Trophodynamics of the scyphomedusae Aurelia aurita. Predation rate in relation to abundance, size and type of prey organism

Abstract

Ephyra larvae and small medusae (1.7-95 mm diameter, 0.01-350 mg ash-free dry wt, AFDW) of the scyphozoan jellyfish Aurelia aurita were used in predation experiments with phytoplankton (the flagellate Isochrysis galbana, 4 μm diameter, ̃6 × 10-6 μg AFDW cell-1), ciliates (the oligotrich Strombidium sulcatum, 28 μm diameter, ̃2 × 10-3 μg AFDW), rotifers (Synchaeta sp., 0.5 μg AFDW individual-1) and mixed zooplankton (mainly copepods and cladocerans, 2.1-3.1 μg AFDW individual-1). Phytoplankton in natural concentrations (50-200 μg C I-1) were not utilized by large medusae (44-95 mm diameter). Ciliates in concentrations from 0.5 to 50 individuals ml"1 were consumed by ephyra larvae and small medusae (3-14 mm diameter) at a maximum predation rate of 171 prey day-1, corresponding to a daily ration of 0.42%. The rotifer Synchaeta sp., offered in concentrations of 100-600 prey I-1, resulted in daily rations of ephyra larvae (2-5 mm diameter) between 1 and 13%. Mixed zooplankton allowed the highest daily rations, usually in the range 5-40%. Large medusae (>45 mm diameter) consumed between 2000 and 3500 prey organisms day"1 in prey concentrations exceeding 100 I-1. Predation rate and daily ration were positively correlated with prey abundance. Seen over a broad size spectrum, the daily ration decreased with increased medusa size. The daily rations observed in high abundance of mixed zooplankton suggest a potential 'scope for growth' that exceeds the growth rate observed in field populations, and this, in turn, suggests that the natural populations are usually food limited. The predicted predation rate at average prey concentrations that are characteristic of neritic environments cannot explain the maximum growth rates observed in field populations. It is therefore suggested that exploitation of patches of prey in high abundance is an important component in the trophodynamics of this species. © 1990 Oxford University Press.