New species of Clade B Symbiodinium (Dinophyceae) from the greater Caribbean belong to different functional guilds: S. aenigmaticum sp. nov., S. antillogorgium sp. nov., S. endomadracis sp. nov., and S. pseudominutum sp. nov.

Molecular approaches have begun to supersede traditional morphometrics in the species delineation of micro-eukaryotes. In addition to fixed differences in DNA sequences, recent genetics-based descriptions within the dinoflagellate genus Symbiodinium have incorporated confirmatory morphological, physiological, and ecological evidence when possible. However, morphological and physiological data are difficult to collect from species that have not been cultured, while the natural ecologies of many cultured species remain unknown. Here, we rely on genetic evidence-the only data consistently available among all taxa investigated-to describe four new Clade B Symbiodinium species. The 'host-specialized' species (S. antillogorgium sp. nov. and S. endomadracis sp. nov.) engage in mutualisms with specific cnidarian hosts, but exhibit differences in our ability to culture them in vitro. The ecologically 'cryptic' species (S. aenigmaticum sp. nov. and S. pseudominutum sp. nov.) thrive in culture, but their roles or functions in the ecosystem (i.e., niches) are yet to be documented. These new species call further attention to the spectrum of ecological guilds among Symbiodinium.