Limitations of laboratory assessments of coelenterate predation: Container effects on the prey selection of the Limnomedusa, Proboscidactyla Flavicirrata (Brandt)

Abstract

Gut-content analyses of field-collected hydrozoan jellyfish, Proboscidactyla flavicirrata (Brandt, 1835), revealed that gastropod and bivalve veligers comprised 65–88% (by number) of the natural diet. In laboratory assays, however, medusae switched their prey selection with container size. Veliger larvae of the gastropod molluscs Haminaea vesicula (Gould) and Alderia modesta (Lovén) were selectively consumed in small (65 ml) and large (2000 and 4000 ml) containers, whereas nauplius larvae of brine shrimp ( Artemia sp.) were selectively consumed in intermediate sized containers (350, 650, and 1000 ml). The results were similar for both standing culture and plankton wheel experiments. Prey selectivity followed the same pattern in all light regimes, but the rate of prey consumption was lower in 14L:10D than in total darkness (OL:24D) or total light (24L:OD) treatments. Differences between the number of veligers and nauplii consumed were greater when a combination of food items was offered simultaneously than would be predicted from consumption rates of prey offered independently. This suggests that medusae may exhibit active prey selectivity. Experiments with containers of different dimensions revealed that prey selection of medusae is influenced by prey type, volume, height, and surface area to volume ratio of containers, but not inside diameter or surface area. Height was found to be the most important parameter in determining medusa prey selectivity, but the cause of selectivity remains unknown. Alteration of prey selection as a function of container size has particular significance in light of previously published studies of gelatinous predator prey selections. Laboratory studies should be conducted in large containers (container size to animal volume ratio ≈ 15000:1 or greater) in conjunction with field analyses to substantiate observed prey selectivity.