Misidentifications of the bloom-forming dinoflagellates Gymnodinium litorale and Margalefidinium polykrikoides in the Mediterranean Sea

Several gymnodinioid dinoflagellates are responsible for blooms in the coastal Mediterranean Sea, coinciding with the greatest tourist arrival in summer. Chain-forming species such as the toxic Gymnodinium catenatum and Margalefidinium (Cochlodinium) polykrikoides, and two innocuous species Gymnodinium impudicum and G. litorale are prone to misidentifications. Recurrent yellow-brownish discolorations were observed during summer 2018 and 2019 in touristic areas of Porto Cesareo, Ionian Sea, Italy. The responsible species was identified as Margalefidinium cf. polykrikoides (Roselli et al. 2020, Mar. Pollut. Bull. 151:110825). However, re-examination of the published light micrographs, in addition to the new molecular analyses of samples collected from these blooms (SSU- and LSU rRNA genes and ITS region) pointed to a different gymnodinioid dinoflagellate, Gymnodinium litorale, which was also subsequently identified in the summer of 2021 from the observations of living cells. We provide light micrographs of Margalefidinium polykrikoides, Gymnodinium catenatum and G. impudicum from their type localities in order to facilitate the comparison and to avoid further misidentifications in the future.