The wild world of barnacle penises

A fun way to round up National Marine Week! A fact that is always fun to tell people while leading a rockpooling session is that barnacles have the longest penis in relation to body size in the entire animal kingdom. 

Most barnacles are hermaphrodites, meaning they can act as either males or females. However, they can’t self-fertilise, so they must find other barnacles to reproduce with. As adults, barnacles live permanently glued to hard surfaces unable to move, making it difficult for them to find a mate. To overcome this challenge, they have evolved a penis that can stretch out to find other barnacles to fertilise. These extra long penises are covered in bristles which can detect chemical signals from neighbouring receptive partners. Once found, the penis will stretch out and deposit their sperm into the other barnacle, where fertilised embryos are then held in egg sacs beneath their exoskeleton.

After mating, the penis degenerates and is then re-grown the following summer ready to do it all again.

Environmental factors affect the size and shape of a barnacle penis. It has been found that in calm water, they grow longer, more flexible penises, while in rougher water, they develop stockier, stronger penises to withstand the choppy conditions.

So now you know....