Many references describe dinoflagellates as photosynthetic; this is wrong, in the same way as describing mosquitoes as feeding on blood is wrong. In terms of number of species, there are probably more non-photosynthetic than photosynthetic dinoflagellates. Protoperidinium is a genus of more than 200 species of mostly non-photosynthetic marine dinoflagellates, many of which possess a single apical horn and two antapical horns as seen in the photo above. Features distinguishing P. grande include the reticulate theca and the compressed cingular area (the cingulum is the groove around the midline; one of the dinoflagellate's two flagella wraps around the cingulum). Unlike some other Protoperidinium species, P. grande does not produce resting cysts; as a result, it is found only in warmer waters around the world. As non-photosynthetic heterotrophs, Protoperidinium species obtain their nutrition by feeding on other micro-organisms such as diatoms, cyanobacteria or other dinoflagellates. Rather than directly engulfing their prey in the way of an amoeba, Protoperidinium extend a large pseudopodial extension called a pallium from their theca's antapical pole to envelop it. The food organism is digested by the pallium, which is then withdrawn back into the dinoflagellate theca carrying a load of nutrients with it. This feeding behaviour was first 'discovered' in the late 1990s, but ironically it had actually been illustrated as long ago as 1895 with later researchers failing to recognise earlier records for what they were (Jacobson 1999*).
*Jacobson's comment on this re-discovery are worth repeating: "the brilliant, detailed observations of Kofoid and Swezy, Schütt, and Biecheler remain a humbling reminder to those of us working in a highly capitalized, high-tech environment that important work can arise from a simple light microscope, coupled with patience, luck and the appropriate search image".
Until relatively recently, Protoperidinium species were included in the genus Peridinium along with a number of freshwater dinoflagellates. The taxonomy of Recent dinoflagellates* has traditionally been dominated by a small number of what might be termed 'super-genera' of hundreds of species that between them encompass the great majority of living taxa. It is probably not surprising that phylogenetic analyses have suggested that many of these super-genera are polyphyletic, but most of those analyses have tended to return very poorly supported results and attempts to subdivide the super-genera have not been entirely successful. The division of Peridinium is one of the more successful examples, based on ecology (Peridinium sensu stricto is freshwater, Protoperidinium is marine), fine details of the arrangement of plates in the theca (Peridinium has five plates around the cingulum; Protoperidinium has four) and the features of cysts produced in some species (Dale, 1978). Molecular analyses have supported the monophyly of Protoperidinium (Yamaguchi & Horiguchi, 2005). The division on ecological grounds has been a common pattern in studies on protists; molecular analyses of a number of other micro-eukaryotic groups such as myxozoans have also produced results that contradict traditional morphological classifications but correlate strongly with ecological features.
*The taxonomy of fossil dinoflagellates is an entirely separate pot of evil.
Dale, B. 1978. Acritarchous cysts of Peridinium faeroense Paulsen: implications for dinoflagellate systematics. Palynology 2: 187-193.
Jacobson, D. M. 1999. A brief history of dinoflagellate feeding research. Journal of Eukaryotic Microbiology 46 (4): 376-381.
Steidinger, K. A., & J. Williams. 1970. Dinoflagellates. Memoirs of the Hourglass Cruises 2: 1-251.
Yamaguchi, A., & T. Horiguchi. 2005. Molecular phylogenetic study of the heterotrophic dinoflagellate genus Protoperidinium (Dinophyceae) inferred from small subunit rRNA gene sequences. Phycological Research 53 (1): 30-42.