Morphology and phylogeny of Prorocentrum venetum Tolomio & Cavolo (Dinophyceae)

Prorocentrum venetum was one of the first species of the genus Prorocentrum described by scanning electron microscopy by Tolomio and Cavolo in 1985. Since the first observation of the species in the Venice Lagoon (Italy) in summer 1981, it has not been reported again in published phytoplankton records of Mediterranean waters or elsewhere. Two strains were isolated from a French Mediterranean lagoon, which were morphologically identified as P. venetum by microscopy. Based on rDNA sequences (spanning the 18S to the D3 region of 28S rDNA), the phylogenetic analysis demonstrated that P. venetum belongs to the same clade as Prorocentrum triestinum and Prorocentrum redfieldii. The analysis of scanning electron micrographs provided an in-depth morphological description of the theca, particularly on the pore pattern of thecal plates and new structural details of the platelets in the periflagellar area. These morphological characteristics were compared with the closely related species within the P. triestinum clade, which showed synapomorphic characters in the periflagellar area (small accessory pore, platelet pattern, shape of the apical wing). Further comparison of characteristics varying between species in this clade and in the sister clade encompassing species related to Prorocentrum micans suggests some features of morphological evolution within this part of the genus.