The enigmatic dinoflagellate Gloeodinium marinum is a Prorocentrum species, P. palmelloides nom. nov. (Dinophyceae).

The identity of a marine planktonic species that forms palmelloid colonies has remained enigmatic since the first observations during the Plankton Expedition in 1889. Initially identified as spores or chlorophyte cells, Gloeodinium marinum was described as an immotile coccoid dinoflagellate with Gymnodinium-like swarmers. In this study, we have reported observations of G. marinum from the Mediterranean Sea, including the type locality, the Southwest Indian Ocean, and the tropical Atlantic Ocean. The sequences of the SSU and LSU rRNA genes and the ITS rRNA region (ITS1-5.8S gene-ITS2) revealed that G. marinum is distantly related to congeneric species, the type species G. montanum, and G. viscum, while it is closely related to Prorocentrum canariense and P. compressum auct. mult., two planktonic species known to produce mucilage envelopes. These phylogenetic and morphological characteristics positioned Gloeodinium marinum as a member of Prorocentrum sensu stricto, and we have proposed to reclassify it with the new name Prorocentrum palmelloides nom. nov. This species differs from its closest relatives in its smaller size (~25 μm), roundly oval shape, and multiple mucilage envelopes. The recently divided cells showed hemispherical shape, and the flagella only appeared in naked swarmers after theca ecdysis. The morphological adaptation to a life within mucilage envelopes has contributed to its cryptic identity for more than 130 years.