Bryozoa

Bryozoa, meaning "moss animals," is a major phylum of common, mostly marine, filterfeeding invertebrate animals. Although there are about 4,500 described living species, recent research suggests that this number may be an underestimate by roughly a factor of five, because many groups may contain large numbers of yet-to-be detected so-called cryptic species.

All bryozoan species are colonial (see Colo-niality)—that is, composed of asexually budded and physiologically connected sets of units called zooids. These zooids, typically less than 1 mm in their longest dimension, serve the basic structural, feeding, defensive, and reproductive functions of the colony. Although some colonies may contain millions of zooids and grow to be as large as 1 m high or wide, most species are relatively small. Colonies may range in shape from flat, encrusting sheets, to vinelike chains, tree- or bushlike branching forms, spiraling corkscrews, or perforated, lacelike meshes. Because small colonies are frequently inconspicuous, and other, more conspicuous colonies are commonly mistaken for small corals or seaweeds by nonzoologists, the Bryozoa remain less well known by the public than other major invertebrate phyla. They may be most noticed as "fouling organisms"

when they grow on the undersides of boats, floats, docks, and water intake pipes.

Bryozoans' box- or tubelike skeletons may be completely or partially mineralized with calcium carbonate, or they may be nonmin-eralized. Depending upon the degree of mineralization, species may be either rigid, semirigid, or completely flexible.

Most colonies are sessile and cemented to their rock, shell, wood, or algal substrates, but a few species form free-living, cone-shaped colonies that can slowly move across sandy bottoms. A few freshwater species can also creep slowly across their substrates. Mineralized bryozoan skeletons are well preserved in the fossil record.

As filter-feeders (that is, animals that remove bacteria, nutrients, and other small particles from water), bryozoans play an ecosystem role in capturing water-borne productivity or nutri-ents—mostly floating phytoplankton and small detrital particles—and converting them to benthic productivity. Some colonies grow large and dense enough to be dominant structural elements in some local ecosystems as well, providing habitats for other species.

Along with Brachiopods and Phoronids, bryozoans are frequently classified into the Lophophorata superphylum. Bryozoa are also sometimes called Ectoprocta, to differentiate "true" (as currently defined) bryozoans from other groups (for example, Entoprocta or Kamp-tozoa) that have been mistakenly lumped with them by some specialists in the past. The phylum is divided into three classes: the Phylac-

tolaemata, the Stenolaemata, and the Gymno-laemata. Phylactolaemates are found exclusively in freshwater habitats. Although they are remarkably widespread across the world, they currently number approximately only fifty species, though some researchers expect numerous cryptic species to be found in this class as well as the others. Stenolaemates, once quite speciose, widespread, and ecologically dominant in shallow seas during earlier geological periods, remain diverse and dominant only in certain ecological refuges, such as dark crevices and caves. Gymnolaemates include the vast majority of living bryozoans, having diversified and gradually replaced the stenolaemates since their apparent origin in the Ordovician.

—Daniel Brumbaugh

See also: Evolutionary Biodiversity; Freshwater;

Oceans

Bibliography

McKinney, F. K., and J. B. C. Jackson. 1989. Bryozoan

Evolution. Chicago: University of Chicago Press;

Ryland, J. S. 1970. Bryozoans. London: Hutchinson;

Biology of Bryozoans. New York: Academic.

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