Barnacles are hermaphrodites which, since they have the sex organs of both male and female, means they can reproduce without the help of a friend. However, barnacles prefer mating with other barnacles rather than inseminating themselves, which poses a problem as they don't get out much, spending their entire lives anchored to the same spot.
To get around this problem, barnacles have evolved very long penises - in fact their penis is about eight times as long as their body, the longest penis-body ratio in the animal world.
In 2008, Christopher Neufeld and Richard Palmer, of the University of Alberta, published research showing that barnacles can change the width and length of their penises to adapt to sea conditions. Their experiments involved transplanting barnacles from rough sea to calmer waters and vice versa to see if their penises changed shape.
A major difficulty of the research, as Neufeld put it, was that "It's hard to get barnacles to extend their penises on demand in the lab". To get round this problem, they artificially inflated the barnacles' genitalia with seawater using a custom-made penis pump built out of tubes and hypodermic syringes.
The results showed that barnacles living in rough waters have shorter, stouter penises that are better suited to withstand strong waves.