Zooplanktivory by Praunus flexuosus (Crustacea : Mysidacea): functional responses and prey selection in relation to prey escape responses

Zooplankton feeding rates and prey selection of the mysid shrimp Praunus fl exuosus were studied in single and multiple prey species experiments. Funct ional responses of P. flexuosus were determined for 2 copepod species, Acar tia spp. (adults) and Eurytemora affinis (adult females), and 2 cladoceran species, Pleopsis polyphemoides and Bosmina longispina maritima. Feeding ra tes were highest with P, polyphemoides, intermediate with B. longispina mar itima and Acartia spp. and lowest with E. affinis. Videofilming of the prey species subjected to an artificial water flow confirmed that E. affinis di splays a stronger escape response than Acartia spp., and that cladocerans c annot withstand a water current. In a prey selection experiment with Acarti a spp. and E, affinis, P. flexuosus clearance rates of Acartia spp. were su ppressed in the presence of E. affinis, suggesting an interference effect f rom E, affinis. In another multiple prey species experiment with Acartia sp p, and P. polyphemoides, the mysids fed intensively on cladocerans, regardl ess of whether the cladocerans occurred alone or with copepods. In contrast , predation on Acartia spp. significantly declined when cladocerans were of fered as alternative food (at high concentration). We suggest that P, flexu osus can capture cladocerans by creating a suspension feeding current, wher eas the evasive copepods need to be perceived and attacked individually. At low prey concentration, P. flexuosus feeds opportunistically by using both feeding modes, and the apparent 'selectivity' mainly reflects the escape c apabilities and morphological defences of the prey species; at high prey co ncentration, P, flexuosus may show true selection by switching from omnivor ous feeding to suspension feeding on cladocerans.