... a new study finds that the ostentatious sea slug, or nudibranch, may take the cake—one species of this marine invertebrate cuts its own penis off after mating and regrows a new reproductive organ within 24 hours, whereupon it’s ready to mate again.
Most nudibranchs are simultaneous hermaphrodites, meaning individuals have both male and female reproductive organs—and they can deliver and accept sperm at the same time.
They can also store sperm from several mates and choose which ones to use in fertilizing their eggs by digesting the sperm they don’t want. (Also see pictures: “Fiery Sea Slug Discovered, Lays Lacy Egg Case.”)
... when researchers examined the discarded penises, they found sperm entangled in the backward-facing spines that cover the organ.
Sekizawa speculated that by removing their penis from their mate, the sea slugs were increasing the chances that their partner would use their sperm to fertilize its eggs, rather than a competitor’s sperm.
“If the sea slug left the penis in the mating partner’s female organ, it could not remove sperm of preceding mates,”
More odd mating: sea hare daisy chains. See video # 4