Bryozoa are a class of simple, aquatic invertebrate creatures, nearly all living in stationary colonies. The majority of marine bryozoans are located in tropical areas, however a few can also be found in oceanic trenches and polar regions. Colonies come in a variety of shapes and sizes, such as fans, bushes, and sheets. Single creatures, known as zooids, roam the colony but are not totally independent. These folks can have unique and different functions. Autozooids are found in all colonies and are responsible for feeding, excretion, and delivering nutrients to the colony via various channels. Some classes have specific zooids such hatcheries for fertilised eggs, colonial defence structures, and root-like attachment structures. Cheilostomata is the most diversified bryozoan order, presumably because its members have the most diverse zooids. They have mineralized exoskeletons and create single-layered sheets which encrust over surfaces, and certain colonies can move very slowly by employing spiky protective zooids as legs. Marine species are numerous in coral reefs, although they rarely constitute a considerable fraction of overall biomass. The skeletons of deceased colonies constitute a prominent component of shell gravels in temperate waters, while living colonies are plentiful in these locations. Marine bryozoans are typically responsible for biofouling on ships' hulls, on docks and marinas, and on offshore buildings. They are among the first to occupy newly constructed or recently cleaned structures. Freshwater species cause minor problems in water pipes, drinking water purification equipment, sewage treatment facilities, and power plant cooling pipes.