Bryozoans are filter-feeding, colonial animals that have formed a significant part of the marine benthos since the Ordovician. They are sometimes known as moss animals and they superficially resemble plants rather than animals. They form a phylum of their own, probably most closely related to brachiopods. Around 20,000 species are recognized, most of these from the fossil record.
Bryozoan zooids are tiny (Fig. 5.1) and feed via a lophophore - an array of tentacles that extract food particles from water. Food is moved through a U-shaped gut to an anus located just outside the ring of tentacles making up the lophophore. Respiration occurs by diffusion as the animals are so small, and they have no circulation system or gills. Bryozoans have a relatively sophisticated nerve system and a complicated series of muscles helping them to move in and out of their skeleton. Male and female characteristics can occur within the same zooid or the same colony. In addition, most bryozoan colonies have a range of specialized zooids that clean or protect the colony, or are dedicated to breeding. The strangest of these specialized zooids are the avicularia. These have highly adapted, toughened areas that are usually used in defense or cleaning, but in some cases can be used as stilts on which the colony "walks".
All of the individuals in a bryozoan colony are genetically identical, regardless of their degree of specialization. They grow by budding from a single individual, called the ancestrula. The colony is built of gelatinous or fibrous protein, aragonite, or calcite, or a mixture of these materials. The shape of the colony reflects the need to feed efficiently and the demands of substrate colonization. A restricted range of shapes have evolved many times, a process known as iterative evolution. These include closely packed mats, elongate shapes, runner forms, erect tubes, disk shapes, and upright fans.
Bryozoans are common in most shallow marine environments. They can also colonize deeper water and a few live in fresh water. In reef environments they can form important sediment baffles that are full of small cavities, providing cryptic niches for a wide diversity of organisms. Bryozoans are rock formers, sometimes to a significant extent, for example during the Carboniferous period. They are able to colonize most substrates, but prefer hard surfaces. In modern seas, bryozoans are the most important carbonate producers on the southern Australian and New Zealand shelves.
Marine bryozoans are divided into two classes, Stenolaemata and Gymnolaemata. The stenolaemates are highly calcified forms, with zooids living in tubes that grow throughout the life of the colony. Gymnolaemates are generally less heavily mineralized, and their zooids grow elegant boxes of fixed size. The stenolaemates were dominant in the Palaeozoic and for most of the Mesozoic, until the late Cretaceous. Since then one group of gymnolaemates, the cheilostomes, have dominated bryozoan faunas.
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