A new method using fluorescent microspheres to determine grazing on ciliates by the mixotrophic dinoflagellate Ceratium furca

Feeding in the mixotrophic dinoflagellate Ceratium furca was investigated using 1 mu m fluorescent microspheres to label prey. Microspheres added to natural plankton assemblages at a concentration of 5 x 10(6) to 10(7) ml(-1) were rapidly ingested by a variety of planktonic organisms including small flagellates, ciliates, dinoflagellates, ebriids and amoebae. Prey thus labeled were in turn phagocytized by C, furca and were easily detected within the predator using fluorescent microscopy. Ingestion rates were calculated by following the appearance of labeled food vacuoles in C. furca over time. C. furca did not ingest microspheres directly. and thus the calculated rates represented feeding on labeled prey only. Food vacuole contents and labeled prey were identified using a modified Protargol staining procedure that retained the integrity and fluorescent properties of the latex microspheres. Data on food vacuole content and potential prey availability indicated that C. furca preyed mainly an choreotrich ciliates (i.e. Strobilidium spp, and tintinnids) 10 to 40 mu m in diameter. In addition, feeding rates were strongly correlated with choreotrich densities and suggested that C, furca preferred ciliates of the genus Strobilidium. Ingestion rates of C. furca ranged from 0 to 0.11 prey h(-1), and varied greatly from one year to the next. Clearance rates ranged from 0 to 12.49 mu l (C. furca)(-1) h(-1), with an average of 2.1 +/- 0.41 ml (C. furca)(-1) h(-1). Both ingestion and clearance rates were comparable to rates previously reported for mixotrophic and heterotrophic dinoflagellates. The method presented here has several advantages for the study of mixotrophy in large, slow-feeding dinoflagellates. Live prey found in natural water samples are used, and surface characteristics of prey remain unaltered. No addition of prey is necessary, and only a minimum amount of handling of the sample is required. Incubation experiments can last for 6 h, allowing sufficient time for the appearance of labeled food vacuoles in predators with low feeding rates.