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Bugula calathus is an erect bryozoan that forms cup-shaped colonies, up to about 3 cm in diameter. The colonies are made up of a number of flat dichotomous branches that arise from a central base and spread out regularly on all sides to form the shallow cup. Bugula calathus appears straw coloured when living and when dried it appears a yellow horn coloured, in contrast to the ash-grey of the dried Bugula flabellata, for which the species is often mistaken.
The species is able colonise stones and other bryozoan species, and ranges from the intertidal zone to shallow subtidal waters. Small modified zooids which resemble rootlets (rhizoids) are used to attach to the colony to the substrate.
The species has been recorded from the south-west of Britain, extending north to Bardsey Island in the Irish Sea, and southwards into the western Mediterranean.
There are two subspecies of Bugula calathus: Bugula calathus calathus, which is described above, and Bugula calathus minor which occurs in the eastern Mediterranean and on the coast of Africa.