A phylum of small, aquatic animals that typically form colonies comprising a few to a million individuals (zooids); each zooid typically has a tentacle-like feeding apparatus around its mouth; colony forms a calcareous, chitinous, or gelatinous skeleton; c.4000 living species, mostly marine, and attached to hard substrates or seaweeds, rarely to soft sediments; some found in fresh water; abundant as fossils; also known as moss animals.
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"Bryozoa", from Ernst Haeckel's Kunstformen der Natur, 1904 |
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Stenolaemata |
Bryozoans are tiny colonial animals that generally build stony skeletons of calcium carbonate, superficially similar to coral.
Ecology
Most species of Bryozoan live in marine environments, though there are about 50 species which inhabit freshwater. In their aquatic habitats, bryozoans may be found on all types of hard substrates: sand grains, rocks, shells, wood, blades of kelp, pipes and ships may be heavily encrusted with bryozoans. Some bryozoan colonies, however, do not grow on solid substrates, but form colonies on sediment. While some species have been found at depths of 8,200 m, most bryozoans inhabit much shallower water. Most bryozoans are sessile and immobile, but a few colonies are able to creep about, and a few species of non-colonial bryozoans live and move about in the spaces between sand grains.
Bryozoans are also colony-forming animals. The colonies range from millimeters to meters in size, but the individuals that make up the colonies are tiny, usually less than a millimeter long. Some individuals are devoted to strengthening the colony (kenozooids), and still others to cleaning the colony (vibracula).
Anatomy
Bryozoan skeletons grow in a variety of shapes and patterns: mound-shaped, lacy fans, branching twigs, and even corkscrew-shaped.
The tentacles of the bryozoans are ciliated, and the beating of the cilia creates a powerful current of water which drives water together with entrained food particles (mainly phytoplankton) towards the mouth. In many bryozoans only the zooids within a few generations of the growing edge are in an actively feeding state;
Because of their small size, bryozoans have no need of a blood system.
Bryozoans can reproduce both sexually and asexually. Asexual reproduction occurs by budding off new zooids as the colony grows, and is the main way by which a colony expands in size. If a piece of a bryozoan colony breaks off, the piece can continue to grow and will form a new colony.
One species of bryozoan, Bugula neritina, is of current interest as a source of cytotoxic chemicals, bryostatins, under clinical investigation as anti-cancer agents.
Fossils
Fossil bryozoans are found in rocks beginning in the early Ordovician. They were often major components of Ordovician seabed communities and, like modern-day bryozoans, played an important role in sediment stabilization and binding, as well as providing sources of food for other benthic organisms. During the Mississippian (354 to 323 million years ago) bryozoans were so common that their broken skeletons form entire limestone beds.
Most fossil bryozoans have mineralized skeletons.
With regard to the bryozoan groups lacking mineralized skeletons, the statoblasts of freshwater phylactolaemates have been recorded as far back as the Permian, and the ctenostome fossils date only from the Triassic.
One of the most important events during bryozoan evolution was the acquisition of a calcareous skeleton and the related change in the mechanism of tentacle protrusion.
Classification
The Bryozoans were formerly considered to contain two subgroups: the Ectoprocta and the Entoprocta, based on the similar bodyplans and mode of life of these two groups.
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