Evaluating potential factors influencing branch diameter and skeletal Mg-calcite using an Antarctic cyclostome bryozoan species

Environmental and biological factors play important roles in determining the skeletal morphology and mineralogy of modular colonial organisms such as bryozoans. The broad bathymetrical ranges of Antarctic bryozoans make them useful organisms for evaluating depth-related changes in colony morphology, including branch diameter, which has been shown to decrease with depth in various ramose colonial animals. The current study focuses on the bryozoan Fasciculipora ramosa, using 32 specimens collected at East Antarctica from 185 to 597 m deep during the CEAMARC cruise (2007–2008). In order to test for the expected inverse relationship between depth and branch diameter, measurements of branch diameters were made. Levels of Mg in the calcite skeleton were determined as this was also predicted to follow changes in branch diameter. A significant negative correlation was found between branch diameter and depth. No significant relationship was detected between branch diameter and skeletal Mg, suggesting that these variables are driven by different factors. The thickest branches were found in current-influenced sites on Adelie Bank where temperature and salinity are both lower and planktonic food supply may also be different from the other sites. Further studies are needed along a wider bathymetrical range to clarify which factors influence branch diameter as well as Mg content in F. ramosa and to test its potential as an environmental indicator.