Bryozoans, or "moss animals," are filter-feeding, colonial animals that superficially resemble plants rather than animals. They have formed a significant part of the marine benthos since the Ordovician. It is the only phylum in which all known species are colonial. A colony consists of very small individuals called zooids that are physically connected. They grow by budding from a single individual called the ancestrula. Around 20,000 species are recognized, most of them from fossil record.
Some bryozoans encrust rocky surfaces, shells, or algae. Others form lacy or fan-like colonies that in some regions may form an abundant component of limestone. Bryozoan colonies range from millimeters to meters in size, but the individuals that make up the colonies are rarely larger than a millimeter.
Bryozoans are considered nuisances by some: over 125 species are known to grow on the bottoms of ships, causing drag and reducing the efficiency of the ships. Certain freshwater species occasionally form great jelly-like colonies so huge that they clog public or industrial water intakes. Yet bryozoans produce a remarkable variety of chemical compounds, some of which may find uses in medicine. One compound produced by a common marine bryozoan, the drug bryostatin 1, is currently under serious testing as an anti-cancer drug.
Organism: The bryozoan animal is called zooid. Zooids within a colony can differ distinctly in morphology and function. Some, if not all, are feeding colonies. However, many colonies possess polymorphic zooids that differ morphologically and perform specialized functions, including reproduction, colony support, cleaning and defence.
Feeding zooids have a fluid-filled body cavity (coelom), a protrusible tentacle-bearing feeding organ called the lophophore, a U-shaped digestive tract, muscles, a nervous system, and tissues attaching the digestive tract to the body-wall. Eggs and sperms-producing organs are present in some feeding and non-feeding zooids in all colonies.
The lophophore consists of a ring of hollow tentacles and a supporting tentacle sheath. The sheath encloses the tentacles when they are retracted, and turns inside out for support when they protrude for feeding. The tentacles are covered with cilia that produce currents directed toward the mouth centered at the base of the tentacular ring. The mouth opens into the U-shaped digestive tract which ends at an anus on the side of the protruded tentacle sheath below the ring. Protrusion of the tentacles involves exerting hydrostatic pressure on the fluid of the body cavity. This pressure is produced in various ways, usually by muscles modifying the shape of the body cavity. Other muscles contract to bring about retraction of the tentacles.
Reproduction in Bryozoa occurs sexually by the union of sperms and eggs; the fertile eggs change into larvae which escape outwards and float in water. The larvae of marine bryozoa are ciliated and planktonic. After settling, a larva undergoes extensive reorganization of tissues to produce the first zooid, which is called the ancestrula of the colony. This first zooid then buds asexually to produce the succeeding individuals (colony).
Skeleton:
The skeleton of bryozoa is external and is made up of CaCo3, usually calcite, sometimes aragonite or both. The skeleton of one individual is called Zooecium and that of the colony is called Zoarium.
Ecology:
Bryozoans are common in most shallow marine environments. They can also colonize deeper water and a few live in fresh water. They are rock-formers, sometimes to a significant extent, for example during the Carboniferous period. They are able to colonize most substrates, but prefer hard substrates. In modern seas, bryozoans are the most important carbonate producers on the southern Australian and
Classification and Taxonomy
Modern classifications of the Bryozoa recognize three classes within the phylum. The marine species are grouped into the classes Stenolaemata and Gymnolaemata. They are distinguished primarily by features of their soft-anatomy. The third class, Phylactolaemata, includes only freshwater species. No hard parts are mineralized by species in this class, but they have left a fossil record of imprints and tubes in deposits of Tertiary age.
Class Stenolaemata
The stenolaemates are highly calcified bryozoa with zooids living in tubes that grow throughout the life of the colony. This class includes four orders:
Order Trepostomata
These are the so-called "stony" bryozoans of the Early Paleozoic. Most formed massive to dendroid calcareous colonies. The feeding zooids were housed in long, circular to polygonal tubes in which transverse partitions, called diaphragms, are present. Trepostomes appeared late in the Early Ordovician and diversified rapidly to a maximum in the Middle and Late Ordovician. They were the most abundant fossil group in some Ordovician beds. They declined markedly to two or three genera in the Triassic, when the trepostomes became extinct.
Order Cryptostomata
These form delicate, less highly-calcified colonies than the trepostomes. They are characterized by short zooidal tubes in which the aperture is hidden or obscured. Diaphragms are commonly absent. A hemiseptum, a short plate that extends part way across the aperture at the top of the zooidal tube, may be present. These bryozoans range from the Ordovician into the Permian.
Order Fenestrata
The zooidal tubes in this order are similar to those in the Cryptostomata, although the chambers for feeding zooids are even smaller and are circular in cross-section. Colony form is sheet-like or fanlike in many species. Fenestrates appeared in the Early Ordovician; their maximum abundance was reached in the Lower Carboniferous and they became extinct during the Triassic.
Order Cyclostomata
Cyclostomes have plain rounded apertures, thin zooidal walls, and usually an absence of diaphragms. Colony form is variable. Most tube walls have very small communication pores. Cyclostomes first appeared in the Ordovician and formed small, insignificant colonies throughout the Paleozoic. During the Cretaceous, the Cyclostomata underwent an evolutionary explosion. The number of genera recognized quadrupled and the order reached maximum diversity. Cyclostomes declined at the end of the Cretaceous, with the number of genera dropping from about 175 to 50, a number that has remained more or less constant to the present. This is the only order of stenolaemates known from Jurassic and younger rocks.
Class Gymnolaemata
The gymnolaemates are generally less heavily mineralized and their zooids grow elegant boxes of fixed size. They occur as flat encrusting colonies dependent upon the substrate. The Class Gymnolaemata includes two orders.
Order Ctenostomata
Ctenostomes generally lack skeletons and are usually preserved as borings on calcareous substrates such as shells, or as encrustations. One genus is free-living. Colonies immersed in shells commonly consist of widely-spaced feeding zooids that are connected by tubular stems, termed stolons. The order ranges from the Upper Ordovician to the present. There is about 50 genera, most of which are known only from living specimens.
Order Cheilostomata
Most cheilostomes have box-like colonies and an aperture closed by an operculum (lid). The growth habit includes encrusting to erect, shrub-like forms. Cheilostomes appear first in Late Jurassic rocks with only a few taxa, but the group radiated markedly in the mid-Cretaceous.
Bryozoan evolution:
Stenolaemates evolved to their greatest diversity during the Paleozoic. They were severely affected by the end-Permian mass extinction event and most orders were extinct by the end of the Triassic. Only Cyclostomata survived into the Mesozoic and radiated to a great abundance, especially during the Cretaceous. Many genera of cyclostomes became extinct at the end of the Cretaceous, but a few survive to the present day.
Gymnolaemates are known from the Ordovician, they are divided into two orders: the minor Ctenostomata which have been a small element of bryozoan fauna throughout the phanerozoic, and the hugely diverse order Cheilostomata. This group first appeared in the Jurassic and have come to dominate bryozoan assemblages.
Bryozoa as environmental indicators
Bryozoans are useful as environmental indicators since they are sensitive to environmental changes and their colony shape is largely controlled by environmental characteristics. In post-Paleozoic rocks and in the modern oceans, bryozoans are dominant members of shallow benthic communities in temperate latitudes, with normal salinity and low to moderate rates of sedimentation. They are abundant in modern oceans at water depth of 40-90 m, and in sedimentation rates of less than 100 cm per thousand years.
Colony shape may also be used as environmental indicators for example encrusting forms (thin laminae on dead shells and hard objects) are common in shallow water. Erect forms (tree-like branches) are common in deep water. Free-living mobile forms (which can roll themselves on sea floor) are typical of sandy sea beds, where the bryozoan colony needs to be able to respond to a mobile substrate.
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