Clarifying confusion – <i>Prorocentrum triestinum</i> J.Schiller and <i>Prorocentrum redfieldii</i> Bursa (Prorocentrales, Dinophyceae) are two different species

The Prorocentrales are a unique group of dinophytes based on several apomorphic traits, but species delimitation is challenging within the group. <i>Prorocentrum triestinum</i> was described by Josef Schiller in 1918 as an important bloom-forming species from Trieste (Mediterranean, Adriatic Sea) with a conspicuous asymmetric outline and a small, asymmetrically located subapical spine. All subsequent records under this name fail to conform to Schiller’s original description. These inconsistencies have their origin in John Dodge’s 1975 revision of <i>Prorocentrum</i>, which placed <i>Prorocentrum redfieldii</i>, a more symmetrical, slender species with a long apical spine, into synonymy under <i>P. triestinum</i>. To clarify this confusion, we collected samples at the type locality of <i>P. triestinum</i> in Trieste and established a strain that is morphologically consistent with the protologue and suitable for use in epitypification. Morphology and rRNA sequence data of this strain were compared with four new strains identified as <i>P. redfieldii</i> from the Mediterranean Sea and the North Atlantic Ocean. Cells of <i>P. triestinum</i> had an asymmetric outline in lateral view and a small, dorso-subapical spine. These features, which are readily resolved by light microscopy, were distinct from those of the nearly symmetrical and slender cells of <i>P. redfieldii</i>, which had a long, apically located spine. The species are nevertheless closely related and share an identical architecture of the periflagellar area with a distinctive, largely reduced accessory pore together with a very small platelet 7. This apomorphy clearly differentiates both species from other species of <i>Prorocentrum</i>. Both species differ in their primary rRNA sequences, and ITS and LSU sequence differences will enable them to be distinguished in future meta-barcoding studies. The present study demonstrates that <i>P. triestinum</i> and <i>P. redfieldii</i> are distinct species and thus contributes to a reliable biodiversity assessment of <i>Prorocentrum</i>. HIGHLIGHTS <i>Prorocentrum triestinum</i> is characterised molecularly for the first time and delimited from <i>P. redfieldii</i>. The identity of important bloom-forming species is clarified. Structural details of the periflagellar area are described.