Bryozoa

Bryozoa (brī'əzō`ə), name of a phylum, in older systems of classification, that included the invertebrate animals now classified in the phyla Entoprocta and Ectoprocta. The term bryozoan (or moss animal) is still commonly used for members of the Ectoprocta.

---

Bryozoa [‚brī·ə′zō·ə]

(invertebrate zoology)

The moss animals, a major phylum of sessile aquatic invertebrates occurring in colonies with hardened exoskeleton.

---

Bryozoa

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.