Higher ingestion rates and importance of ciliates in the diet of a large, subarctic copepod revealed by larger volume incubations

Despite ecological importance, the feeding ecology of the large calanoid copepod, Neocalanus cristatus is little known, as its ingestion rate under experimental conditions consistently falls short of its metabolic demand. The conventional incubation bottle size (similar to 2 liters) used in previous studies may have affected its feeding behavior. Here, we examined the effect of bottle size (2.4 vs. 13liters) and copepod density on the feeding of N. cristatus Stage C5. Under similar copepod density (0.4 ind. L-1) ingestion rates decreased significantly for tests conducted in small bottles and thus, small bottles inhibited copepod feeding. In experiments with large bottles, ingestion rates were highest in the single-individual treatments (1 ind. 13 L-1), decreasing exponentially to < 1% of the maximum ingestion rate with increasing copepod densities (up to 20 ind. 13 L-1). Considerable food limitation occurred in the higher copepod density treatments with lower initial prey allocation per individual. Moreover, in the single-individual treatments, clearance rates on naked ciliates (11-20 L ind.(-1) d(-1)) were remarkably higher than in multiple-individual treatments. Grazer-grazer interaction, thus, interfered with copepod feeding on microzooplankton in multiple-individual treatments. The ingestion rate in the single-individual treatments (62-102 mu gC ind.(-1) d(-1)) exceeded the daily metabolic requirement; feeding rates noted in previous studies were, thus, underestimations, affected by bottle size and copepod mutual interference. Efficient feeding by N. cristatus on naked ciliates emphasizes the importance of the ciliate-copepod link in the grazing food chain in the subarctic Pacific.