The Bryozoa are a phylum of small, colonial, almost exclusively sessile, aquatic organisms. More than 4000 living species are known, and the fossil record comprises about 20 000 species. Most species encrust stones or algae, others form erect bushes, and a few form free-living colonies. Bryozoans are found in all oceans and many freshwater areas.
Each zooid (individual) in the colony (Figure 1 and 2) consists of a cup- or box-shaped cystid, which is the main part of the thickened body wall, and a polypide, which consists of a retractile introvert with the mouth region surrounded by a ring of tentacles (usually called the lophophore) and the gut suspended in the body coelom. The cystid has a one-layered ectodermal epithelium with a chitinous cuticle. The cuticle of cystids of the living groups Cheilostomata and Cyclostomata and of some of the fossil groups (see below) has an inner, calcified layer, which is very ornate in many species. The term zooecium is sometimes used for the calcified skeleton of a zooid. The inner side of the ectodermal epithelium is lined by a mesodermal epithelium in gymnolaemates and phylactolaemates, whereas the mesodermal layer is detached forming a membranous sac in the Cyclostomata, separating a pseudocoel between ectoderm and membranous sac from a coelom inside the membranous sac. The zooids in a colony are connected: in cyclostomes, interzooidal pores are open so that the pseudocoel is continuous throughout the colony; in gymnolaemates, the interzooidal pores are plugged by special cells; and in the phylactolaemates there are incomplete walls between zooids.
The introvert with the tentacles can be retracted into the cystid by paired bundles of muscle cells extending from the basal part of the cystid to a thickened ring of basement membrane around the mouth. In phylactolaemates and ctenostomes, which have flexible cystids, protrusion of the polypide is caused by contraction of muscles in the cystid wall. In cheilostomes, a membranous, uncalcified part of the frontal cystid wall can be pulled in by groups of muscles extending from the lateral cystid wall to the membranous part and this causes the protrusion (Figure 1). In cyclostomes, contractions of annular muscle cells in the membranous sac squeeze the polypide out (Figure 2). When the polypide is retracted, the opening of the cystid, called the orifice, becomes constricted by a muscle or, in the Cheilostomata, closed by an operculum (Figure 1).
The tentacles are almost cylindrical, consisting of ectodermal cells in fixed patterns, a thickened basement membrane, and a mesodermal layer with longitudinal muscles surrounding a coelomic canal. Gymnolaemate and phylactolaemate tentacles carry frontal, laterofrontal and lateral ciliary bands; the cyclostomes lack frontal cilia. The lateral cells are multiciliate, whereas each laterofrontal cell carries one stiff cilium originating in a depression surrounded by microvilli, which indicates that the cell is sensory.
The U-shaped gut comprises pharynx, stomach and rectum. The pharynx is triradiate in gymnolaemates and cyclostomes, consisting of a myoepithelium with radiating myofilaments; the phylactolaemate pharynx has only circular muscles. Stomach and rectum have areas of ciliated and unciliated cells of varying morphology, but their digestive functions have not been elucidated.
The spacious body coelom is connected to the small coelom around the mouth through a pore. Mesodermal tissue in the shape of hollow strands or layers covers the basal part of the stomach and extends as funicular tubes through the body coelom to the cystid wall and between the interzooidal pores.
A small neural ganglion is situated at the anal side of the pharynx near the mouth. Peripheral nerves go to the tentacles, gut, body wall muscles, polypide retractors, and in the gymnolaemates to a delicate plexus, which probably functions as a colonial nervous system. See also Invertebrate Neuropeptides
There are no excretory organs.
All bryozoans have hermaphroditic colonies and most species have hermaphroditic zooids; all cyclostomes have special female zooids, and several gymnolaemates have special female or male zooids. Ovaries are usually located in the funicular tissue at the cystid wall, whereas the testes are located in the funiculus.
All zooids in phylactolaemate colonies are very similar, but polymorphism is pronounced in cyclostomes and cheilostomes. The cyclostomes' large female gonozooids initially have a polypide, which is apparently necessary for fertilization, but which loses the tentacles when the embryo begins to develop; special cleaning and anchoring zooids are found in some species. Erect ctenostome colonies have feeding zooids rising from tubular stolons. Many cheilostomes have highly specialized heterozooids in the shape of cleaning zooids (vibracula) with a setiform operculum, defence zooids (avicularia) with a mandible-shaped operculum, and rhizoids.
Three classes of living bryozoans are usually recognized: Gymnolaemata, Stenolaemata and Phylactolaemata, but as will be seen below, the stenolaemates may be specialized gymnolaemates.
Gymnolaemata have circular tentacle crowns, and the musculature of the cystid is restricted to distinct parietal muscles (Figure 1). Colonies are of many shapes, usually white or grey, but some species are vividly coloured in orange or violet. Two orders are usually recognized, Ctenostomata with uncalcified cystid and without operculum, and Cheilostomata with calcified cystid and with operculum; but the cheilostomes are probably an ingroup of the ctenostomes (see below). The systematics is in an unstable phase.
Stenolaemata, with the only living order Cyclostomata, have circular tentacle crowns and the mesoderm with the circular musculature of the cystid is detached as the membranous sac. Colonies are mostly small, delicate and white, but a few species form more robust, branched colonies with dull colours.
Almost all phylactolaemates have large tentacle crowns, which are shaped like a horse-shoe; each of the two arms carries a row of tentacles on each side. A flap of tissue (sometimes called epistome) with an extension of the coelom around the mouth partially covers the anal side of the mouth. The musculature of the cystid consists of circular and longitudinal layers. Colonies are small, oval or branched or large and fleshy.
Bryozoan colonies are abundant on most hard substrates, such as stones, rocks, pilings, hydroids, dead corals and shells, and several species typically occur on algae or sea grasses (Figure 3 and 4).
Gymnolaemata are the most speciose group with more than 3500 described living species; most species are marine, but a number of species occur in brackish water and a very few are limnic. Many species form sheet-like colonies on various substrates, and erect, branching colonies are also common; a few species are free-living and can move by means of special zooids called vibracula. Some species, for example some growing on algae, form small, short-lived colonies, which begin reproduction at an early age, whereas others, for example some species growing on corals, grow very slowly and can be more than a hundred years old; older parts of these colonies die off, fragmenting the colonies, which reproduce very occasionally. A number of fouling species cause problems for shipping.
Cyclostomata are exclusively marine, and about 500 living species have been described. The colonies are found on stones, shells, hydroids and algae and are usually not more than 1 cm.
Phylactolaemata occur exclusively in freshwater, with about 50 species occurring in clear-water lakes, ponds and streams. Most colonies grow on stones or plant material, but one species forms floating colonies, with a diameter of more than half a metre. Some species may occur in great masses and have been known to clog water pipes. The freshwater bryozoans may die off seasonally or as the result of unfavourable environmental conditions, but recolonization and dispersal to new localities takes place with the resistant statoblasts (see below).
The bryozoans are ciliary filter-feeders. The lateral ciliary bands on the tentacles create a water current towards the mouth and out between the tentacles. In cyclostomates and gymnolaemates, particles in the current contact the stiff laterofrontal cilia, which form a mechanical filter, and this triggers a flicking of the tentacles bringing the particles to the central part of the current, leading to the mouth. In gymnolaemates, the frontal ciliary band may also transport particles. Larger particles may be captured by special movements of the tentacles. Feeding in phylactolaemates is in need of further study. The captured particles are swallowed through sudden expansions of the pharynx and transported to the stomach, where both extracellular and intracellular digestion take place. The particles form a long cord, which is kept in rotation by ciliated areas of both stomach and rectum.
Bryozoans lack excretory organs, and waste materials accumulate in special zones of the gut. The polypide degenerates after a while to form a 'brown body', and a new polypide buds from the orificial area. Series of brown bodies are retained in the body cavity of some species, whereas the brown body in other species becomes overgrown by the developing polypide and discarded as the first faecal pellet when the new polypide protrudes.
Mature sperm are spawned through a minute pore at the tentacle tips. The free-swimming spermatozoa are elongate and spindle-shaped. Fertilization takes place in the ovary, but it is uncertain how the sperm reaches the ovary. The three living groups show very different developmental types.
Gymnolaemate development takes place in the coelom in a few ctenostomes, whereas cleavage does not begin before the egg has been shed in most other species. A few genera spawn the small eggs into the water, where they develop into cyphonautes larvae, which are planktotrophic, laterally compressed with a triangular, chitinous shell on each side. A ciliated apical organ is situated at the anterior pole of the larva and the opposite side of the triangular body carries an oval band of locomotory cilia, the corona. A U-shaped ciliated ridge across a ventral (posterior) concavity is a filter-feeding structure. An invagination of thickened, glandular epithelium, the adhesive sac, is found on the ventral side. Most of the brooding gymnolaemates deposit large, yolky eggs, but some cheilostomes spawn small eggs, which are nourished through a placenta formed by the maternal zooid. Most of the nonfeeding larvae lack the shells and any trace of a gut. Many cheilostomes brood the embryos in a globular brood structure, the ovicel, formed by the zooid distal to the maternal zooid (Figure 1).
Larvae that are ready to settle test the substratum with a ciliated frontal organ, and when a place has been selected, the adhesive sac everts and secretes its adhesives onto the substratum. The basal side of the fully metamorphosed cyphonautes larva is covered by epithelium from the adhesive sac, whereas the upper side is covered by the pallial epithelium, which had carried the shells. Other types of larvae show much variation in metamorphosis.
The cyclostome egg is fertilized in a very young zooid, which develops into a large, nonfeeding gonozooid. The egg develops into a primary embryo which buds off secondary embryos, which may form tertiary embryos. The embryos differentiate into gutless, ciliated larvae, with an anterior invagination and a posterior adhesive sac. At metamorphosis the adhesive sac everts on the substratum and the anterior invagination everts to cover the upper side of the body.
Phylactolaemate fertilization has not been studied, and the fertilized egg enters an invagination from the ectoderm in an unknown way. The embryo develops in this 'embryo sac' into a stage that lacks almost all larval organs except the ciliated ectodermal covering and has one or more precociously budded polypides.
After metamorphosis, the first polypide or group of polypides forms through budding, and the first adult stage is called the ancestrula. It consists of one zooid in cyclostomes, ctenostomes and most cheilostomes, but other cheilostomes have compound ancestrulae with two to several polypides formed almost simultaneously. The ancestrula is often morphologically different from all the other zooids in the colony, but a few species show complicated changes of zooid morphology during the growth of the colony. The phylactolaemate 'larvae' are in fact small, free-swimming colonies with one to many polypides.
Polypide buds, including those of the ancestrula, those at the growing edge of the colony, and those formed at the germination of statoblasts, develop from an invagination of a narrow area of ectoderm and mesoderm from the body wall.
Phylactolaemates form resting bodies with chitinous shells (statoblasts) in the funiculus, which are important for dispersal and for survival through periods of drought or freezing.
Bryozoans have an extensive fossil record. The earliest gymnolaemates were apparently of the noncalcified ctenostome type and have a poor fossil record beginning in the Ordovician; some species boring in calcareous substrates have left tracks and other species have been preserved by bioimmuration, i.e. as moulds in calcareous matter secreted by cemented bivalves or tube-building polychaetes. Highly calcified stenolaemates flourished with five orders in the Palaeozoic, of which Trepostomata, Cryptostomata, Cystoporata and Fenestrata died out in the Upper Permian or Lower Triassic. Only the Cyclostomata survived with a radiation in the Cenozoic. The calcified cheilostomes evolved in the Jurassic and there is a very extensive fossil record from the Cretaceous to present. See also Fossils in Phylogeny Reconstruction, and Fossil Record
Phylactolaemate statoblasts have been reported from the Jurassic, but the reports appear questionable; more certain fossils are reported from the Tertiary.
The monophyly of bryozoans is usually not questioned, although it has not been supported by some molecular studies. The three living classes Gymnolaemata, Phylactolaemata and Stenolaemata appear very well defined, but new information indicates that Gymnolaemata is paraphyletic with the stenolaemates evolving from Early Ordovician gymnolaemates (ctenostomes). Early cheilostomes with planktotrophic larvae probably evolved from ctenostomes during the Jurassic, but the large radiation began in the Mid-Cretaceous with the evolution of lecithotrophic larvae and brooding. The relationships of the limnic phylactolaemates are uncertain.
The phylogenetic position of the Bryozoa is under dispute. The fact that all known bryozoans are colonial and that the larval organs of planktotrophic larvae degenerate at metamorphosis makes the dorsoventral orientation of the polypides formed through budding impossible to ascertain, and this makes comparisons with other phyla difficult. Traditionally, bryozoans are combined with phoronids and brachiopods in the superphylum Lophophorata, but this is based on similarities, especially in the ciliated tentacle crowns, which may not reflect homology.
Molecular data, almost exclusively obtained from 18S rRNA sequences, have indicated a number of different positions, but most results place the bryozoans within the 'Lophotrochozoa' together with other protostomes with ciliated larvae; phoronids and brachiopods are included in that group, but a closer relationship between the three phyla is not indicated. See also Phoronida, and Brachiopoda (Lamp Shells)
First-formed zooid in a colony, originating from a metamorphosed larva.Bioimmuration
Fossilization of soft organisms as moulds in calcareous matter secreted by cemented bivalves or tube-building polychaetes.Coelom
Secondary body cavity, surrounded by mesodermal cells.Cyphonautes larva
Planktotrophic larva with paired, lateral, chitinous shells.Cystid
Skeletal plus cellular parts of the zooid wall into which the polypide can be retracted.Epistome
Tissue flap extending from the anal side of the mouth.Funiculus
Mesodermal strands connecting the polypide with the cystid wall across the coelomic cavity.Introvert
Retractile part of the polypide, carrying the tentacle crown.Polymorphism
Presence in a colony of zooids of different morphology and function, for example feeding zooids (autozooids), cleaning zooids (vibracula), defence zooids (avicularia), attachment zooids (rhizoids) and female reproductive zooids (gonozooids).Polypide
Movable parts of the zooid, including tentacle crown, introvert and gut.Pseudocoel
Primary body cavity, surrounded by ectodermal or by ectodermal and mesodermal cells.Statoblast
Resting body consisting of a mass of cells with yolk surrounded by two chitinous shells.Stolon
Slender, tubular, branching 'root system' from which zooids are budded.Zooecium
Calcified layer of the body wall in cheilostome and cyclostome bryozoans.Zooid
Individual in a colony.
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