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THE BARNACLE FILE
Which sexual athlete glues its head to a rock and kicks food into its mouth? Paul Naylorknows.
Got some air left at the end of a shore dive? Take a close-up look at a rock covered in barnacles and see their feeding limbs, like grasping hands, flick out and sweep through the water to catch planktonic food. You may also see small blennies trying to munch the limbs before they can be withdrawn.
A barnacle has been described as being like a shrimp that glues its head to a rock, lives in a house and kicks food into its mouth. This is entirely accurate, as the settling young barnacle's head produces a special cement that attaches it permanently to a rock or other hard surface.
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For their size, the humble barnacles are possibly the best-endowed members of the animal kingdom. They are hermaphrodites (male and female simultaneously) but still have to reach a neighbour for internal fertilisation to occur. A long penis does the trick. Barnacles that are isolated from others cannot reproduce.
Barnacles are food for fish that have the necessary chisel-like teeth for removing them. Dog-whelks prey on them intensively, drilling through their plates or forcing open the "lid" in the top of the barnacle "house". Empty "houses" left behind by dead barnacles provide homes for a range of tiny creatures.
Looking rather like limpets or similar shellfish, barnacles were classified as molluscs until the 19th century. Examination of their planktonic larvae revealed that they are crustaceans like crabs, lobsters and shrimps. The usual crustacean armour is modified to form flat plates that make up the adult barnacle's volcano shape.
A few barnacle species have a different lifestyle and live as soft-bodied parasites, forming branching roots within the tissues of crabs. They eventually produce a pale yellow bulge under the crab's tail, distinguishable from an egg mass because it is smooth rather than granular. Parasitised crabs are prevented from moulting and often have heavy encrusting growth on their carapaces.
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