by M E Skeel

Created on: December 20, 2008   Last Updated: May 23, 2009

Barnacles are so strange that they were misidentified as molluscs by taxonomists until 1830. After all, they had a shell and live a sessile life, so surely they were related to snails and clams? Their larvae finally gave them away. Barnacle larvae are free-living in the plankton and have jointed appendages and other arthropod characteristics, which reveal their Crustacean natures. But these are no ordinary prawn or shrimp larvae, destined to grow into active crawling or swimming lifestyles. Instead barnacle larvae leave their wandering ways behind, settle on their heads on some hard surface and turn into wannabe molluscs. The eyes and brain disappear. Shells are formed around the body and the former swimming antennules do the work of tentacles, waving about in the water, picking up microscopic food particles and feeding them to the mouth.

This may seem like a strange life choice but it has been successful for the barnacle clan. Clad in solid plates, adult barnacle are safe from predators and can concentrate on the two things they do best: eating and reproducing. After that, lifesyles are mostly related to substrate choice. Some barnacles settle on inanimate surfaces such as rocks, or on human-made surfaces such as boat hulls and piers. If they can't find a lifeless surface, they settle on other organisms such as oysters, and often smother them, creating the desired lifeless hard substrate that the barnacles need to mature. At this point barnacles develop into one of two basic forms: those with a stalk or peduncle and those without. Stalked barnacles get some advantages in height and maneuverability but may be a little more vulnerable to predation without the harder shells of the stolid, stalkless forms.

Having a stalk is useful for sex also. Barnacles are monoecious, with both sexes are found in a single individual. This enables them to self-fertilize if there are no other barnacles in reach. If there are, (as there usually are because barnacles quickly fill up available hard spaces), then orgies occur when the sexually mature hermaphrodites take turns fertilising each other. Barnacles have penises which they insert into the ovaries of neighbouring barnacles. This internal fertilisation technique is useful because it increases fertilisation rates considerably as well as probably being more fun for the barnacles in what otherwise would be a rather boring existence.

The female half of the barnacle then keeps the eggs safe until they hatch into

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