The sea slug, Chromodoris reticulata, is a type of soft-bodied marine mollusk, and its disposable penis is very rare in the animal kingdom.
As for what happens after sex ... "The penis just falls off," lead author Ayami Sekizawa told Discovery News.
Sekizawa, a researcher at Osaka City University, and colleagues made the discovery after studying C. reticulata individuals that they collected during scuba diving trips in shallow coral reef areas near Okinawa, Japan.
The researchers set up an experiment tank and watched as the sea slugs copulated 31 times.
These animals are "simultaneous hermaphrodites," meaning each performs both the "male role" of donating sperm to a mating partner and the "female role" of receiving sperm from the partner simultaneously during copulation.
A typical mating episode involves two individuals touching each other with their genital orifices. They then "project" their penises and each insert them into the other’s vagina and start copulation. After a short time, one removes its penis from the partner. Later, the other mate removes its penis too.
Both individuals then crawl away, with their elongated penises still dangling. The sexual organs, which feature backward-pointed spines for possibly trapping rival sperm, would then suddenly sever from their bodies and float away.