Feeding selectivities of the marine cladocerans Penilia avirostris, Podon intermedius and Evadne nordmanni

We conducted grazing experiments with the three marine cladoceran genera Penilia, Podon and Evadne, with Penilia avirostris feeding on plankton communities from Blanes Bay (NW Mediterranean, Spain), covering a wide range of food concentrations (0.02-8.8 mm(3) l(-1), plankton assemblages grown in mesocosms at different nutrient levels), and with Podon intermedius and Evadne nordmanni feeding on the plankton community found in summer in Hopavagen Fjord (NE Atlantic, Norway, 0.4 mm(3) l(-1)). P. avirostris and P. intermedius showed bell-shaped grazing spectra. Both species reached highest grazing coefficients at similar food sizes, i.e. when the food organisms ranged between 15 and 70 mum and between 7.5 and 70 mum at their longest linear extensions, respectively. E. nordmanni preferred organisms of around 125 mum, but also showed high grazing coefficients for particles of around 10 mum, while grazing coefficients for intermediate food sizes were low. Lower size limits were >2.5 mum, for all cladocerans. P. avirostris showed upper food size limits of 100 mum length (longest linear extension) and of 37.5 mum particle width. Upper size limits for P. intermedius were 135 mum long and 60 mum wide; those for E. nordmanni were 210 mum long and 60 mum wide. Effective food concentration (EFC) followed a domed curve with increasing nutrient enrichment for P. avirostris; maximum values were at intermediate enrichment levels. The EFC was significantly higher for P. intermedius than for E. nordmanni. With increasing food concentrations, the clearance rates of P. avirostris showed a curvilinear response, with a narrow modal range; ingestion rates indicated a rectilinear functional response. Mean clearance rates of P. avirostris, P. intermedius and E. nordmanni were 25.5, 18.0 and 19.3 ml ind.(-1) day(-1), respectively. Ingestion rates at similar food concentrations (0.4 mm(3) l(-1)) were 0.6, 0.8 and 0.9 mug C ind.(-1) day(-1).