Size-based prey selectivity and dietary shifts in the jellyfish, Aurelia aurita

Gut content analyses on field-caught Aurelia aurita showed both quantitative and qualitative change in diet as a function of medusa size. Larger medusae tended towards greater numbers and diversity of prey (up to 1550 individual prey representing as many as 13 different prey groups). We also found that medusa size was a good predictor of prey diversity recovered from the medusa gut. While a shift toward greater prey diversity in larger medusae might be explained by increased contact rates with 'rare' prey taxa, we also found size-based prey selectivity changes in A. aurita. We used in situ gut content data to describe selectivity by A. aurita for three prey types representing varying degrees of swimming or escape velocity. Fish eggs were used as a non-swimming prey, and small (< 1 mm) and large (> 1 mm) copepods were used as slow and fast swimming/escaping prey, respectively. Size-dependent increases in selectivity of small copepods can be explained by increased capture efficiency by larger medusae. Selectivity indices of fish eggs and large copepods did not vary with medusa size. Based on these data, we argue that growth to a large size by scyphomedusae can provide a means to enhance diet which, in turn, may positively influence growth and reproduction.