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

Gut content analyses on field-caught Aurelia aurita showed both quantitativ e and qualitative change in diet as a function of medusa size. Larger medus ae tended towards greater numbers and diversity of prey (up to 1550 individ ual prey representing as many as 13 different prey groups). We also found t hat medusa size was a good predictor of prey diversity recovered from the m edusa gut. While a shift toward greater prey diversity in larger medusae mi ght 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 rep resenting varying degrees of swimming or escape velocity. Fish eggs were us ed as a non-swimming prey, and small (< 1 mm) and large (> 1 mm) copepods w ere used as slow and fast swimming/escaping prey, respectively. Size-depend ent increases in selectivity of small copepods can be explained by increase d capture efficiency by larger medusae. Selectivity indices of fish eggs an d large copepods did not vary with medusa size. Based on these data, we arg ue that growth to a large size by scyphomedusae can provide a means to enha nce diet which, in turn, may positively influence growth and reproduction.