For many species of terrestrial slugs, sex can end with a loss of genitals. It’s a process called Apophallation. Slugs are hermaphrodites, so they are capable of fertilizing themselves, but they also have the option to mate with a partner. When they do mate, their penises that emerge from the genital opening in their heads, and proceed to intertwine in a corkscrew-like fashion over several hours. Once the sperm transfer is complete, the slugs will attempt to free their phalluses. When this becomes unsuccessful, one of the slugs may bite its partner’s penis off.
Bitten slugs do not lose their penis entirely. They just go from euphallic – having a fully functional penis to hemiphallic – having a truncated and nonfunctional penis. Apophallation is actually relatively common for slugs, and victims can still partake in sexual activities. Hemiphallic slugs, however, are no longer capable of fertilizing themselves.