Molecular and phylogenetic characterization of Ostreopsis (Dinophyceae) and the description of a new species, Ostreopsis rhodesae sp. nov., from a subtropical Australian lagoon.

Cryptic and pseudo-cryptic species are common amongst marine phytoplankton, and may cause misleading inferences of ecological and physiological data of plankton community studies. Deciphering the diversity and distribution of species of the benthic dinoflagellate Ostreopsis is one example, as there are many morphologically indistinct clades that differ greatly genetically and toxicologically from one another. In this study, a new species, Ostreopsis rhodesae from the southern Great Barrier Reef was described. While it initially appeared to be highly similar to several other Ostreopsis species, we found O. rhodesae can be distinguished based on the relative size of the second apical plate (2'), which is twice as long as the APC plate, and separates the third apical (3') from the third precingular (3'') plate. Phylogenetic trees based on the SSU, ITS/5.8S and D1-D2 and D8-D10 regions of the LSU rRNA were well supported, and showed a clear difference to other Ostreopsis clades. Compensatory base changes (CBCs) were identified in helices of the ITS2 between O. rhodesae and O. cf. ovata and O. cf. siamensis, which were also present in the same habitat. Fish gill cell lines were toxic to O. rhodesae, cell extracts but no palytoxin-like analogues were found in them. The findings highlight a case of pseudo-cryptic speciation, found in sympatry with closely related and morphologically similar species, but biologically and functionally distinct.