Zooplankton feeding ecology: bacterivory by metazoan microzooplankton

Bacterivory by the rotifer Brachionus plicatilis Müller, nauplii and copepodites of the copepods Centropages Krøyer sp. and Acartia tonsa Dana, and the tintinnid Favella panamensis Kofoid & Campbell was examined using fluorescently labelled bacteria (FLB) and epifluorescence microscopy. FLB were < 1 μm in diameter, and were offered at environmental concentrations (1.47?9.08 × 10⁶ cells·ml^?1). FLB were visible within rotifers, nauplii, copepodites, and tintinnids, confirming ingestion. Rotifer clearance rates (32–418 μl·animal^?1·h^?1) exhibited no relation with FLB concentration. In some cases rates of clearance of FLB by rotifers were different with alternative phytoplankton food (Nanochloris Naumann sp.) than in replicates with FLB alone, whereas in other cases presence of alternative food exhibited no clear effects on rates of ingestion of FLB. Clearance rates for all six naupliar stages of A. tonsa nauplii (0–320 μl·animal^?1·h^?1) were stage-related, with higher rates by NIII-VI nauplii than NI-II nauplii. Nauplii had higher rates of clearance of FLB in the absence of alternative phytoplankton food (Isochrysis Parke sp.). Clearance rates of FLB by a single stage of Centropages sp. nauplii, A. tonsa CI copepodites and F. panamensis (each obtained at only a single food concentration of either 1.5 or 5.0 × 10⁶ cells·ml^?1) were within the range of 85–142 μl·animal^?1·h^?1. These ranges were similar to those of rotifers and A. tonsa nauplii. This is the first report of FLB ingestion by metazoan marine microzooplankton. Although rotifers and ciliates might be expected to ingest small particles such as FLB using ciliary induced feeding currents, the means by which nauplii and copepodites eat FLB is less clear. We propose that they may “eat” bacteria as they “drink” to osmoregulate.