Morphological observations of the marine planktonic dinoflagellate Phalacroma turbineum Kofoid et Michener (Dinophyta), with discussion on its taxonomy and distribution

The marine planktonic thecate dinoflagellate Phalacroma turbineum Kofoid et Michener was found at two oceanic stations in the southern Gulf of Mexico and studied by light and scanning electron microscopy. This species is extremely rare and has not been reported since 1911. The terms reticules and septa are introduced here for structures of theca and cingulum of the species. The peculiar morphological characters of P. turbineum are described: (1) two separate pores that might represent flagellar pores, (2) right and left sulcal lists are united, (3) left sulcal list showed no apparent ribs, (4) a parasagittal list occurs in the hypotheca as continuation of the left sulcal list, (5) structural complexity of the cingulum, with septa along the cingulum, (6) strong ribs supporting the two cingular lists, (7) large and spaced reticules in the theca, with elevated ridges, and (8) an elongation of the hypotheca. No chloroplasts were detected. The morphology and taxonomy of P. turbineum are discussed, especially the possibility that the species may be considered as belonging to a new genus. Finally, although information is yet limited, we suggest that the species is distributed in equatorial and tropical zones of both the Atlantic and Pacific Oceans, in oceanic environments.