Found in the Pacific Ocean, the Chromodoris reticulata (not pictured) are believed to be the first known organism with a "disposable" penis, reports Nature.com.
It was previously known that the species had both male and female sexual organs, allowing them to simultaneously give and receive sperm.
However, when scientists began observing the 108 pairs of slugs in Okinawa, they realised that the creatures would only have sex once every 24 hours.
After having sex, the slugs would leave behind the used penis on the floor of the tank before taking a day to regenerate it.
Scientists also noted that the penis had backwards-facing spines on them.
The research has been published on the Royal Society's Biology Letters.