Bryozoa
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| Bryozoa Fossil range: Ordovician - Recent |
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 "Bryozoa", from Ernst Haeckel's Kunstformen der Natur, 1904
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Ectoprocta |
Bryozoans are tiny colonial animals that generally build stony skeletons of calcium carbonate, superficially similar to coral (although some species lack any calcification in the colony and instead have a mucilaginous structure). Members of the Phylum bryozoa are known as "moss animals" or "moss animacules" (which is the literal translation of the Greek term βρυόζωα, "bryózoa") or as "sea mats". They generally prefer warm, tropical waters, but are known to occur worldwide. There are about 8,000 living species, with several times that number of fossil forms known.
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[edit] Description
[edit] Types of zooid
All bryozoans are colonial except for one genus, Monobryozoon.[1][2] Individual members of a bryozoan colony are about 0.5 millimetres (0.020 in) long and are known as "zooids",[3] since they are not fully-independent animals.[4] All colonies contain feeding zooids, and those of some groups also contain non-feeding specialist zooids;[2] since they are genetically identical, colony members co-operate, rather like the organs of larger animals.[3]
The bodies of all types have two main parts. The "cystid" consists of the body wall and whatever type of exoskeleton is secreted by the epidermis. The exoskeleton may be organic (chitin, polysaccharide or protein) or made of the mineral calcium carbonate. The body wall consists of the epidermis, basal lamina (a mat of non-cellular material), connective tissue, muscles, and the mesothelium which lines the coelom (main body cavity).[3] The other main part, known as the "polypide" and almost entirely within the cystid, contains the nervous system, digestive system, some specialized muscles and the feeding apparatus or or other specialized organs that take the place of the feeding apparatus.
[edit] Feeding zooids
The most common type of zooid is the feeding form, in which the polypide bears a "crown" of hollow tentacles called a lophophore, which captures food particles.[2] The "crown" may be a full circle or U-shaped.[3] The sides of the tentacles bear fine hairs called cilia, whose beating drives a water current from the tips of the tentacles and out between their bases. Food particles that collide with the tentacles are trapped by mucus ("slime"), and further cilia on the inner surfaces of the tentacles drive the particles towards the mouth, which lies in the center of the base of the "crown".[5]
The lophophore and mouth are mounted on a flexible tube called the "invert" because it can be turned inside-out and withdrawn into the polypide,[3] rather like the finger of a rubber glove; in this position the lophophore lies inside the invert and is folded like the spokes of an umbrella. The invert is withdrawn, sometimes within 60 milliseconds, by a pair of retractor muscles that are anchored at the far end of the cystid. Sensors at the tips of the tentacles may check for signs of danger before the invert and lophophore are fully extended. Extension is driven by by an increase in internal fluid pressure, which species with flexible exoskeletons produce by contracting circular muscles that lie just inside the body wall. Some species with rigid exoskeletons have a flexible membrane that replaces part of the exoskeleton, and transverse muscles anchored on the far side of the exoskeleton increase the fluid pressure by pulling the membrane inwards.[3] In others there is no gap in the protective skeleton, and the transverse muscles pull on a flexible sac which is connected to the water outside by tiny pores; the expansion of the sac increases the pressure inside the body and pushes the invert and lophophore out.[3]
In some species the retracted invert and lophophore are protected by an operculum ("lid"), which is closed by muscles and opened by fluid pressure.[3]
[edit] Colony forms and composition
[edit] Ecology
Most species of bryozoa live in marine environments, though there are about 50 species which inhabit freshwater. In their aquatic habitats, bryozoans may be found on all types of hard substrates: sand grains, rocks, shells, wood, blades of kelp, pipes and ships may be heavily encrusted with bryozoans. Some bryozoan colonies, however, do not grow on solid substrates, but form colonies on sediment. While some species have been found at depths of 8,200 m (27,000 ft), most bryozoans inhabit much shallower water. Most bryozoans are sessile, but a few colonies are able to creep about, and some non-colonial bryozoans live and move about in the spaces between sand grains. One remarkable species makes its living while floating in the Southern Ocean. Fossil bryozoans are common throughout the world in sedimentary rocks representing shallow marine habitats, especially in rocks of post-Cambrian Paleozoic age.
Almost all bryozoans are colony-forming animals. Many millions of individuals can form one colony. The colonies range from millimeters to meters in size, but the individuals that make up the colonies (the zooids) are tiny, usually less than a millimeter long. In each colony, different individuals assume different functions. Some individuals gather up the food for the colony (autozooids), others depend on them (heterozooids). Some individuals are devoted to strengthening the colony (kenozooids), and still others to cleaning the colony (vibracula). There is only a single known solitary species, Monobryozoon ambulans, which does not form colonies.
[edit] Anatomy
Bryozoan skeletons grow in a variety of shapes and patterns: mound-shaped, lacy fans, branching twigs, and even corkscrew-shaped. Their skeletons have numerous tiny openings, each of which is the home of a minute animal called a zooid. They also have a coelomate body with a looped alimentary canal or gut, opening at the mouth and terminating at the anus. They feed with a specialized, ciliated structure called a lophophore, which is a crown of tentacles surrounding the mouth. Their diet consists of small microorganisms, including diatoms and other unicellular algae. In turn, bryozoans are preyed on by grazing organisms such as sea urchins and fish. Bryozoans do not have any defined respiratory, or circulatory systems due to their small size. However, they do have a simple nervous system and a hydrostatic skeletal system. Several studies have been undertaken on the crystallography of bryozoan skeletons, revealing a complex fabric suite of oriented calcite or aragonite crystallites within an organic matrix - see for example Hall et al. (2002).
The tentacles of the bryozoans are ciliated, and the beating of the cilia creates a powerful current of water which drives water together with entrained food particles (mainly phytoplankton) towards the mouth. The gut is U-shaped, and consists of a pharynx which passes into the esophagus, followed by the stomach, which has three parts: the cardia, the caecum, and the pylorus. The pylorus leads to an intestine and a short rectum terminating at the anus, which opens outside the lophophore. In some groups, notably some ctenostomes, a specialized gizzard may be formed from the proximal part of the cardia. Gut and lophophore are the principal components of the polypide. Cyclical degeneration and regeneration of the polypide is characteristic of marine bryozoans. After the final polypide degeneration, the skeletal aperture of the feeding zooid may become sealed by the secretion of a terminal diaphragm. In many bryozoans only the zooids within a few generations of the growing edge are in an actively feeding state; older, more proximal zooids (e.g. in the interiors of bushy colonies) are usually dormant.
Because of their small size, bryozoans have no need of a blood system. Gaseous exchange occurs across the entire surface of the body, but particularly through the tentacles of the lophophore.
Bryozoans can reproduce both sexually and asexually. All bryozoans, as far as is known, are hermaphroditic (meaning they are both male and female). Asexual reproduction occurs by budding off new zooids as the colony grows, and is the main way by which a colony expands in size. If a piece of a bryozoan colony breaks off, the piece can continue to grow and will form a new colony. A colony formed this way is composed entirely of clones (genetically identical individuals) of the first animal, which is called the ancestrula.
One species of bryozoan, Bugula neritina, is of current interest as a source of cytotoxic chemicals, bryostatins, under clinical investigation as anti-cancer agents.
[edit] Fossils
Fossil bryozoans are found in rocks beginning in the Early Ordovician as part of the Ordovician radiation. They were often major components of Ordovician seabed communities and, like modern-day bryozoans, played an important role in sediment stabilization and binding, as well as providing sources of food for other benthic organisms. During the Mississippian (354 to 323 million years ago) bryozoans were so common that their broken skeletons form entire limestone beds. Bryozoan fossil record comprises more than 1,000 described species. It is plausible that the Bryozoa existed in the Cambrian but were soft-bodied or not preserved for some other reason; perhaps they evolved from a phoronid-like ancestor at about this time.
Bryozoans are important members of sclerobiont (organisms which dwell on hard substrates such as shells and rocks) communities in the fossil record and in the Recent. For a review of sclerobiont evolution, history and ecology, see Taylor & Wilson (2003).
Most fossil bryozoans have mineralized skeletons. The skeletons of individual zooids vary from tubular to box-shaped and contain a terminal aperture from which the lophophore is protruded to feed. No pores are present in the great majority of Ordovician bryozoans, but skeletal evidence shows that epithelia were continuous from one zooid to the next.
With regard to the bryozoan groups lacking mineralized skeletons, the statoblasts of freshwater phylactolaemates have been recorded as far back as the Permian, and the ctenostome fossils date from the Triassic.
One of the most important events during bryozoan evolution was the acquisition of a calcareous skeleton and the related change in the mechanism of tentacle protrusion. The rigidity of the outer body walls allowed a greater degree of zooid contiguity and the evolution of massive, multiserial colony forms.
[edit] Classification
The bryozoans were formerly considered to contain two subgroups: the ectoprocta and the entoprocta, based on the similar bodyplans and mode of life of these two groups. (Some researchers also included the Cycliophora, which are thought to be closely related to the entoprocta.) However, the ectoprocta are coelomate (possessing a body cavity) and their embryos undergo radial cleavage, while the entoprocta are acoelemate and undergo spiral cleavage. Molecular studies are ambiguous about the exact position of the entoprocta, but do not support a close relationship with the ectoprocta. For these reasons, the entoprocta are now considered a phylum of their own.[6] The removal of the 150 species of entoprocta leaves bryozoa synonymous with ectoprocta; some authors have adopted the latter name for the group, but the majority continue to use the former.
The closest relations of the bryozoans appear to be the brachiopods. The sister group to this clade is still unclear but this seems most likely to be the phoronids.
[edit] References
- Hall, S.R., Taylor, P.D., Davis, S.A. and Mann, S., 2002. Electron diffraction studies of the calcareous skeletons of bryozoans. Journal of Inorganic Biochemistry 88: 410-419. [1]
- Hayward, P. G., J. S. Ryland and P. D. Taylor (eds.), 1992. Biology and Palaeobiology of Bryozoans, Olsen and Olsen, Fredensborg, Denmark.
- Robinson, R. A. (ed.), 1983. Treatise on Invertebrate Paleontology, Part G, Bryozoa (revised). Geological Society of America and University of Kansas Press.
- Sharp, J.H., Winson, M.K. and Porter, J.S. 2007. Bryozoan metabolites: an ecological perspective. Natureal Product Reports 24: 659-673.
- Taylor, P.D. and Wilson, M.A., 2003. Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews 62: 1-103. [2]
- Woollacott, R. M. and R. L. Zimmer (eds), 1977. The Biology of Bryozoans, Academic Press, New York.
- ^ Giere, O. (2009). "Tentaculata". Meiobenthology (2 ed.). Springer Verlag. p. 227. ISBN 9783540686576. http://books.google.co.uk/books?id=an9ncYOxkUoC&pg=PA227&lpg=PA227&dq=solitary+bryozoan&source=bl&ots=0oQTXPYm31&sig=JVs6tDVhn-yNVJJoA8d016prOgk&hl=en&ei=zcVTSpOdNJarjAf274mOCQ&sa=X&oi=book_result&ct=result&resnum=2. Retrieved on 2009-07-07.
- ^ a b c Doherty, P.J. (2001). "The Lophophorates". in Anderson, D.T.. Invertebrate Zoology (2 ed.). Oxford University Press. pp. 363–373. ISBN 0195513681.
- ^ a b c d e f g h Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). "Lophoporata". Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 829-845. ISBN 0030259827.
- ^ Little, W.; Fowlwer, H.W, Coulson, J. and Onions, C.T. (1964). "Zooid". Shorter Oxford English Dictionary. Oxford University Press.
- ^ Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). "Lophoporata". Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 817. ISBN 0030259827.
- ^ James W. Valentine (2004). On the origins of phyla. University of Chicago Press.
[edit] See also
[edit] External links
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Wikimedia Commons has media related to: Bryozoa |
- Index to Bryozoa Bryozoa Home Page, was at RMIT; now bryozoa.net
- Other Bryozoan WWW Resources
- International Bryozoology Association official website
- Bryozoan Introduction
- The Phylum Ectoprocta (Bryozoa)
- Phylum Bryozoa at Wikispecies
- Bryozoans in the Connecticut River
- Bryozoa Fact Sheet
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