Bryozoans are colonial animals with a horseshoe-shaped feeding organ called the lophophore. They have a stony exoskeleton and lack circulatory, motor, or respiratory systems. There are about 8,000 living species, mostly marine, and they serve as top-level consumers in the aquatic food pyramid. They have been found in the fossil record since the early Ordovician.
Bryozoans, also known as moss animals or sea mats, are encrusting colonial animals found in oceans around the world. 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 distinctive horseshoe-shaped, cylinder-shaped, or spiral-shaped ciliated feeding organ called the lophophore. Lophophores are used for filter feeding and evolved from a simple ring of cilia around the mouth. Most bryozoans are stationary, although some colonies may crawl around and at least one species is free-floating.
To protect themselves, most bryozoans surround their soft parts with a stony exoskeleton of calcium carbonate, as coral does. This skeleton can often be found encrusted with mollusc shells found on a beach and can be scraped off by rubbing the shell with a finger under running water. Some bryozoan species do not 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 in diameter, although a few centimeters are more typical. Individual members in a bryozoan colony are tiny, usually between 0.5 and 5 mm.
Bryozoans are coelomous animals, meaning they have a body cavity and a simple intestine with a mouth and anus. Apart from the lofoforo, that’s it. Bryozoans lack circulatory, motor, or respiratory systems, due to their small size and stationary lifestyle. The oxygen diffuses directly into the animal’s cells because it is so small. They also have an extremely simple nervous system. Bryozoans have been found in the fossil record since the early Ordovician (488 million years ago), but may have existed earlier, in the Cambrian, but lacked a hard skeleton at that time. They probably evolved from phoronid-like ancestors. The phoronids are another simple group of lophophorates.
There are about 8,000 living species of bryozoans, with 50 freshwater species and the rest marine. They are sometimes colorful – blue, brown, purple or red – and their colonies can be seen while snorkelling. Bryozoans serve as top-level consumers in the aquatic food pyramid, consuming small bacteria and single-celled organisms and providing food for grazing animals such as sea urchins and fish. They form an important part of the post-Cambrian Paleozoic fossil record. Sometimes their skeletons are found in thick layers in Paleozoic strata, making it difficult to locate fossils of anything else.
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