Who is Oxyrrhis marina? Morphological and phylogenetic studies on an unusual dinoflagellate

Oxyrrhis marina is an extensively studied morphospecies and a common protist model used to examine a range of ecological processes. Further, as a result of a number of unusual cytological and genetic features, Oxyrrhis is increasingly a target for the study of evolutionary patterns and genome organization within the Alveolata. However, a small number of early morphological studies and recent phylogenetic data suggest that O. marina represents more than one species. As different research groups employ different O. marina isolates (which are potentially highly divergent strains or different species), the context in which comparisons between isolates can be made is difficult to assess. In this paper, we explore the literature that has contributed to the definition of O. marina, highlighting the unusual characteristics possessed by O. marina that have motivated much of the study on this organism and informed its key phylogenetic position. In addition, we assess historical and contemporary evidence for multiple Oxyrrhis species. Based on this assessment, in particular recent molecular genetic data, we assert that O. marina represents two species: O. marina and O. maritima. Based on historical observations, we also indicate that a third species (O. tenticulifera) may occur, although there are no contemporary data to support or refute this designation. Extensive cryptic diversity has important implications for researchers studying Oxyrrhis: caution must be exercised in characterizing Oxyrrhis isolates for experimental study (i.e. it is inappropriate to report assessments concerning poorly characterized isolates), and comparative studies of multiple isolates are required to assess individual, population and species level variation in the genus. Finally, in a broader context, the ecological and evolutionary processes driving diversity in free-living protists remains poorly understood. Model protists such as O. marina and O. maritima for which we are beginning to recognize and characterize an extensive pool of variation present ideal opportunities to unravel these fundamental processes.