Detachable penises

Detachable penises are the way to go.
The sea slug Chromodoris reticulata can
dispose of its penis after mating and spare
"penis tissue" can extend to take the place
of the penis during the next mating.
Researchers aren't completely sure why a
disposable penis is useful, but they have
some ideas. First, the detachable penis has
backwards facing barbs, and when
researchers examined the detached penis,
the barbs held sperm. Researchers think
that the barbs may act as a way of
removing rival sea slug sperm, and once
its removed, the sea slug disposes of the
penis. A second idea suggests that the
slug may need to get away quickly if there
is a predator, so being able to detach from
the penis could save the sea slug's life
while not entirely wasting the mating