Morphology, ultrastructure and feeding behaviour ofProtoperidinium voraxsp. nov. (Dinophyceae, Peridiniales)

Protoperidinium vorax sp. nov. is a small heterotrophic dinoflagellate with a plate formula of Po, X, 4′, 3a, 7′′, 3C, 5S, 5′′′, 0p, 2′′′′. P. vorax has an ortho-hexa thecal plate arrangement. Its peculiar features are the presence of only three cingular plates and the marked extension of the second intercalary plate (2a), which abuts the second precingular plate (2′′). TEM sections revealed the presence of a large sac pusule, several convoluted vesicular structures and electron-dense accumulation bodies scattered in the cell periphery. These accumulation bodies probably represent the Periodic Acid-Schiff (PAS) bodies, which fluoresce yellow–green under blue light excitation. P. vorax is a pallium feeder and was observed feeding on diatoms of different sizes. When fed with the chain-forming diatom Skeletonema pseudocostatum under non-limiting food concentrations, doubling rates up to 1.28 divisions d−1 were recorded. Ingestion rates of 35.95 cells dinoflagellate−1 h−1 and clearance rates of 18.34 µl dinoflagellate−1 h−1 were reached, the highest ever recorded for a small-sized dinoflagellate. These data support the hypothesis that small heterotrophic species might contribute to diatom bloom decay.