Bryozoa (Ectoprocta: ‘Moss’ Animals)


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)).

Keywords: colonial; zooid; cystid; polypide; cyclostomes; eurystomes; phylactolaemates; cyphonautes larva

Figure 1.

Part of a colony of the cheilostomate bryozoan Membranipora membranacea on a kelp frond; the white line along the upper part of the colony is the growing edge.

Figure 2.

Spirally coiled colonies of the cheilostomate bryozoan Bugula plumosa. Photo courtesy of Groeneveld R (

Figure 3.

Protruded polypides of the cheilostomate bryozoan M. tuberculata. The colony has been cut along one row of zooids, so that the polypides are seen in profile. Courtesy of Zimmer RL, University of Southern California.

Figure 4.

Diagram of two zooids in a cheilostome colony. The right zooid has protruded the polypide by contracting the parietal muscles, whereas the left zooid has retracted the polypide. The left zooid has induced the right zooid to produce an ovicel, and the left zooid has then deposited a large egg, which is protected by the operculum. The polypide is orange and the coelom blue. Reproduced with the permission of The Natural History Museum of Denmark (University of Copenhagen).

Figure 5.

Diagrams of a cyclostome zooid with protruded (left) and retracted (right) polypide. The polypide is orange and the coelom is blue. The protruded zooid shows the contracted basal part of the membranous sac. Reproduced with the permission of The Natural History Museum (University of Copenhagen).

Figure 6.

Cyphonautes larva. Courtesy of Slotwinski A, University of Tasmania.



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Mukai H, Terakado K and Reed CG (1997) Bryozoa. In: Harrison FW (ed.) Microscopic Anatomy of Invertebrates, vol. 13, pp. 45–206. New York: Wiley‐Liss.

Nielsen C (1970) On metamorphosis and ancestrula formation in cyclostomatous bryozoans. Ophelia 7: 217–256.

Nielsen C (2012) Animal Evolution. Interrelationships of the Living Phyla, 3rd edn. Oxford: Oxford University Press.

Nielsen C and Riisgård HU (1998) Tentacle structure and filter‐feeding in Crisia eburnea and other cyclostomatous bryozoans, with a review of upstream‐collecting mechanisms. Marine. Ecology Progress Series 168: 163–186.

Nielsen C and Worsaae K (2010) Structure and occurrence of cyphonautes larvae (Bryozoa, Ectoprocta). Journal of Morphology 271: 1094–1109.

Riisgård HU and Manríquez P (1997) Filter‐feeding in fifteen marine ectoprocts (Bryozoa): particle capture and water pumping. Marine Ecology Progress Series 154: 223–239.

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Further Reading

Hayward PJ (1985) Ctenostome Bryozoans. In: Synopses of the British Fauna (New Series), vol. 33, pp. 1–168. London: Brill/Backhuys.

Hayward PJ and Ryland JS (1985) Cyclostome Bryozoans. In: Synopses of the British Fauna (New Series), vol. 34, pp. 1–147. London: Brill/Backhuys.

Hayward PJ and Ryland JS (1995) Handbook of the Marine Fauna of North‐West Europe, pp. 629–661. Oxford: Oxford University Press.

Hayward PJ and Ryland JS (1998) Cheilosomatous Bryozoans, Part 1. Aeteoidea–Cribrilinoidea. In: Synopses of the British Fauna (New Series), 2nd edn, vol. 10, pp. 1–366. Shrewsbury: Field Studies Council.

Hayward PJ and Ryland JS (1999) Cheilostomatous Bryozoans, Part 2. Hippothooidea–Celleporoidea. In: Synopses of the British Fauna (New Series), 2nd edn, vol. 14, pp. 1–416. Shrewsbury: Field Studies Council.

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How to Cite close
Claus, Nielsen(Mar 2013) Bryozoa (Ectoprocta: ‘Moss’ Animals). In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001613.pub2]