The known global distribution of the distinctive PSP-producing dinoflagellate Gymnodinium catenatum has increased dramatically in the last few decades and it is now reported from every continent. Whether this represents increased recognition of a widespread cryptic species or recent dispersal from a source population is not clear. Recent studies have shown that the distinctive microreticulate cysts characteristic of G. catenatum are also produced by two distinct but related non-toxic species, G. nolleri Ellegaard et Moestrup and G. microreticulatum Bolch et Hallegraeff. Consequently, previous cyst reports may have been mistakenly attributed to G. catenatum or include more than one species. Resolving these distinct species has not only clarified the current distribution of G. catenatum but also provides an opportunity to examine the evolution and biogeography of the complex.

Interbreeding, biochemical, and molecular genetic comparison of G. catenatum strains from Japan, Spain and Portugal and Australia studies indicate a high level of variation within populations, consistent with a sexually outbreeding species. Small-scale geographic and temporal clustering of Tasmanian strains by isolation location and bloom year indicates that genetic exchange between neighbouring estuaries is limited and that Tasmanian G. catenatum blooms are composed estuary bound sub-populations. Japanese and Spanish/Portuguese strains can be resolved into distinct clusters which are, surprisingly, more closely related to each other than either population are to the Australian cluster. The probable source for Australian G. catenatum remains unclear, however, genetic and biochemical data support the secondary relocation of Tasmanian populations to mainland Australia, possibly via a domestic shipping vector.

Studies of LSU-rDNA sequences show that the G. catenatum complex is a distinct monophyletic lineage arising from the base of the loop-apical grooved group of gymndinioids. Within the complex, G. microreticulatum diverges earliest and shows abundant LSU-rDNA sequence variation between populations; G. nolleri and G. catenatum diverged much more recently, with G. catenatum exhibiting no sequence variation among strains from Australia, Hong Kong, Japan, Spain and Uruguay. Comparing nuclear volumes indicates that the complex may have evolved from a G. microreticulatum-like ancestor by a series of polyploidy events. A broad re-examination of the fossil dinoflagellate cyst record in light of recent molecular phylogenetic data suggests a common ancestor of the G. catenatum complex evolving around the end of the Cretaceous, with the G. catenatum lineage diverging around 130-140 Mya and that the G.nolleri and G. catenatum lineages diverging about 16-19 Mya. The known modern distributions of the complex suggest a European origin for G. catenatum, followed by dispersal to other continents within the last 25 thousand years, either by natural processes, human means, or a combination of both.