Barnacles are arthropods. That means they are invertebrate animals with an exoskeleton -- a skeleton on the outside of the body. Although they are animals, barnacles are non-mobile, which means they attach themselves to a hard surface and remain there all their lives. At first view, they look more like plants than animals.
What They Look Like
Sea barnacles are tiny things. The largest ones are less than an inch in diameter. They vary in color from dirty pink and off white to a greenish or brownish tint. The color depends on the environment, as evolution has helped barnacles adapt.
Where They Live
Sea barnacles live everywhere, from deep sea areas to coastal shallow waters. Young sea barnacles can swim and move around, so their habitat is somewhat larger. Once they reach adulthood, the barnacles will attach themselves to a rocky surface, where they remain for the rest of their lives. To eat, they stretch their feathery barbed legs out of their shell to catch plankton to eat.
Picking a Spot
Location is everything for sea barnacles. That's because once they've attached to a place, they can't change their minds and move somewhere else. To pick a place, sea barnacles look for others of the same species so they can attach themselves to the same area. The reason is simple; if other barnacles are there and are alive, chances are the place is safe and provides enough food to survive. That's not to say sea barnacles don't have predators -- sea stars, snails and human intervention all threaten barnacles -- but their chances of survival are better in group settings.
Since sea barnacles can't move, reproduction would be tricky -- except for the fact that barnacles are hermaphrodites, which means they have both female and male organs. As a result, each barnacle can reach out to a nearby barnacle for breeding. Once the mating has been completed, the "female" lies eggs, which are held in a sac over the winter. In order for reproduction to take place, the temperature of the water must be low, so it always happens in late fall or early winter.