GROWTH AND GRAZING RATES OF PROTOPERIDINIUM-HIROBIS ABE, A THECATE HETEROTROPHIC DINOFLAGELLATE

Growth and feeding rates of a laboratory-reared small thecate heterotrophic dinoflagellate, Protoperidinium hirobis Abe, grown on the diatom Leptocylindrus danicus, were measured in batch cultures. Ingestion rates were determined directly by the enumeration of empty diatom frustules produced by dinoflagellate feeding. Both growth and feeding rates saturated at diatom concentrations of approximately 10(4) cells ml-1, and reached maximum values of 1.7 divisions day-1 and 23 diatoms grazer-1 day-1, respectively. This rate of cell division is notably high compared to photosynthetic dinoflagellates, which seldom grow faster than 1 division day-1. A maximal clearance rate of 0.5 mul h-1 was measured. Mean cell size varied proportionally with food abundance, with food-saturated cells having double the mean volume of food-depleted cells. Timing of cell division and grazing rate patterns were also examined; while mitosis occurred chiefly during the dark period, no diel variations in feeding rate were detected. These rates represent the first direct growth and ingestion measurements to be made for a thecate heterotrophic dinoflagellate. They serve to underscore one function these dinoflagellates perform within the microzooplanktonic food web: that of transforming large diatoms into particles more easily ingested by microzooplankters.