Detection of Prorocentrum shikokuense in the Mediterranean Sea and evidence that P. dentatum, P. obtusidens and P. shikokuense are three different species (Prorocentrales, Dinophyceae) Author Gómez, Fernando Author Zhang, Huan Author Roselli, Leonilde Author Lin, Senjie text Acta Protozoologica 2021 2022-02-14 60 47 59 http://dx.doi.org/10.4467/16890027ap.21.006.15380 journal article 268357 10.4467/16890027AP.21.006.15380 bb0ec03e-dbbf-4995-8364-4eb71b5563fa 1689-0027 8357003 Prorocentrum shikokuense in the Mediterranean Sea Marampouti et al. (2021) listed Prorocentrum shikokuense among the twenty alien harmful microalgae in the Mediterranean Sea. However, the analyses of the available molecular data reveal that the Mediterranean or Atlanto-Mediterranean ribotypes of most of these ‘alien’ microalgae are genetically distinct from the supposed original exotic populations. This suggests that most of the Mediterranean populations of these microalgae are genetically independent populations rather than recent introductions from exotic ocean regions ( Gómez and Galil 2021 ). The port of Brindisi receives a considerable ship traffic from Asia, apparently supporting the hypothesis of the introduction of exotic phytoplankton via ballast waters. Molecular data are needed, however, to resolve the biogeographical affinity of the Mediterranean population of P. shikokuense . The problem is that available rRNA gene sequences show a strong bias towards isolates from the Pacific Ocean. Another problem is the limited genetic divergence between the isolates because their rRNA gene sequences were almost identical even using variable molecular markers such as the ITS rRNA gene sequence. Nearly all the available sequences of P. shikokuense are from the Pacific Ocean ( Figs 2–4 ). An exception in the SSU- and LSU rRNA gene phylogeny is the strain K-1260 of the Norwegian Culture Collection of Algae (accession numbers MK713637-9) that was isolated in a port of the La Gomera Island in the Canary Archipelago (subtropical North Atlantic). This small port does not receive big ships, and the Canary Archipelago is not known as a route of ship traffic from Asia. The LSU and ITS rRNA gene sequences of the strain K-1260 were 100% identical to the Mediterranean ribotype of P. shikokuense . This suggests a distinct population of P. shikokuense exists in the tropical Atlantic and Mediterranean Sea, at odds with the hypothesis of a recent introduction from Asian waters. However, because most of the available sequences are from isolates from the Pacific Ocean, and the resolution of the molecular marker is insufficient, an unequivocal conclusion on the biogeographical affinities of this species remain to emerge. Percopo et al. (2011) reported a scanning electron microscopy (SEM) image of Prorocentrum shikokuense (identified as P. donghaiense ) from the offshore waters of the Gulf of Lions, NW Mediterranean Sea. Cell dimensions were 16.5 µm long and 9.5 µm wide. The individual was shorter, and relatively wider (lower length/wide ratio) than the cells of P. shikokuense during a bloom in the port of Brindisi (mean 21.6 µm long, 9.3 µm wide). It should be noted that the drying in the SEM treatment reduces the cell size ( Pertola et al. 2003 ). The robust appearance of the individuals reported in Percopo et al. (2011) matches well with smaller individuals of P. donghaiense in the species original description (16–22 µm long, 9–14 µm wide; Lu and Goebel 2001 ). This suggests that the morphotype observed in offshore waters differed from the blooming cells in neritic eutrophic waters ( Fig. 1 AH). Percopo et al. (2011) hypothesized that Prorocentrum maximum (Gourret) J.Schiller , described from the Gulf of Lions, could be a senior synonym of P. shikokuense . However, the shape of the cell illustrated by Gourret (1883) as Postprorocentrum maximum may correspond to an individual of P. micans without apical spine. Later, Schiller (1931) transferred it into Prorocentrum as P. maximum . Schiller (1931) reported a line drawing of P. maximum that is considered an earlier illustration of P. mexicanum B.F.Osorio ( Gómez et al. 2017 ).