Japanese researchers just published a paper in the Royal Society journal Biology Letters titled "Disposable penis and its replenishment in a simultaneous hermaphrodite." Slugs typically have both male and female sex organs, and mate by coming together such that each male and female organ aligns with the other slug's opposites, donating sperm to one another.
Other species shed the male sex organ after copulation, however researchers believe that Chromodoris reticulata is the first creature known that can re-grow its male appendage.
The Japanese team says that in the first act of copulation the penis may be used to remove any sperm left by any competitors that its partner has mated with, giving a distinct sexual advantage. After shedding the first penis and the competing sperm, the second penis can be used to inject the sea slug with another dose of its own sperm, ensuring that their genes are the ones that are passed on.
Examination of the animals' anatomy revealed that the sea slugs had a large portion of their penis coiled up in a spiral inside their bodies, which they would then use to regrow a new appendage. The scientists also noted the penis was equipped with spines.