Add this to the huge list of strangest animal mating habits we've posted on Neatorama a while back: the hermaphrodite Chromodoris reticulata sea slug has a disposable penis that snaps off every time it finishes mating. (But don't fret, Neatoramanauts, it has got spare penises)
Like almost all nudibranchs, C.reticulata is a hermaphrodite. Every individual is both male and female. When they have sex, both partners sidle up to each other (always on the right side) and penetrate each other at the same time. From afar it looks like a single white thread unites them. Zoom in closer, and you can make out two distinct parallel penises. They fill each other’s vaginas with sperm for around ten minutes.
Ayami Sekizawa from Osaka City University watched the mating slugs, she noticed something odd. After they finished mating, they would crawl away with their penises still extruded from their bodies. Twenty minutes later, these trailing genitals broke off completely.
That ought to be a fairly ruinous end to the slug’s sex life, but after a day, C.reticulata is ready to mate again. Its secret is a penis that’s much longer than it lets on. When it mates, it extrudes around a centimetre of penis, but the full organ is three centimetres long. The rest remains coiled up inside the animal. Each segment may be disposable, but C.reticulata has three of them.
Ed Yong of Not Exactly Rocket Science has the story: Link