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 (Fig. 1). The colonies are variable in size and habit (Figs. 2 and 3). Some are known as lace corals and others as sea mats, but the only general name is bryozoans (sea mosses).
Fig. 1 Morphological features of autozooids, in frontal view.
Fig. 2 Stenolaemate morphologic features, some of which are found only in living colonies and others only in fossil colonies. The enlarged oblique view is from the colony at the lower right.
Fig. 3 Morphological features of gymnolaemate colony, and enlarged oblique view of block from it.
Byrozoans form colonies by asexual budding from a primary zooid, or ancestrula, formed by metamorphosis of a sexually produced larva or from some kind of resting bud. Structurally the zooids are metazoan, triploblastic, unsegmented, and bilaterally symmetrical, with a regionated fluid-filled body cavity that is considered to be a coelom. The body wall comprises epidermis underlain by a feebly developed peritoneum, between which muscle may be present. The epidermis secretes a chitinlike cuticle or gelatinous layer, and the cuticle may become calcified as a rigid exoskeleton.
Much of the zooid consists of the lophophore, alimentary canal, and associated musculature, together known as the polypide. The lophophore comprises a circle or crescent of slender, ciliated tentacles plus their supporting ridge. When spread for feeding, the tentacles form a funnel with the mouth at its vertex. During withdrawal the tentacles close, and are pulled downward from their base through the simultaneously in-rolling introvert. They then lie within the introvert, called the tentacle sheath. The open distal end of the in-rolled tentacle sheath is termed the orifice, and it may be closed simply by a sphincter muscle or by more elaborate structures.
The alimentary canal is deeply looped, and regionated into pharynx, stomach, and rectum. The limb descending from the mouth consists basically of the pharynx and stomach cardia; the central stomach and its dilatation, the cecum, form the base of the loop; and the stomach pylorus and the rectum constitute the ascending limb. The anus opens outside the lophophore. The nerve ganglion, center of a system that may be complex, lies between the mouth and the anus. There are no special excretory or respiratory organs, and no circulatory system. Colonies, but not all zooids, are hermaphroditic. Zooids are generally not more than 0.04 in. (1 mm) long.
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 1.5 ft (0.5 m) in length, or patches 0.67 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 4 in. (10 cm) in the largest dimension. In many colonies much of the bulk consists of the zooid exoskeletons, termed zooecia, 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.
All bryozoans are epibenthic, and are generally attached to firm substrata, less often anchored in or resting on sand. Most are marine, but one complete class (Phylactolaemata) is confined to fresh water. In the latter habitat, statoblasts and hibernacula, resting bodies resistant to cold and desiccation, are produced. Sexual reproduction leads either to a distinctive, planktotrophic larva, called the cyphonautes, or more usually to a subspherical, nonfeeding, short-lived type that has had no particular name until recently distinguished as “coronate,” because of its extensive ciliary corona.
Taxonomy and classification
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 Calyssozoa) 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. See also: Entoprocta
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.
See also: Gymnolaemata; Phylactolaemata; Stenolaemata
Until somewhat over 200 years ago bryozoans went unrecognized. When discovered they were thought, with coelenterates, to be plants—a confusion that led Linnaeus to invent the term zoophyte. In 1830 J. Vaughan Thompson characterized bryozoans, separating them from other zoophytes, but at the same time ambiguously introducing a class Polyzoa. G. C. Ehrenberg's name Bryozoa followed in 1831. See also: Cryptostomata; Ctenostomata; Cyclostomata (Bryozoa); Cystoporata; Hederellida; Trepostomata
Comparative morphologists link the Bryozoa with the Phoronida and the Brachiopoda, characterized especially by possession of a lophophore, which L. H. Hyman defined as “a tentaculated extension of the mesosome that embraces the mouth but not the anus, and has a coelomic lumen.” The tentacle circlet of the Entoprocta is postanal and noncoelomic. The designation of the bryozoan lophophore as mesosomal, and its body cavity as mesocoelic, rests entirely on the interpretation of a supraoral flap of tissue, the epistome, present in phylactolaemates, as the protosome or first region of a tripartite body. In addition to the lophophore and presumed tripartite body, all three phyla have a U-shaped alimentary tract. The lophophore is crescentic in some members of all three groups. Even if the relationship is correct, however, it sheds little light on bryozoan origins. The soft-bodied Phylactolaemata have left no fossil record, but comparative anatomy suggests that they, the Stenolaemata, and the Gymnolaemata must already have been distinct by the Ordovician. The view that Bryozoa arose from entoprocts, perhaps by internal budding from the larva, seems tenuous, and implies at most no more than a similarity between larvae such as that which links Mollusca to Annelida. Certainly the immediate ancestor must have been vermiform and sedentary, for cylindrical zooids, terminal lophophores, and deeply recurved digestive tracts with the anus rather close to the mouth represent the primitive state in all classes.
Functional morphology of zooid
Zooids in all three bryozoan classes display the same ground plan: eversible lophophore, U-shaped gut, fluid-filled coelom, and deformable body wall. The lophophore is everted by increase in hydrostatic pressure caused by inflexion of the body wall; withdrawal is achieved by the direct pull of retractor muscles anchored to the wall. The flexible phylactolaemate wall incorporates circular and longitudinal muscle, and its contraction everts the lophophore. Recent work indicates that stenolaemates, as evidenced by the extant Cyclostomata, have a remarkably modified system. Throughout this class the zooid is cylindrical (often inaccurately described as tubular; only the exoskeleton is tubular). Except in the immediate vicinity of the orifice, the cuticle is calcified and totally rigid. The peritoneum, with a hypertrophied basal membrane and associated bands of circular muscle, has detached from the epidermis and lies freely as a bag, the membranous sac, anchored at some points to the body wall. The endosaccal cavity is coelomic, but the exosaccal cavity, external to the mesoderm, is not. The sac contracts to evert the lophophore; exosaccal fluid must be displaced proximally as the polypide is eased toward the orifice.
The simplest gymnolaemates (Ctenostomata) have cylindrical zooids and deformable walls, although the musculature is extrinsic. The circular (now renamed transverse parietal) muscles lie inside the peritoneum, and the longitudinal muscles are reduced and specialized. Mechanically the system works as in Phylactolaemata. An early trend of zooidal evolution in the Gymnolaemata was toward a flat, membranous, coffinlike shape, adherent to the substratum. It seems probable that the cheilostomes arose from ctenostomes like that by calcification of the lateral walls and the differentiation of a cuticular lid, the operculum, to close the orifice. Cheilostomes of this plan, classified as the suborder Anasca, preserve a wholly or partially flexible front wall. Transverse parietal muscles, relocated to span the coelom from the rigid lateral walls to the frontal membrane, bring about its inflexion.
Natural selection in cheilostomes has favored modifications which better protect the polypide while preserving the function of the frontal membrane. Thus the frontal membrane can be underlain by a shelf, at a distance sufficient to permit inflexion; or it can be overarched by flattened spines or by a meshwork of partially fused spines. In the subclass Ascophora the membrane may be overgrown or replaced by a calcified frontal wall, but the flexible membrane either remains or is replaced more deeply by a functional equivalent. A saclike cavity or ascus, opening just behind the orifice, is created either by overgrowth of the wall or by involution from the orifice underneath the wall. Transverse parietal muscles insert on the lower face of the ascus, which is pulled down to evert the lophophore, while water to compensate for the volume change enters the ascus through its opening.
The expanded lophophore of marine bryozoans forms an almost radially symmetrical funnel of 8–30 tentacles, although fixed or transient bilateral symmetry may occur. In gymnolaemates the base of this funnel stands free above the surface of the zooid, but in stenolaemates it lies concealed within the orificial region of the exoskeleton. The top diameter is in the range of 0.01 to 0.05 in. (0.25 to 1.25 mm). In primitive phylactolaemates 20–30 tentacles form a funnel, but in most members of this class the large lophophore comprises 100 or more tentacles disposed in a horseshoe, the lobes of which project from the adanal side.
The base of the lophophore is the most complex part of the polypide. Essentially it is a hollow epidermal annulus, with thickened basement membrane. The lumen (mesocoel) opens into the main body cavity (metacoel) by an adanal pore. The cerebral ganglion partly occludes this pore, and a nerve tract parallels the mesocoel. Radial and circular muscles open and close the mouth. The tentacles rise from the annulus as slender hollow cylinders. Their central coelom is surrounded by a thick, flanged collagenous tube, the hypertrophied basement membrane of the epidermal cells. The lumen, which opens terminally through a minute pore, is lined by peritoneum and contains frontal (that is, facing into the funnel) and abfrontal longitudinal muscles. The lateral epidermal cells bear long cilia (about 25 micrometers) and the frontal cells shorter cilia (about 15 μm). Laterofrontally is a line of static sensory cilia. The tentacles contain motor and sensory nerves.
The lateral cilia generate a water current that enters the top of the funnel and flows downward and outward between the tentacles. Particles such as bacteria and phytoplankton are separated from the water and directed at the mouth by the frontal cilia without the use of mucus. The mechanism of separation is uncertain, but a likely explanation is that particles passing out between the tentacles, perhaps sensed by the rigid laterofrontal cilia, are flicked back into the funnel by localized reversal of ciliary beat. Small particles accumulate just above the mouth; heavier particles may be projected down the funnel and directly into the partly open mouth.
Unwanted particles are rejected by closing the mouth, flicking the tentacles, closure or concerted movements of the funnel, and ejection from the pharynx (see below). A few bryozoans with large lophophores have rejection tracts leading centrifugally between the ventromedial (abanal) tentacles. Bryozoans are vigorous feeders, and it has been calculated that the lophophore of even a small zooid filters about 0.3 ml/h (and a colony may contain thousands of zooids).
Bryozoan colonies may have exhalant chimneys or set points of water outflow. The reason is clear: if the colony is large, solid, flat, and uniformly covered by tentacular funnels pumping toward its surface, the filtered water requires a means to get away. Small colonies may have a single, central chimney; large colonies may have a mamillate surface, the summit of each mamilla marking a chimney; and wholly flat colonies produce chimneys by an apparently coordinated bending away of lophophores from predetermined points. Filtered water flows toward the chimneys, through which it is expelled. Lophophores surrounding a chimney display strong bilateral symmetry, with the bordering tentacles standing tall and upright instead of curving into the chimney. A consequence of the posture is that the beat of the current-generating lateral cilia becomes centripetal to the chimney rather than being unhelpfully directed toward its base.
The pharynx is a remarkable organ. It is short, thick-walled, and situated immediately below the mouth. Its exterior is circular in section, its interior deeply furrowed. The frontal ciliation from the lophophore continues through the mouth and may cover the distal part of the pharynx: the midventral groove is ciliated, even when the rest of the pharynx is not, and its cilia beat upward, whether there is a ventromedial rejection tract or not. Between the grooves, the inwardly bulging walls are made up of vacuolated cells which incorporate intracellular myofibrils. The simultaneous contraction of all the myofibrils, acting against the fluid-filled vacuole, causes a brief convulsive dilation of the pharynx, engulfing the particles accumulated outside the mouth.
A sphincter and valve separate the pharynx from the cardia (descending arm of the stomach). In some ctenostomes, and a few other bryozoans, part of the cardia is differentiated as a gizzard, armed internally with teeth or keratinized plates, presumably for crushing the frustules of diatoms. The unmodified wall of the cardia contains secretory cells. The floor of the central stomach is ciliated, evidently to carry particles into the cecum, which is the main locus of extra- and intracellular digestion and absorption. The epithelium of the pylorus is ciliated and serves to compact the undigested remains into a revolving cord. A sphincter separates the pylorus from the rectum, which has an absorptive function and modifies the cord of reject material into fecal pellets. The pellet is moved through and expelled from the rectum by peristalsis. At summer temperatures (72°F or 22°C) the gut transit time is less than 1 h but at 43°F (6°C) it is 2 h.
Metabolic integration and colony evolution
The simplest bryozoan colonies presumably consisted of loosely interconnected, monomorphic feeding zooids. In the most ancient fossil stenolaemates, confluence of coeloms had been achieved either by means of internal pores or by the so-called double wall, in which a superficial (or hypostegal) coelom unites the colony outside the calcification. The hypostegal coelom must have been reasonably effective as a means of distributing metabolites, since the colonies of trepostomes were large by bryozoan standards, and the fenestrates evolved colony forms of considerable complexity. However, rapid growth and the proliferation of nonfeeding zooid polymorphs requires a transport system more efficient than aqueous diffusion. The funiculus provides such a system.
In living single-walled stenolaemates (cyclostomes) zooids communicate via small, open interzooidal pores. The funiculus is a small muscular strand linking the proximal end of the cecum with the base of the membranous sac at its attachment to the zooid wall. The testis is positioned where the funiculus joins the cecum. The phylactolaemate funiculus similarly is muscular, supports the testis, and links the cecum to the proximal end of the ventral body wall. More importantly, it contains a core of blastogenic tissue from which the statoblasts arise. These are the future resting buds and become richly supplied with yolk. The obvious inference is that the products of digestion are transported, perhaps to the testis but certainly to the developing statoblasts, along the funiculus. Communication between zooids in phylactolaemates is by open pores; indeed in many of them the dividing walls have almost completely broken down, so that the polypides are suspended in a communal coelom.
The adaptive radiation of zooid form has been very slight in both Cyc