Gymnolaemata

(invertebrate zoology) A class of ectoproct bryozoans possessing lophophores which are circular in basal outline and zooecia which are short, wide, and vaselike or boxlike.

(Marine bryozoans)

Phylum: Ectoprocta

Class: Gymnolaemata

Number of families: About 130

Thumbnail description
Colonial bryozoans, usually polymorphic zooids bearing a lophophore that protrudes through the action of muscles pulling on a frontal wall of the body

Evolution and systematics

Uncalcified gymnolaemates are known as fossils from the late Ordovician on, almost exclusively as distinctive borings in carbonate substrates such as shells. Nonboring, noncalcified gymnolaemate bryozoans are extremely rare as fossils and are known from the Jurassic and Cretaceous only. Calcareous gymnolaemates did not appear in the oceans until the Cretaceous, during which they diversified rapidly. There were very few species of gymnolaemates in the early Cretaceous, but by the end of the period, there were more than 100 genera. Gymnolaemates continued to diversify in the Cenozoic era. There are two gymnolaemate orders—Ctenostomata and Cheilostomatida—and more than 1,000 genera. The Ctenostomata are stoloniferous or compact colonies in which the uncalcified exoskeleton is membranous, chitinous, or gelatinous and the usually terminal orifices lack an operculum. Order Cheilostomatida contains colonies composed of boxlike zooids that are adjacent but have separate calcareous walls. The orifice of cheilostomes is covered with an operculum.

Physical characteristics

Gymnolaemates are morphologically varied. The simplest have no skeletons, no polymorphs, and no change in zooidal form during colony development. The most complex gymnolaemates have elaborately calcified skeletons and at least two kinds of polymorphic zooids and display considerable variation in zooidal form during colony development (astogeny). Zooids are cylindrical or flattened. The zoecium, which covers the zooid, consists of an organic cuticle composed of protein and chitin or of cuticle overlying calcium carbonate. In many species, the zoecium is heavy and rigid. Some impregnation of the chitinous layer with calcium carbonate may be present, even when a calcareous layer is absent. An orifice enables the lophophore (food-catching organ) to protrude. The lophophore is circular and consists of a simple ridge bearing eight to 30 or more tentacles. Just within the orifice is a chamber, the atrium. An epistome and intrinsic musculature in the body wall are lacking. Protrusion of the lophophore depends on body wall deformation. Within the body wall is a large coelom surrounding the U-shaped digestive tract. The mouth at the center of the lophophore opens into the digestive tract. The anus opens through the dorsal side of the tentacular sheath outside the lophophore, hence the name Ectoprocta ("outside anus"). Interzooidal communication occurs through a funicular network of tissue-plugged pores in vertical walls.

Gymnolaemates are colonial, sessile animals. Individuals composing the colonies usually are approximately 0.02 in (0.5 mm) long.

Distribution

All seas; attached to rocks, pilings, shells, algae, and other animals in coastal waters. Some species have been found in depths as great as 26,900 ft (8,200 m).

Habitat

Any type of hard surfaces (rock, shells, coral, and wood) and Gymnolaemates are capable of using extremely restricted spaces.

Behavior

Nothing is known.

Feeding ecology and diet

Gymnolaemates are suspension feeders, although they sometimes use supplemental methods.

Reproductive biology

Asexual reproduction is responsible for colony growth and regeneration of zooids. Each colony begins from a single, sexually produced larva that settles and metamorphoses into a founding zooid, called the ancestrula. The ancestrula undergoes budding to produce a group of daughter zooids, which themselves subsequently form more buds, as do succeeding generations. Budding involves only elements of the body wall. The developing bud originates from the parent zooid. The bud initially includes only components of the body wall, or cystid, and an internal coelomic compartment. A new polypide is then generated from the living tissues of the bud, that is, the epidermis and the peritoneum. The epidermis and peritoneum invaginate, the former producing the lophophore and the gut. The peritoneum produces all of the new coelomic linings and the funiculus. Most gymnolaemates are hermaphroditic. Testes and ovaries develop either within the same zooid (zooidal hermaphroditism) or in different zooids within the same colony (zooidal gonochorism). In species that exhibit zooidal gonochorism, the colonies may be protandrous, protogynous, or simultaneous hermaphrodites, and the male and female zooids usually exhibit sexual dimorphism. Sexual polymorphism in hermaphroditic zooids usually is a seasonal specialization involving the polypide. In gonochoric zooids, sexual polymorphism is permanent and involves specialization of the cystid, zoecium, and sometimes the polypide.

Gymnolaemates have a variety of brooding methods, usually involving formation of an external brooding area called an ovicell, or ooecium.

Conservation status

No species are listed by the IUCN.

Significance to humans

Members of this class are used for medicinal and research purposes.

Species accounts

Resources

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 1990.

McKinney, F. K., and J. B. C. Jackson. Bryozoan Evolution. Chicago: University of Chicago Press, 1989.

Reed, Christopher G. "Bryozoa." In Reproduction of Marine Invertebrates. Vol. 6, edited by A. C. Giese, J. S. Pearse, and V. B. Pearse. Pacific Grove, CA: Boxwood Press, 1991.

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

Ryland, J. S., and P. J. Hayward. "British Anascan Bryozoans." In Synopses of British Fauna. London: Academic Press, 1977.

Bayer, M. M., and C. D. Todd. "Evidence for Zooid Senescence in the Marine Bryozoan Electra pilosa." Invertebrate Biology 116 (1997): 331-340.

Hermansen, P., P. S. Larsen, and H. U. Riisgard. "Colony Growth Rate of Encrusting Marine Bryozoans (Electra pilosa and Celleporella hyalina)." Journal of Experimental Marine Biology and Ecology 263 (2001): 1-23.

Bryozoan Orders. RMIT University, Civil and Geological Engineering. 6 July 2000 [7 July 2003]. "Cheilostome Bryozoans Taxonomic List." NMITA: Neogene Biota of Tropical America. Department of Geoscience, University of Iowa. 5 Aug. 1998 [7 July 2003]. .

Tyler, W. "A Sea Mat: Electra pilosa. " MarLIN: The Marine Life Information Network for Britain and Ireland. Marine Biological Association of the United Kingdom. 2003 [7 July 2003]..

Tyler-Waters, H. "An Erect Bryozoan: Bugula turbinata." MarLIN: The Marine Life Information Network for Britain and Ireland. Marine Biological Association of the United Kingdom. 2002 [7 July 2003]. .

[Article by: Michela Borges, MSc]

A class of bryozoans. Predominantly marine, gymnolaemates possess lophophores which are circular in basal outline and zooecia which are short, wide, vaselike or boxlike. Highly diverse in size and shape, most gymnolaemate colonies are small and delicate, but a few are large conspicuous growths. The individual zooids may be relatively isolated or loosely grouped side by side.

The gymnolaemates include several thousand species—mostly marine, some brackish, and avery few fresh-water—belonging to the two orders Ctenostomata and Cheilostomata. Consequently, in line with the recent approach to classification of Bryozoa, there is a tendency now touse the name Eurystomata for this class instead. Appearing early in the Early Ordovician, when they included the possible ectoproct stem group, gymnolaemates remained quite inconspicuous until the Cretaceous, when they rose to the position of dominance among bryozoans which they stillmaintain. See also Bryozoa; Ctenostomata.

Members of the extinct order Cheilostomata have body walls that are negligibly to heavily mineralized by calcium carbonate; most are polymorphic. Feeding zooids possess tentacles that form a circular, bell-to-funnel-shaped lophophore centered on the mouth, atop a cuticular tube enclosing the initial portion of the U-shaped digestive tract. The anus is located part of the wayup the cuticular tube. When the lophophore is retracted from the feeding position above the colony surface, the cuticular tube inverts and surrounds the lophophore, and the orifice is closedby a protective operculum. The operculum is a proximally hinged cuticular plate that is a thickened, sometimes mineralized part of the body wall.

Polymorphism is more prevalent in cheilostomes than in any other bryozoans. Avicularia are common cheilostome polymorphs that may be larger or much smaller than feeding zooids. They have reduced polypides and hypertrophied opercula called mandibles. Mandibles may have any shape from broad fans to bristles, but many are tapered distally to an acute point.

Colonies can be cemented to a continuous substratum, attached by rhizoids to continuous substrata or sediments, or free-living. Cemented colonies include threadlike, sheet, mound, or various erect branched morphologies; rhizoid-attached colonies are predominantly erect; and most free-living colonies are small, broadly flaring, inverted cones.

Most cheilostomes are marine, some are estuarine, and none live in fresh water.

Gymnolaemata
Scientific classification
Kingdom: Animalia Phylum: Bryozoa Class: Gymnolaemata Allman, 1856
Orders
Ctenostomata Cheilostomata

Gymnolaemata is a class of the phylum Bryozoa. Gymnolaemata typically live under seawater and grow on surfaces of rocks, kelps, and even in some cases, on animal species like fish. They are mostly marine bryozoans with cylindrical or flattened zooids. The lophophore is protruded by action of muscles pulling on frontal wall. This order includes the majority of living bryozoan species.

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