Bryozoans, also known as moss animals or sea mats, are encrusting colonial animals found throughout the world's oceans. They prefer warm, tropical water. Bryozoans have their own phylum, Bryozoa, which is a member of the superphylum Lophotrochozoa, the lophophorates. What all members of this group have in common is that they use a characteristic horseshoe-shaped, cylinder-shaped, or coiled ciliated feeding organ called a lophophore. Lophophores are used for filter feeding and evolved from a simple ring of cilia around the mouth. Most bryozoans are stationary, though some colonies can creep around, and at least one species is free-floating.
To protect themselves, most bryozoans surround their soft parts with a stony calcium carbonate exoskeleton, like coral does. This skeleton can often be found encrusting mollusk shells found on a beach, and can be scraped off by rubbing the shell with a finger under running water. Some species of bryozoan don't build skeletons, and are instead held together by mucus. Bryozoans are highly colonial, again like coral, to which they are only distantly related, and form colonies up to a few meters across, though a few centimeters across is more typical. The individual members in a bryozoan colony are tiny, usually between 0.5 and 5 mm in size.
Bryozoans are coelomate animals, meaning they have a body cavity, and a simple gut with a mouth and anus. Aside from the lophophore, that's about it. Bryozoans lack circulatory, motor, or respiratory systems, due to their small size and stationary lifestyle. Oxygen diffuses directly into the cells of the animal, because it is so small. They also have an extremely simple nervous system. Bryozoans are found in the fossil record beginning the in Early Ordovician (488 million years ago), but they may have existed earlier, in the Cambrian, but lacked a hard skeleton at this time. They probably evolved from a phoronid-like ancestors. Phoronids are another simple group of lophophorate.
There are about 8,000 living species of bryozoan, with 50 freshwater species and the remainder marine. They are sometimes colorful - blue, brown, purple, or red -- and their colonies can be seen while snorkeling. Bryozoans serve as first-level consumers in the aquatic food pyramid, consuming small bacteria and unicellular organisms and providing food to grazing animals such as sea urchins and fish. They form a prominent part of the post-Cambrian Paleozoic fossil record. Sometimes their skeletons are found in thick sheets in Paleozoic strata, making it difficult to locate fossils of anything else.
