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BRYOZOA

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(Ehrenberg, 1831 )

Sections
  • Background
  • General Description
  • Development
  • Zoogeography
  • Background

    Bryozoa, along with the phoronids and brachiopods, are sometimes grouped together as lophophorates, based on the shared presence in the adults of a specialized feeding structure called a lophophore, an extension of the body wall into a tentaculate feeding structure that surrounds the mouth. There are approximately 300 European bryozoan species and around 5000 worldwide, with thousands still to be described. The majority are marine, although there are brackish-water and freshwater forms. Most marine species are found in shallow waters, where they can be very abundant, but some occur to abyssal depths. The adults are filter feeders, almost exclusively sessile and colonial, although pelagic colonial species are recorded.
     
    Colonies are composed of a few to many million individuals, forming gelatinous or firm, encrusting mats, or raised coral-like structures on rocky surfaces, shells, sediments, algae, piers etc. The colonies have an economic impact as fouling organisms on ship hulls. Colonies can range from millimetres to metres in size, but the individuals that make up the colonies are rarely larger than a millimetre. Their appearance has stimulated several common names including ‘sea mats’, ‘moss animals’ or ‘lace corals’. Most have skeletons of calcium carbonate and in some regions fossil bryozoa may form an abundant component of limestone. Their larvae can be common in inshore plankton samples.
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    General Description

    There are about 5000 living species of Bryozoa (sometimes called Ectoprocta). They, along with the phoronids and brachiopods, are grouped together as lophophores, based on the presence of a specialized feeding structure called a lophophore, an extension of the body wall into a tentaculate structure that surrounds the mouth. The majority are marine, although there are brackish-water and freshwater forms. The adults are filter feeders, almost exclusively sessile and colonial. The colonies are composed of a few to many million individuals, forming gelatinous or firm, encrusting mats, or raised coral-like structures on rocky surfaces, shells, sediments, algae, piers etc. They have an economic impact as fouling organisms on ship hulls. Colonies can range from millimetres to metres in size, but the individuals that make up the colonies are rarely larger than a millimetre. Their appearance has stimulated several common names including ‘sea mats’, ‘moss animals’ or ‘lace corals’. Their skeletons are of calcium carbonate and in some regions fossil bryozoa may form an abundant component of limestone. Most are found in shallow waters, but some occur to depths of >8000 m.
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    Development

    Bryozoans can reproduce both asexually and sexually and are all hermaphroditic, although exact reproductive strategy can vary between species. Asexual reproduction occurs by budding and is the primary way by which a colony expands in size. Sexual reproduction can be by release of eggs and sperm directly into the water column, but most species fertilize and brood their eggs within the colony. The eggs of non-brooding species are small and the cyphonautes larva that emerges (Figure 1A, B) is considered to be the most primitive type of bryozoan larva. They filter-feed on phytoplankton and small particulates and increase in size during larval life. Brooded eggs are generally produced in lower numbers and are larger and heavily yolked. The larval types emerging are either coronate larvae (Figure 1E) or pseudocyphonautes (Figure 1F), both of which are non-feeding. Bryozoan colonies are typically formed from a single founding individual, the ancestrula, and result when a sexually produced larva settles, metamorphoses and commences asexual reproduction.Identification of bryozoan larvae is a specialised task, so only some examples are given here, but a key to the identification of some cyphonautes larvae is given in Hayward & Ryland (1998).

    Cyphonautes larvae: Cyphonautes larvae (Figure 1A, B) can be found in plankton samples throughout the year and are widely recognised as being the larvae of bryozoans, but actually only occur in a few genera. The larva is compressed laterally between two chitinous shell valves, roughly triangular in outline, held together by an adductor muscle. At their apex the valves are cut away and often flared, to accommodate the apical sense organ (Figure 1C). At their base the valves gape (Figure 1D), the opening encircled by the locomotory organ, the ciliary corona (Figure 1A). There is an internal chamber termed the vestibule, divided by two lateral ciliated ridges into an anterior inhalent chamber and a posterior exhalent one, through which water is drawn by cilia and filtered. The valves can be transparent or coloured and the surface may be granulated. The internal organs usually become shrunken and disrupted during preservation (Figure 1B). The pelagic phase generally lasts around 4-8 weeks. Size:Cyphonautes larvae ~0.2-0.7 mm total length. 

    Figure 1A-F. Bryozoan larvae

    Coronate and pseudocyphonautes larvae: Coronate and pseudocyphonautes larvae typically spend only a few hours in the plankton, so are much less likely to be sampled than cyphonautes larvae. Coronate larvae (Figure 1E) are usually spherical, but may be elongated or shortened along the median axis, so have a much more variable body plan. They all bear a tuft of apical cilia, a posterior tuft of plume cilia and a well marked corona of short cilia that can cover the majority of their surface. Pseudocyphonautes larvae (Figure 1F) have paired shells with a flattened oral surface surrounded by a ciliated corona. They are bean-shaped in lateral view, with an apical sense organ opposite the corona.

    Further information: Atkins, 1955;Temkin & Zimmer, 2002; Larink & Westheide, 2006; Ryland, 1965, 1970; Todd et al., 1991; Zimmer & Woollacott, 1977).

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    Zoogeography

    Particularly coastal waters, but widely distributed.

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