Barnacles show that "nature finds a way"

Nature via Scientific American: Barnacles Mate via "Spermcasting"
Oversize penises are not always enough to let these immobile crustaceans mate if the animals live in solitude, so they release sperm into the sea, which allows other barnacles to capture it and thus fertilize eggs
By Daniel Cressey and Nature magazine

It can be hard to find a sexual partner when you are glued to a rock.

Barnacles famously get around this problem by having penises longer than their bodies, so that they can seek out relatively distant mates. But now it seems that some adopt another strategy, entrusting their precious bodily fluids to the currents.

Some of these crustaceans live alone, with no neighbors near enough to have sex with. In the case of the gooseneck-barnacle species Pollicipes polymerus, this presented a mystery: although some barnacles are thought to self-fertilize, scientists have never been able to witness reproduction of solitary P. polymerus, so these animals were thought to have to mate.

But Richard Palmer, a marine biologist at the University of Alberta in Edmonton, Canada, and his team have now shown that the barnacles are capable of capturing sperm released into the sea, which gives them another route to procreation.

In case this article makes for dry reading, try Friday Weird Science: Slutty Sloppy Barnacle Spermcasting at Scientopia.  It has photos and a video.