Phylogeography, morphological variation and taxonomy of the toxic dinoflagellate Gambierdiscus toxicus (Dinophyceae)

Gambierdiscus toxicus Adachi et Fukuyo 1979 is a toxin-producing marine dinoflagellate responsible for the syndrome known as ciguatera, which sickens thousands of people every year. Despite its importance, little is known regarding the global genetic structure of this species; therefore, it is unclear whether documented variation in toxin production and outbreaks of ciguatera are the result of ecological triggers or the presence of different genetic strains across the geographic range of G. toxicus. We examined the molecular phylogeny and morphological characteristics of 28 globally distributed G. toxicus isolates by sequencing part of the large subunit (LSU) and small subunit (SSU) ribosomal DNA and analyzing the thecal architecture using scanning electron microscopy (SEM). Our analyses showed that G. toxicus is comprised of at least four distinct lineages (Clades A–D) separated by substantial genetic distances. One of the clades recovered in the phylogenetic analysis (Clade B) is morphologically distinct, the characteristics of which agree with the species description of Gambierdiscus belizeanus Faust 1995. The remaining three lineages; however, are morphologically homogeneous and may represent cryptic species. SEM analyses showed that an important morphological feature used to distinguish Gambierdiscus morphospecies, the apical pore (Po) plate shape, was variable among isolates of G. toxicus and did not reflect phylogenetic groupings; hence, its utility as a diagnostic feature is questionable. Our analyses suggest that G. toxicus is not a single cosmopolitan species with a worldwide circumtropical distribution but instead comprises a wide-ranging species complex. The presence of multiple cryptic species could partly explain the seemingly random patterns of ciguatera toxicity across geography.