Statolith diameter as an age indicator in the planktonic tunicate Oikopleura vanhoeffeni: Variability in age-specific growth patterns in Conception Bay, Newfoundland

We explored the use of statolith diameter as an age indicator for the larvacean Oikopleura vanhoeffeni. Laboratory studies indicated that variability in statolith diameter-at-age is substantially lower than that of trunk length-at-age, and that variability in statolith diameter-at-age remains constant as statolith diameter increases with age, while variability in trunk length-at-age increases with age. These results suggest that statolith diameter is the better indicator of age at all body sizes and ages. Using statolith diameter as a proxy for age, trunk length-at-age of O. vanhoeffeni was observed in Conception Bay, Newfoundland, for two years. Seasonal variation in trunk length-at-age was observed in older age groups, and this variability increased with age, suggesting that body size is not a reliable indicator of age in field populations. Length-at-age increased in older individuals during the spring phytoplankton bloom, suggesting variation of age-specific growth in relation to food availability. This new method to determine age of animals in the field provides opportunities to define age structure and age-specific life history characters that are essential for understanding the population dynamics of larvaceans.