Interpreting seawater temperature range using oxygen isotopes and zooid size variation in Pentapora foliacea (Bryozoa)

This is the first investigation of how two independent proxies for seawater temperature inference (zooid size variation and oxygen isotope ratios of skeletal carbonate) relate to the actual measured ranges of temperature experienced by cheilostome bryozoan colonies. Nine specimens of the bimineralic marine cheilostome bryozoan Pentapora foliacea (Ellis and Solander, 1786) were analysed, collected from ~18-m depth at two localities in Wales, UK—four from Skomer Island (51°42′510″N, 5°13′42.60″W) and five from Porth Ysgaden, Lleyn Peninsula (52°54′6.75″N, 4°38′47.34″W). The annual range of temperature implied by zooid size variability provides a good approximation of the actual range of temperature recorded by a datalogger. However, annual ranges of temperature reconstructed from skeletal oxygen isotope ratios were narrower, typically not showing the lowest temperatures experienced by the colonies. This can be explained by progressive thickening of zooid skeletal walls during the life of the colony that homogenises the temperature signal by time-averaging over the lifetime of the colonies. Our study provides evidence that a combined morphological isotope approach has great potential in the reconstruction of annual ranges in seawater temperatures from historical and fossil bryozoans, particularly for species that lack ontogenetic skeletal wall thickening and bimineralic skeletal composition. As cheilostome bryozoans have been common in benthic communities since the Late Cretaceous, they represent a valuable and underutilised resource for the interpretation of environmental regimes

Effects of temperature, size, and food on the growth of Membranipora membranacea in laboratory and field studies

In the rocky subtidal ecosystem of the western North Atlantic outbreaks of the introduced epiphytic bryozoan Membranipora membranacea cause defoliation of kelp beds and facilitate the introduction of other non-native benthic species. We quantified size- and temperature-dependent growth rates of M. membranacea colonies in the field and the laboratory for durations of 8-23 days. Also, we examined the interaction between food abundance and temperature on growth rates of newly settled colonies in the laboratory. Growth rates were positively related to temperature and increased exponentially with size of colonies over the ranges examined (5.7-16.2°C and 0.5-192 mm, respectively), and were significantly higher in the field than in the laboratory. There was an interactive effect between food and temperature on the size and growth rates of colonies, with the most pronounced effects of food limitation on colonies grown at the warmest temperatures, and no effect of food on colonies grown at the coldest temperatures. Quantifying the growth rates of introduced species is essential to understanding their population dynamics, particularly when outbreaks can have severe impacts on the native community