The utility of statoliths and bell size to elucidate age and condition of a scyphomedusa (Cassiopea sp.)

Scyphomedusae play important roles in marine ecosystems and are of economic significance. However, no reliable techniques for estimating scyphomedusa age have been documented. This study focused on the utility of Cassiopea sp. (Cnidaria: Scyphozoa) statoliths, statocysts, and body size as proxies for age of medusae. Reared medusae of known age and a manipulative experiment were used to assess the accuracy and reliability of four measures of age: number of statoliths, size (diameter) of statoliths, area of statocyst (housing statoliths), and bell diameter. Bell diameter provided the most accurate measure of age under constant conditions, but was increasingly inaccurate under varying environmental conditions. In contrast, the average number of statoliths per medusa reflected age with relatively low accuracy, but did not vary with changes in food availability and salinity. Only temperature influenced the average number of statoliths. Comparisons of bell diameter to the number of statoliths in medusae under low food availability to those fed well showed that the ratio of medusa size to the number of statoliths can be used to recognise medusae that are relatively poorly conditioned. Statoliths, therefore, provide a tool for studying both population ecology and the influence of environmental variation on medusa growth.