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A phylum of sessile aquatic invertebrates (also called Polyzoa) which form colonies of zooids. Each zooid, in its basic form, has a lophophore of ciliated tentacles situated distally on an introvert, a looped gut with the mouth inside the lophophore and the anus outside, a coelomic body cavity, and (commonly) a protective exoskeleton. The colonies are variable in size and habit. Some are known as lace corals and others as sea mats, but the only general name is bryozoans (sea mosses).
The colony may be minute, of not more than a single feeding zooid and its immediate buds, or substantial, forming masses 3 ft (1 m) in circumference, festoons 1.6 ft (0.5 m) in length, or patches 2.7 ft2 (0.25 m2) in area. Commonly the colonies form incrustations not more than a few square centimeters in area, small twiggy bushes up to about 1.2 in. (3 cm) in height, or soft masses up to about 0.3 ft (0.1 m) in the largest dimension. In many colonies much of the bulk consists of the zooid exoskeletons which may persist long after the death of the organism and account for the abundance of fossilized bryozoan remains.
Many bryozoans display polymorphism, having certain zooids adapted in particular ways to perform specialized functions, such as protection, cleaning the surface, anchoring the colony, or sheltering the embryo. The evolution of nonfeeding polymorphs is dependent upon some form of intercommunication between zooids.
Bryozoa is the name of a phylum for which Ectoprocta is generally regarded as a synonym, these names being used by zoologists according to personal preference. Entoprocta (synonym Callyssozoa) is likewise regarded as an independent phylum. A minority regard Ectoprocta and Entoprocta as subphyla within the Bryozoa, while others maintain Ectoprocta and Entoprocta as phyla but link them under Bryozoa as a name of convenience.
The phylum contains some 20,000 described species, one-fifth of them living. These are distributed among three classes and a somewhat variable number of orders:
- Phylum Bryozoa
- Class Phylactolaemata
- Order Plumatellida
- Class Gymnolaemata
- Order Ctenostomata
- Suborder Cheilostomata
- Class Stenolaemata
- Order Cyclostomata
- Suborder Cystoporata (extinct)
- Suborder Trepostomata
- Suborder Cryptostomata
- Suborder Hederellida
Fresh-water bryozoans are present on submerged tree roots and aquatic plants in most lakes, ponds, and rivers, especially in clear water of alkaline pH. Most other bryozoans are marine, although some gymnolaemates inhabit brackish water. They are common in the sea, ranging from the middle shore to a depth of over 26,000 ft (8000 m), and are maximally abundant in waters of the continental shelf. Most attach to firm substrata, so that their distribution is primarily determined by the availability of support. Mud is unfavorable and so is sand unless well provided with stone, dead shells, hydroids, or large foraminiferans.
Colony form in bryozoans is to some extent related to habitat. Encrusting and bushy flexible species are adapted to wave exposure; brittle twiglike and foliaceous species are found deeper; some erect branching species tolerate sediment deposition. One group of tiny discoid species lives on sand in warm seas, and in one genus the colonies are so small that they live actually among the sand grains; a few species live anchored in mud. A number of stolonate ctenostomes bore into the substance of mollusk shells; other species are associated only with hermit crabs, and a few are commensal with shrimps or polychaete worms.
Bryozoans have few serious predators. Nudibranch mollusks and pycnogonids (sea spiders) specialize in feeding on zooids but are rarely destructive of entire colonies. Loxosomatids (Entoprocta) and a hydroid (Zanclea) are common commensals.
Life spans vary. Small algal dwellers complete their life cycle in a few months. Many species survive a year but have two overlapping generations; others are perennial, with one known to survive for 12 years.
Bryozoans may be a nuisance in colonizing ship hulls and the insides of water pipes, and one species has caused severe dermatitis in fishers. Recently some delicate kinds have been used in costume jewelry, and green-dyed clumps of dried Bugula are often sold as “everlasting plants.”
Fossil Bryozoa have a long geological history, from early in the Ordovician Period [500 million years ago (Ma)] to the Recent. Individual fossils range in size from a few millimeters to several meters in maximum dimension. Various encrusting or erect growth forms are common, though some were free-living. Representatives of the marine orders that secreted calcareous skeletons (Cryptostomata, Cyclostomata, Cystoporata, Trepostomata, and Cheilostomata) commonly are abundant in sedimentary rocks formed where benthic organisms flourished. Skeletons generally are calcite, though some are aragonite or mixed calcite and aragonite. Ctenostomata have nonmineralized skeletons, so they have been preserved only as excavations or borings in marine shells or on the undersides of other organisms that overgrew them. The fresh-water Phylactolaemata have gelatinous skeletons, but their tough statoblasts (dormant reproductive bodies) have been reported from sediments as old as the Jurassic (at least 150 Ma). During the Ordovician, Carboniferous, and Permian periods, bryozoans were important parts of many fossil reefs, reef flanks, and other carbonate buildups in shallow (less than 100 m depth) tropical waters. Bryozoans commonly dominate and may reach very high diversities in post-Paleozoic cool-temperate carbonate deposits, indicating a shift in primary environment after the Paleozoic.
Although colonies of many bryozoan species are large, the individual skeletons of each zooid (unit of the colony) range from less than 0.1 to about 1 mm in diameter. The smaller diameters are typical for cross sections of elongate tubes that characterize zooids in stenolaemate bryozoans, and the larger diameters are typical for the more equidimensional zooids of cheilostomes. Identification is based on numerous external and, for most stenolaemates, internal features that require study with a microscope. Features of the colonial skeletons (zoaria) as well as the morphology of the individual zooidal skeletons (zooecia) are used to classify bryozoans. Many fossil bryozoans had only one type of zooid (autozooids), which apparently could feed and carry out all other necessary biological functions of the colony. Others were polymorphic, with various types of specialized zooids supplementing the autozooids. Number, types, and morphology of polymorphs is important in classification. Other characters important in classification of fossil bryozoans are wall structure, reproductive chambers, general growth habit or specific shape of colonies, and for some, surface topography of the colony.
(also Polyzoa), a class of invertebrates of the phylum Tentaculata. These aquatic, predominantly marine, invertebrates are sessile, colonial animals. Some bryozoan colonies overgrow their substrates (stones, algae). In such cases, some colonies resemble encrustations or clumps, and others are treelike, leaflike, or shrublike. Still others are in the form of gelatinous strands.
A bryozoan colony consists of microscopic individuals, or zooids, which are enclosed in a calcareous, chitinous, or gelatinous cell (cystidium). The anterior part of the body (polypide), on which lies the mouth in the midst of a corolla of tentacles, can move out through an opening in the cystidium. The movement of the cilia on the tentacles creates a current of water, by means of which small plankton and detritus, the principal food of bryozoans, are drawn into the mouth.
Because of the sessile life of bryozoans, their organ systems are simplified. The digestive system consists of a mouth, pharynx, esophagus, stomach, and U-shaped gut. The anus lies on the dorsal side of the body near the mouth but not in the tentacle circle. In most bryozoans, respiratory, circulatory, and excretory systems are absent. Respiration is effected through the tentacles and the body surface. There is a coelom, whose fluid fulfills the functions of blood. Substances to be excreted are accumulated in the phagocytes and eliminated through the intestine. The nervous system consists of a ganglion and nerves departing from it. There are longitudinal muscles to retract the polypide, and parietal muscles to protrude it.
Reproduction is asexual and sexual. The majority of bryozoans are bisexual. The eggs develop in water, in the coelom, or in brood pouches, ovicells, or gonozooids. Larvae of the trochophore type are variously shaped and equipped with a circlet of cilia; Cyphonautes larvae have a bivalve shell. The larva settles on the bottom, where it attaches itself and, after undergoing a series of simplifications, forms the first individual—the ancestrula. This new individual gives rise to a new colony by budding. In many freshwater bryozoans, internal buds known as stato-blasts develop, which germinate during the winter and form new colonies in the spring.
The colonies of many marine Bryozoans are characterized by polymorphism, that is, the presence of individuals that differ in structure and function from normal autozooids. These individuals include gonozooids, avicularia, vibracula (which protect and cleanse the colony), and kenozooids (which fasten a colony). Some individuals are characterized by a short life-span. An entire zooid or, sometimes, part of a zooid may rapidly atrophy and degenerate, with subsequent resurgence.
Bryozoans are widely distributed in seas and freshwaters, where they are found from the tidal zone to depths of 200–300 m (more rarely up to 6, 000 m). There are approximately 4, 500 species. The USSR has about 450 marine species and about 30 freshwater species.
The most ancient remains of bryozoans are known from the Lower Ordovician, but it is conjectured that bryozoans existed as early as the Cambrian. Paleontologists classify Bryozoa as a special phylum with two classes, Gymnolaemata and Phylactolaemata. The latter class is almost unknown in fossil form. In all, there are about 15, 000 extinct species. Bryozoans were particularly diverse in the Paleozoic. In a number of cases, these invertebrates participated in reef formation. Bryozoans are significant in solving problems of stratigraphy, paleoecology, and paleogeography.
REFERENCESKliuge, G. A. “Mshanki (Bryozoa).” In Zhizn’ presnykh vod SSSR, vol. 2. Edited by V. I. Zhadin. Moscow-Leningrad, 1949.
Kliuge, G. A. Mshanki severnykh morei SSSR. Moscow-Leningrad, 1962. (Opredeliteli po faune SSSR, vol. 76.)
Osnovy paleontologii, vol. 7. Edited by T. G. Sarycheva. Moscow, 1960.
“The Lophophorate Coelomates—Phylum Ectoprocta.” In L. H. Hyman, The Invertebrates, vol. 5. New York-London-Toronto, 1959.
E. I. ANDROSOVA