In the ocean ecosystem, competition for food and living space is often very intense, and some of the adaptations of marine organisms for carving out a spot to live on, or in, the sea floor are remarkable. One very special adaptation is bioerosion, or the biological process of excavating holes in rocks or shells. A wide spectrum of organisms, ranging from bacteria to sponges to barnacles, are able to bioerode quite effectively. Some animals, such as parrot fish, sea urchins, and chitons (polyplacophoran mollusks), bioerode by scraping the surface of rocks or shells in order to eat encrusting algae. Others, such as certain groups of sponges, worms, and bivalves (pelecypods), bioerode by creating holes (borings) in hard substrates, which they occupy permanently.