Bryozoan growth forms

Bryozoans exhibit an impressive range of colony form, a fact noted and much used by non-bryozoologists (as well as bryozoologists), particularly paleontologists who have sought to correlate colony form with environment in fossil assemblages in which bryozoans have been found in rock-forming abundance. Colonies can be two-dimensional encrusters (the majority), or, through frontal budding and/or self-overgrowth, these can become mounded (well exemplified by species of Celleporaria) or erect. Hence, erect bryozoans may be firmly fixed to the substratum (like lace corals of the family Phidoloporidae), but many are not, having rootlike rhizoids that attach them to the substratum. Owing to the fact that rhizoids can spread out, and in a few cases even become finely divided and attach to sand grains, some species are adapted to live on soft sediments. Some soft-sediment dwellers can be tiny conical forms, scarcely more than a millimetre in size and may have only a single anchoring rootlet. In some others, the rootlet is a robust tube that anchors a large 'platey' colony above the sediment. Bryozoan colonies may be thickly calcified, lightly calcified, or (in the case of ctenostomes) uncalcified. If erect colonies are lightly calcified they can bend in a current and are said to be flexibly erect (members of the families Bugulidae and Candidae). But well-calcified colonies can also be flexible if they are articulated by uncalcified joints (Cellariidae, Catenicellidae, Poricellariidae, Quadricellariidae, Savignyellidae, Tetraplariidae) or/and are basally rooted. (Examples of all these forms can be seen on the website in Box 25.3.)

Apart from encrusting, fixed-erect, and flexible-erect colonies, there are two other main categories. One is free living (vagrant), that is, the colony is unattached to the substratum and, owing to bristle-like avicularian mandibles around the periphery of the colony (cap- or discus-shaped), has the power of mobility (lunulariids, otionellids, and selenariids). One other distinctive category is that of shell-borer/shell-eroder (found among several families of Ctenostomata). None has yet been reported from the GBR but they must exist. Their small size, transparency, and cryptic habit render them easily overlooked, however.

For the entire GBR Province, 55% of the bryozoan species are two-dimensional encrusters, 20% are flexibly erect, and 15% are firm three-dimensional structures that are rigidly attached to the substratum. These categories of colonial morphology are very broad. They can be subdivided, and more than 20 such categories have been recognised and named (for example, encrusting colonies may be runners, spots, or sheets; erect colonies may be sticks, trees, or fronds). Such categories based on colony form potentially allow for the possibility of paleoenvironmental analysis, but it must be said that correlations between morphology and environmental conditions in the literature have typically been inferential and not backed up by experimental or ecological observations based on living colonies. For this reason, a new, integrated classification scheme based on growth-habit characteristics and the disposition of modules (zooids) has been developed to allow a testable common ground for systematic comparison of character states among varied bryozoan growth habits. It would be fruitful applying this system to the varied range of colony forms represented in the GBR Province.

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