Gymnolaemata

(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

List of Species

No common name

Bugula turbinata

order

Cheliostomata

family

Bugulidae

taxonomy

Bugula turbinata Alder, 1857

other common names

None known.

physical characteristics

Bugula turbinata forms an erect, orange to brown, tufted colony approximately 1.25–2.5 in (3–6 cm) high. The branches are arranged spirally around the main axis. There are two proximal rows of zooids and three or four distal rows. Individual zooids are rectangular, narrowing slightly at the proximal end and bearing a single short spine at each corner of the distal end. The front of the zooid is almost entirely membranous. The polypide bears 13 tentacles. Short, plump avicularia originate just below the spines. These structures resemble birds' heads with rectangularly hooked beaks. Inner avicularia are smaller than marginal avicularia. Brood chambers (ooecia) are globular and conspicuous. Colonies are attached to the substratum by extensions of the basal zooids (rhizoids).

distribution

Britain to Mediterranean Sea (eastern Atlantic).

habitat

Walls of gullies. Under boulders on lower shore and on bedrock, boulders, stones, and shells in the shallow subtidal zone.

behavior

Nothing is known.

feeding ecology and diet

Phytoplankton, macroalgal spores, detritus, and bacteria.

reproductive biology

Reproduction is by protogynous hermaphroditism. The developmental mechanism is lecithotrophic viviparity (no care). Bugula species are placental ovicell brooders, producing small embryos that are brooded in conspicuous hyperstomial ovicells. The size of embryos increases considerably during development owing to nutrition derived from the inside of the ovicell, which functions as a placenta.

conservation status

Not listed by the IUCN.

significance to humans

Medicinal and research use.

Sea mat

Electra pilosa

order

Cheliostomata

family

Electridae

taxonomy

Electra pilosa Linnaeus, 1767

other common names

None known.

physical characteristics

A calcareous encrusting species, sea mats form star-shaped or broad sheet colonies on the fronds of large algae, small irregular patches on stones and shells, narrow tufts (independent of the substratum), or cylindrical encrustations around the fronds of small red algae. The zooids are ovate to oblong. Approximately one half of the front of the zooid is calcified but translucent and perforated by large pores that leave a distal oval, membranous, frontal area surrounded by four to 12 (often nine) spines. Spines vary in length, but the median, proximal spine is always present and usually larger than the others. In some cases the proximal spine becomes well developed and longer than the zooid, giving the colony a hairy appearance.

distribution

All temperate seas; cosmopolitan.

habitat

A variety of substrata in marine habitats from low water into the shallow sublittoral zone (living at maximum water depths of approximately 164 ft [50 m]). Particularly common on macroalgae, such as Fucus serratus and laminaran kelp. Sea mats also encrust shellfish such as mussels and are a common fouling organism.

behavior

Nothing is known.

feeding ecology and diet

Phytoplankton, algal spores.

reproductive biology

Reproduction is by budding and hermaphroditism. The developmental mechanism is planktotrophic. The maternal lophophore may actively collect sperm. The ovaries produce up to 31 oocytes, which are released into the coelomic cavity. Fertilization is internal. Eggs come into contact with sperm (either as aggregates or singly) in the coelomic cavity, and fertilization occurs at or near ovulation. Embryos are shed into the water and develop into planktonic cyphonautes larvae.

conservation status

Not listed by the IUCN.

significance to humans

Nothing is known.

Resources

Books

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.

Periodicals

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.

Other

Bryozoan Orders. RMIT University, Civil and Geological Engineering. 6 July 2000 [7 July 2003]. <http://www.civgeo.rmit.edu.au/bryozoa/orders.html>.

"Cheilostome Bryozoans Taxonomic List." NMITA: Neogene Biota of Tropical America. Department of Geoscience, University of Iowa. 5 Aug. 1998 [7 July 2003]. <http://porites.geology.uiowa.edu/database/bryozoa/systemat/bryotaxa.htm#cupula>.

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].<http://www.marlin.ac.uk/species/Elepil.htm>.

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]. <http://www.marlin.ac.uk/index2.htm?species/Bugtur.htm>.

Michela Borges, MSc