The common barnacle is a small crustacean that lives in the "upper zone", an area of the beach that the water reaches during high tide. They are arthropods and have a close kinship to crabs, prawns, and lobsters, and in a more general way, insects and spiders.
A barnacle is an incredibly interesting sea creature. At first glance, it looks deceptively like a mollusk, and like mollusks, they attach themselves to rocky surfaces. They can usually be found affixed to boats, piers, and any other uneven surface that may be in contact with shallow water at regular intervals. Another remarkable trait of the barnacle is that it is hermaphroditic, containing both male and female reproductive organs.
Barnacles reproduce by fertilizing the eggs of its neighbors. The fertilized eggs remain within its parent's shell until they develop into nauplius larvae. There can be over 10,000 naupli within one parent's shell.
When the naupli are ready, they are released into the ocean as plankton. The nauplius will then develop through various molt stages until it reaches another larval stage and becomes a cyprid, where it develops a carapace or a kind of shell.
As a cyprid, this complex marine animal will cease to feed, choosing instead to simply wait for the right time to attach itself to an acceptable surface. When this is done, the barnacle will molt and rotate, making sure that its appendages face forward, which will gradually change into feathery cirripeds. These long limbs will act as a kind of plankton sweeper, which is how it gathers its food. Once the barnacle reaches this stage in its development, it will have graduated into adulthood. The barnacle will then live the rest of its life attached to one spot and will not be able to move or swim about. In short, it is now at the mercy of the elements.
Some kinds of barnacles are the rock barnacle, honeycomb barnacle, six-plated barnacle, and rosette barnacle.