GROWTH-CHECK-BASED GROWTH RATES AS CLIMATE PROXIES IN ERECT CHEILOSTOME BRYOZOANS FROM THE SOUTHERN OCEAN

Bryozoans, as colonial marine animals, provide a unique method for recording environmental change. Growth rate can be measured in bryozoans using annual winter growth checks. Growth checks are visible constrictions in colony width analogous to tree rings. This study determines the spacing of winter growth checks in three erect Southern Ocean cheilostome bryozoan species. The three species are Cellarinella nutti, Melicerita chathamensis, and M. obliqua which were collected from the sea floor from 148-500 m depth at 48-77°S. We used X-ray and SEM images to measure branch width, autozooid length and width, and autozooid count for each zooid generation up through the colonies across multiple growth checks. Mean annual growth rate was determined to be 3.63 mm/yr for C. nutti, 3.68 mm/yr in M. chathamensis, and 3.21 mm/yr in M. obliqua. As expected, M. chathamensis had the fastest growth rate while living in the warmest water as inferred from depth and latitude. M. obliqua had the slowest growth rate but lived at intermediate depth and latitude. These data when combined with results from previously published studies reveal a negative correlation between growth rate and latitude so that species living in higher latitudes in colder waters have slower growth rates. We also found a negative correlation between growth rate and water depth so that species living in deeper colder waters also have slower growth rates. This indicates that erect bryozoan growth rates are related to temperature and may be useful as a proxy for temperature.