Prey selection mechanism of ambush-foraging hydromedusae

The widespread occurrence and frequent abundance of small hydromedusae suggests that they may play an important role in planktonic communities. However, rather than exhibiting dominant impacts on any specific planktonic group, field studies have demonstrated diverse dietary niches and only modest trophic impacts by small hydromedusae. To understand the functional bases for these patterns, we exposed 2 hydromedusae (Cladonema californicum and Leuckartiara sp.) to a variety of prey types (dinoflagellates, rotifers, barnacle nauplii, copepods and the hydromedusa Obelia sp.) while video- recording predation sequences (encounter, capture, ingestion). Both C. californicum and Leuckartiara sp. ambush prey and possess penetrating nematocysts (stenoteles and euryteles, respectively). Although similar prey selection patterns might be expected based on encounter models or nematocyst complements, the 2 species exhibited some markedly different ingestion patterns. For example, C. californicum positively selected copepod prey and negatively selected hydromedusae, whereas Leuckartiara sp. exhibited the opposite pattern. Quantification of predation sequences demonstrated that hydromedusan dietary variations resulted from species-specific differences in prey capture efficiencies as well as efficiencies in post-capture transfer to the gut. Species-specific prey selection patterns and limited ingestion capacities may explain the diverse prey selection patterns and limited trophic significance observed in field studies of ambush-foraging hydromedusae.