Barnacle

The barnacle is a hardy animal that is found in or very closely to sea water. Although it is frequently confused for a mollusc because of its hard outer shell, it is actually a crustacean, closely related to crabs and lobsters.

Barnacles are most often seen as roughly circular sessile invertebrates( which means that they cannot move on their own), and are permanently attaches to the substrate they live on. In their juvenile form they are free-floating, but eventually they attach themselves to any nearby rock, shell, or other object and stay there for the rest of their lives. Their shells are composed of calcite.

Barnacles are often seen on crabs, whales, boats , rocks and on the shells of sea turtles. Although some species of barnacle are parasitic, most barnacle species are harmless, because they are filter feeders and do not interfere with an animal's normal diet and do not harm that animal that they live on in any way. Many species of barnacle are so harmless that in fact, an animal that is covered in them, may not even notice !
Barnacles are filter feeders (also known as suspension feeders) that feed on food particles that they strain out of the water. The shell of the barnacle is made up of a number of plates , with feathery leg-like appendages that draw water into their shell so that they can feed.