Bryozoa (Ectoprocta: ‘Moss’ Animals)

Abstract The Bryozoa are a phylum of small, colonial, filter‐feeding, almost exclusively sessile, aquatic organisms. Approximately 5600 living species have been described, and the fossil record comprises approximately 20 000 species. Bryozoans are found in all oceans and many freshwater areas. Most species encrust stones or algae, others form erect bushes and a few form free‐living colonies. The individuals in the colonies are almost always smaller than 1 mm, but colonies of freshwater species may grow to 0.5 m in diameter, and some of the marine species form tufts up to more than 25 cm in length. Each individual (zooid) in a colony consists of rather stiff, often calcified, main body wall (cystid) and a tentacle crown plus gut (polypide), which can be retracted into the cystid. Colonies are hermaphroditic, in some groups with hermaphroditic zooids, but a number of groups have separate male and female zooids. Development is indirect with triangular shelled cyphonautes larvae in a few groups, but brood protection and lecithotrophic larvae of many types are more widespread. Key Concepts: Bryozoans are colonial aquatic organisms most often growing on stones, shells or algae. Each individual (zooid) in a colony consists of a more or less stiff main body wall (cystid) and a retractable tentacle crown around the mouth and a gut (polypide), which can be retracted into the cystid. The almost cylindrical tentacles have bands of cilia which create a water current towards the mouth and between the tentacles. Food particles are strained from this water current. Colonies are hermaphroditic; many species show hermaphroditic zooids, but some show male and female zooids. Most species brood the embryos in more or less elaborate brooding structures, called ovicels in the cheilostomes; cyclostomes have large female zooids, which are brood chambers with a rudimentary polypide. The Bryozoa can be classified as follows: Bryozoa (Phylactolaemata+Gymnolaemata (Eurystomata (Cheilostomata+Ctenostomata)+Cyclostomata)).