Zooid size and colony growth of the estuarine bryozoan Coilopeum seurati (Canu) (order: Cheilostomatida; suborder: Malacostegina) were examined over 15 ma at Avonmouth Dock, Avon, England. Data were analysed in conjunction with synchronous measurements of temperature, salinity and food availability. Zooid length, width and area were strongly temperature-dependent, while both food availability and colony growth rate had no significant effect on zooid length, width or area. Salinity and the interaction of temperature and-salinity significantly influenced zooid length and area, suggesting that changes in zooid size may result from oxygen limitation in warm waters. The validity of a number of other mechanisms proposed to account for temperature-related changes in zooid size is discussed. The results support the use of zooid size as an indicator of both long-term trends and seasonal variations in temperature in Recent and fossil assemblages as long as data sets are large and the effects of other factors on zooid size: are considered. Colony growth rate was found to be significantly influenced by both the amount of food available to the colonies and the combined effect of temperature and food availability, suggesting that growth rate increases as food increases, but that the former may be limited at low temperatures when metabolic rates are low.
