ZOBODAT - www.zobodat.at Zoologisch-Botanische Datenbank/Zoological-Botanical Database Digitale Literatur/Digital Literature Zeitschrift/Journal: Denisia Jahr/Year: 2005 Band/Volume: 0016 Autor(en)/Author(s): Ryland John S. Artikel/Article: Bryozoa: an introductory overview 9-20 © Biologiezentrum Linz/Austria; download unter www.biologiezentrum.at Bryozoa: an introductory overview J.S. RYLAND Introduction REED (1991) and, comprehensively, by MUKAl et al. (1997), whose survey should be Bryozoa, sometimes called Ectoprocta, consulted by those seeking information on constitute a phylum in which there are zooidal soft-part morphology, cells, and ul- probably more than 8,000 extant species trastructure in extant bryozoans of all class- (the much quoted figure of 4000, made es. The account also includes much original nearly 50 years ago (HYMAN 1959), is cer- description based on the phylactolaemate tainly a serious underestimate). Some au- Asajirella (formerly Pectinatella) gelaanosa. thorities regard the phylum Entoprocta to This work essentially provides the English be related to Bryozoa, but the evidence is language update for HYMAN's (1959) classic conflicting and opinion divided (see text, incorporating a thorough review of 40 NIELSEN 2000). The bryozoans are a widely years' significant research. The linkage of distributed, aquatic, invertebrate group of the Bryozoa with Brachiopoda and Phoroni- animals whose members form colonies com- da, as 'lophophorates' (following HYMAN posed of numerous units known as zooids. 1959) may no longer be tenable; certainly, Until the mid 18th century, and particular- the feeding mechanisms may not be as simi- ly the publication of John ELLIS' "Natural lar as once supposed (NIELSEN & RllSGÄRD History of the Corallines" (1755), bry- 1998, and references therein). ozoans, like corals and hydroids, were re- garded as plants. This is reflected both in the name of the phylum, which translates as General features: size range 'moss animals' and in the term 'zoophyte' and diversity of structure which was used by LINNAEUS (1758) to em- Bryozoan colonies vary in size. Among brace both bryozoans and hydroids. Seven- Gymnolaemata, colonies of Monobryozoon, ty-five years after ELLIS, bryozoan zooids which live between particles of marine sand, were distinguished from those of cnidarians consist of little more than a single feeding by possessing both a mouth and an anus (DE zooid less than one millimetre in height. BLAINVILLE 1820) and, a decade later, Colonies of the coralline genus Pentapcrra of EHRENBERG (1831) formalized the distinc- European seas, however, can reach 1 m or tion by introducing the names Bryozoa and more in circumference; in the warm-water Anthozoa. Bryozoans are separated into Zoobotryon, which hangs from harbour pil- three classes: Phylactolaemata (freshwater ings, and the phylactolaemate Pectinateüa, dwelling); Stenolaemata (exclusively ma- massive colonies may exceed 0.5 m in diam- rine); and Gymnolaemata (mostly marine). eter. Colonies that form crusts generally cov- The order Cheilostomatida (class Gymno- er only a few square centimetres; erect laemata), containing over 600 genera, is the colonies may rise only 2-5 cm, though the ge- most successful bryozoan group. General ac- latinous Akyonidium (Fig. 1), washed ashore counts of the phylum have been provided by during autumn gales, may exceed 15 cm. CORI (1941), HYMAN (1959), BRIEN (1960), RYLAND (1970), and WOOLLACOTT & ZIM- The texture of colonies is variable. Some, MER (1977), though all are now, in certain especially in fresh water and on seashores, are respects, outdated. More specific but impor- gelatinous or membranous; others are tufted, tant reviews have been provided by RYLAND with flat, leaf-like fronds or whorls of slender Denisia 16, zugleich Kataloge (1976a), MCKINNEY & JACKSON (1989), der OÖ. Landesmuseen branches, whose horny texture results from Neue Serie 28 (2005), 9-20 © Biologiezentrum Linz/Austria; download unter www.biologiezentrum.at opens into a digestive tract that is divided into several regions and terminates at an anus, which is outside (but near) the tenta- cles (hence the name alternative name Ec- toprocta, meaning 'outside anus'). If zooids are disturbed, they withdraw their tentacles inside the body cavity. Only if the zooids have transparent walls, as in the gymnolae- mate genera Eowerbanlaa and Membranipo- ra, is the digestive tract visible. The internal living parts of each zooid - i.e., the nervous and muscular systems, the tentacles, and the digestive tract - are called the polypide, while the surrounding walls and their asso- ciated tissues constitute the cystid (Fig. 2). Form and function: zooids Fig. 1: Erect, gelatinous colonies of lightly calcified zooid walls. Still other Although zooid appearance and struc- Alcyonidium diaphanum (Ctenostomatida). colonies are hard and have strongly calcified ture vary considerably from class to class, Many zooids in the colonies contain skeletons. Such colonies may form rough-sur- and even between orders, all conform to a clusters of developing oocytes and/or larvae in zooidal brood chambers derived faced patches or may rise in slender branch- common basic plan (Fig. 2). The predomi- from the tentacle sheath. Scale bar 5 x 1 ing twigs (such as those that form a network nant, feeding zooid in a bryozoan colony is cm. Origin: west coast of Scotland, Junde in the 'lace corals', e.g. Reteporelia). the autozooid. Autozooids are rarely longer 1998. than one millimetre and the primitive shape The colonies, diverse and complex in appears to have been cylindrical (as e.g. in structure, are composed of individual mod- Fig. 6). The skeleton is external, ranging ules, or zooids, each of which has consider- from a thin cuticle to a thick, calcified lay- able individuality. A bryozoan colony usual- er. The tentacles, collectively termed the ly has many zooids, which may be of one lophophore, can be raised above the zooid type or of types that differ both functionally on a slender extension of the body wall (the and structurally. Neighbouring zooids are usually firmly joined and communicate via introvert or tentacle sheath clearly apparent tiny pores in the dividing walls. Zooids ca- in Fig. 6) and outspread for feeding. If dis- Fig. 2: A stained zooid of Membranipora pable of feeding have a protrusible ring of turbed, the tentacles can be withdrawn into membranacea, the polypide brown and slender tentacles at their distal end, on the body cavity in a movement that in- the cystid walls pale. Mural pore chambers which are found cilia that propel tiny parti- volves inrolling the tentacle sheath, with can be seen (Preparation made by Dr. the mouth and tentacles being pulled down Genevieve Lutaud). cles of food toward the mouth. The mouth within it by the action of paired retractor muscles. Eversion of the withdrawn tenta- cles is effected by raising the hydrostatic pressure of the body fluid, so compressing the tentacle sheath; but the precise mecha- nism differs from class to class. Phylactolae- mates have a muscular and contractile body wall to achieve this function; in gymnolae- mates the wall is non-muscular but in whole or part flexible, so that it can be pulled in- ward by the body musculature associated with it (parietal muscles). In most extant gymnolaemates the zooids are not cylindri- cal but flat, with rigid side walls as seen in Fig. 9. The upward facing or frontal wall ei- ther remains flexible or has concealed below its calcified surface a membranous cavity, 10 © Biologiezentrum Linz/Austria; download unter www.biologiezentrum.at the ascus (sac), which can be inflated with to the autozooids, as in Schizoporelia. In sea water, thereby compressing the body flu- Bugula the avicularia are movable on short id. At the free end of a cylindrical zooid or stalks and closely resemble miniature birds' near the distal end of a flat zooid is an open- heads; hence the name avicularium (cp. ing known as the orifice, through which the contribution of WÖSS, this volume). An- tentacle sheath and tentacles emerge; in other specialized form of zooid is the vibrac- cheilostome gymnolaemates the orifice has ulum, in which the operculum has become a a closable lid, the operculum. Stenolaemate whip-like seta. The functions of avicularia zooids are different, and the walls form a and vibracula are not clearly known, but slender calcareous tube, no part of which both types of zooids may help to keep the can be inflected to evert the tentacles; in- colony free from particles and epizoites (i.e., stead, body fluid is forced from one part of organisms that attach to the surface of the the zooid to another by muscles (NIELSEN & colony but do not parasitize it). A recent PEDERSEN 1979). summary may be found in McKlNNEY & JACKSON (1989). The alimentary canal forms a deep loop (Fig. 2); the pharynx, ciliated distally, de- scends to the stomach, the anterior part of Form and function: colonies which is termed the cardia or forms a gizzard Despite their ill-defined shape, colonies, in a few genera, such as the gymnolaemate at least in extant bryozoans, are not just ag- Bowerbankia (MARKHAM & RYLAND 1987); gregations of zooids but whole organisms the main stomach section is the caecum, having a physiology and behaviour that ap- and the slender posterior region, which is pear to be coordinated to some extent. Inte- ciliated, the pylorus; the rectum continues gration is made possible by a system of in- from the pylorus; and the anus is situated terzooidal pores (Fig. 2) and the cells that just outside the lophophore. A detailed and traverse them (see BOBIN 1977 and MUKAI fully illustrated account of the lophophore et al. 1997 for details). Most conspicuous are and gut has been provided by MUKAI et al. those of the funiculus, which in gymnolae- (1997). Respiratory, circulatory, and excre- mates becomes a colonial network capable tory systems are absent in bryozoans. The re- of distributing nutrients to nonfeeding productive organs (ovary, testes) are situat- zooids (such as ovicells or gonozooids) or ar- ed on the lining of the body wall or on the eas (such as the growing edge).