BRYOZOANS

From Eldredge (1991)


INTRODUCTION

The Bryozoa (moss animals) are a geologically important group of small animals; some that superficially resemble corals. All bryozoans are colonial and most are marine. Bryozoans are most abundant in temperate-tropical waters that are not too turbid. They require a hard or firm substrate on to which the attach to or encrust, and clear agitated water from which they obtain their suspended food.

Enclosed within a skeleton of calcite, bryozoans have a sac-like coelomate body with a well defined mouth, anus, and other specialized organs. One such organ is the lophophore (a ciliated structure used in food gathering) that is attached to tentacles that surround the mouth (see Figure 1 below). The lophophore is a structure shared by the phylum Brachiopoda leading some to construct the Phylum (or Superphylum) Lophophorata to include both brachiopods and bryozoans. The classification follows your text in treating the Bryozoa and Brachiopoda as separate phyla.


Figure 1 - Section of a Bryozoan Feeding Zooid

From Boardman et al (1987)


CLASSIFICATION AND GEOLOGIC RANGES

Phylum Bryozoa

Class Stenolaemata (Ordovician-Recent)

Order Trepostomata (Ordovician-Triassic)

Order Fenestrata (Ordovician-Permian)

Order Cyclostomata (Ordovician-Recent)

Class Gymnolaemata (Ordovician-Recent)

Order Cheilostomata (Jurassic-Recent)

 


GENERAL MORPHOLOGY AND ECOLOGY

Morphologic Terms (see Figures 1 & 2)

Zooid: Individual animal or member in a bryozoan colony.

Zooaria: The colony of bryozoan animals.

Zooecia (Zooecium singular): Living chambers constructed by a colony of zooids. The zooecium is the living chamber constructed by one individual.

Autopore: Zooecium for the feeding zooid called autozooid which is usually the largest of the various zooecia.

Ancestrula: The ancestral founding zooecium from which other zooecia in the colony bud.

Diaphragm: A partition in a tubular zooecium, transverse to tube length similar to the tabulae in some corals.

Aperture: The opening through which the living animal could extend from its zooecium.

Operculum: Small disk-like cover or lid of an aperture, commonly found in cheilostomes.

Frontal: In zooecia with considerable wall area exposed at the colony surface, a frontal is the exposed part of any one zooecium.

Immature and Mature regions: In trepostome bryozoans, the distal or last formed (most recently grown) end of a zooecium has close-spaced diaphragms and is called the immature area. The proximal end (oldest part) of the zooecium has few diaphragms and is called the mature region.


Figure 2 - Section of a Colonial Bryozoan Zooecia

From Boardman et al (1987)


General Morphology and Ecology

As bryozoan individuals are quite small, they are commonly observed under the microscope from longitudinal or transverse thin-sections. A longitudinal orientation is parallel to zooecium wall, whereas a transverse section is perpendicular to apertural face.

Bryozoans come in a variety of colonial growth habits that can easily be observed without thin-sections. Like corals, the growth habit of bryozoans can be classified as encrusting, - massive or domal, or erect. - Generally speaking, byozoan growth-habits are a function of water energy similar to corals which lived in level-bottom communities; encrusting and massive forms are found in high-energy environments whereas delicate branching and erect forms lived in quite environments.


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CLASSIFICATION


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