I was feeling slightly uninspired about what marine animal to post about today. Since I’ve just got back from Wales where I was teaching rocky shore ecology to undergrads, I thought I’d write a brief article on something a saw every day there – barnacles. You might take some convincing that barnacles are interesting, but they are a very unique and diverse group of animals. Charles Darwin spent 8 years studying and classifying barnacles after returning home from his voyage on the Beagle, after following the suggestion by a friend that he needed to throughly understand at least one group of species before making generalisations in his theory of evolution by natural selection.
Barnacles are crustaceans from the class Cirripedia. This can catch some people out. The hard shell and sessile lifestyle of barnacles put people in mind of molluscs, not crabs or lobsters. There are several different types of barnacles, including the stalked barnacles, which have a long stalk (naturally), and parasitic barnacles that bear little resemblance to other barnacles except at the larval level. The most commonly found barnacle is the acorn barnacle, which I’ll describe in a little more detail.
Acorn barnacles have a free swimming larval stage. When the larvae is ready to settle they cement themselves by their head to a suitable surface, and calcareous plates form around the body. The plates act as a layer of protection for the barnacle, with an operclum at the apex, fro
m which the legs of the barnacle can flick out. Barnacle legs are known as cirri. They are very long and feathery and function in suspension feeding, gathering particles from the water. The photo to the right shows what a barnacle looks like when feeding, with its cirri extended out from its operculum. The most commonly known fact about barnacles is the extremely large length of their penises in comparison to their body size. Barnacles possess the largest penis size to body length ratio in the entire animal kingdom. If barnacles were approximately human in size, it would equate to having something the height of the Empire State Building in your trousers. Why so long? Barnacles are completely sessile and unable to leave their shells, so the penis needs to be extremely long in order to fertilise other nearby individuals.
