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About Bryozoans

The Bryozoa are a phylum of aquatic, colonial invertebrates. They have no widely-known popular name although they are sometimes referred to as moss animals, sea mats or lace corals. According to molecular evidence, bryozoans are closely-related to brachiopods, molluscs and annelids within a group of phyla called the Lophotrochozoa. About 6000 species of bryozoans are estimated to live at the present-day, mostly in the sea but with a hundred or so species inhabiting freshwater environments. The overwhelming majority of bryozoan species secrete skeletons of the calcium carbonate minerals calcite or aragonite. This resistant skeleton accounts for the rich fossil record of the Bryozoa, with the oldest known bryozoans dating from the Early Ordovician.

Bryozoans are very abundant over some areas of continental shelf, for example off southern Australia. Branching bryozoan colonies can be ecologically important in providing habitats and food for other animals, including commercially exploited fishes and crustaceans. Dense communities of bryozoans in the geological past are responsible for the bryozoan limestones that are common in the geological record. A recent focus of interest in bryozoans has been their potential as sources of natural products of pharmaceutical value, notably a chemical called bryostatin which has anti-cancer properties.

Living colony of Flustra foliacea, Sussex.

Living colony of the bryozoan Flustra foliacea photographed off the Sussex coast of England

Colonial animals like bryozoans have a modular construction. The individual modules forming a bryozoan colony are called zooids and are generally less than a millimetre in size. Within a colony the zooids are genetically identical and the colony grows by adding new zooids in an asexual process termed budding. Despite their genetic identity, colonies of most bryozoan species contain two or more types of zooids that differ in appearance and function. All colonies contain feeding zooids (autozooids), and many in addition have larval brooding zooids (gonozooids), defensive zooids (avicularia or eleozooids), and space-filling zooids (kenozooids).

Tentacle cowns of Flustrellidra hispida

Part of a living colony of Flustrellidra hispida showing the tentacle crowns of several feeding zooids

Bryozoan autozooids are each equipped with a lophophore consisting of a ring of tentacles surrounding the mouth. The beating of cilia on the tentacles creates a flow of water that drives food particles (mainly phytoplankton) towards the mouth. When not feeding the lophophore can be withdrawn into the protection afforded by the walls of the tubular or box-shaped zooid. All bryozoans have an opening (orifice or aperture) in the zooidal skeleton through which the lophophore passes. Withdrawal of the lophophore is brought about by contraction of the retractor muscle whereas protrusion is brought about hydrostatically by muscles that squeeze the coelom and cause the lophophore to 'pop-out'.

Sexual reproduction, and also dispersal, is usually achieved by means of a short-lived, free-swimming larval stage. Following settlement on a hard (e.g., rock or shell) or firm (e.g. seaweed) surface, the larva metamorphoses into the founder zooid - ancestrula - of the new colony. New colonies in some species can also be formed by fragmentation and regrowth.

Branching colony of Adeonella from the Adriatic Sea

Branching colony of the bryozoan Adeonella from the Adriatic Sea

Colony-form in bryozoans varies greatly between species and sometimes even within a single species. Encrusting colonies are attached over the entire basal surface and can have zooids arranged in branching chains or in a continuous sheet. Erect colonies grow away from the substrate into bushy or frond-like shapes. These are usually preserved as broken fragments in Research & Curation. A few bryozoans have cap-shaped colonies that live freely on fine sands and silts, and yet others bore into shells.

Fossil bryozoans have failed to receive the attention from palaeontologists that they deserve. Nevertheless, the main features of the evolutionary history of the group are evident. A rapid diversification of bryozoans in the Ordovician resulted in a plateau of diversity that persisted through the Palaeozoic. Dominant Palaeozoic orders all belong to the Class Stenolaemata and include trepostomes, cystoporates, cryptostomes and fenestrates. The last of these orders became extinct at the end of the Permian and the other three did not survive beyond the Triassic. Cyclostomes, a stenolaemate order of minor importance in the Palaeozoic, radiated in the Jurassic and are extant today. Another extant order - cheilostomes - first appeared in the Late Jurassic and by the end of the Cretaceous had surpassed cyclostomes in both diversity and abundance in fossil assemblages. Cheilostomes belong to the Class Gymnolaemata and represent an independent origination of a calcareous skeleton from a non-mineralized ancestor belonging to the paraphyletic Order Ctenostomata. Bryozoan diversity today may be greater than at any time in the geological past, except perhaps during the Pliocene before the cooling and other changes that accompanied Pleistocene glaciation.

Scanning electron micrograph of Microporella from the Pleistocene of New Zealand.Pleistocene

Scanning electron micrograph showing the complex skeletal morphology of a group of zooids of a Pleistocene fossil bryozoan (Microporella) from New Zealand

Bryozoan taxonomy, including identification and classification, usually depends on the structure of the skeleton in both fossil and recent species. Large-scale skeletal characters such as colony-form are easily observed unaided or with the help of a hand lens. A microscope is needed to appreciate smaller scale characters of the zooid. The scanning electron microscope is a vital tool used by bryozoan taxonomists, especially for enabling clear photographic illustration. Thin sections are also employed to study Palaeozoic bryozoans which often have fewer diagnostic external characters than younger species.

For further information on bryozoans it is worth visiting Phil Bock's web-site The Bryozoa Home Page, or consulting the following references:

McKinney, F.K. and Jackson, J.B.C. 1989. Bryozoan Evolution. London: Unwin Hyman.

Ryland, J. S. 1970. Bryozoans. London: Hutchinson.

Taylor, P.D. 1999. Bryozoa. In: Savazzi, E., ed., Functional Morphology of the Invertebrate Skeleton, pp. 623-646. Chichester: Wiley.