Barnacles are crustaceans that attach themselves to hard surfaces, including other marine animals such as crabs, lobsters, and whales. They are encrusters, attaching themselves temporarily to a hard substrate or a symbiont such as a crab or a lobster. Barnacles are suspension feeders, and they draw plankton and detritus into their shell for consumption using their feathery appendages.

When barnacles attach themselves to a lobster, they can be seen as white, hard, volcano-like structures on the lobsters shell. These barnacles are not harmful to the lobster, but they can be considered a form of parasitism since the barnacle gains a surface to attach to and one that is usefully mobile for defense and food collection, while the lobster loses by having to carry the barnacles and appears to have no benefit.