The identity of Centrodinium elongatum, type species of the dinoflagellate genus Centrodinium (Dinophyceae), and a review on the synonymy of allied species.

The planktonic dinoflagellate genus Centrodinium has been understudied, with the type species C. elongatum remaining undocumented since the original description. Here, we report C. elongatum isolated from Mazatlán, Mexican Pacific. In the chains, the posterior daughter cell with an incomplete apical horn shows the morphology of C. elongatum, while the anterior daughter cell with complete epitheca corresponds to C. pulchrum. For the first time, a species of Centrodinium sensu stricto (highly laterally flattened species with horns) was cultured. An unarmored life stage, known as Murrayella ovalis, derived from the spheroplast after ecdysis. In the rDNA molecular phylogenies, C. elongatum (=C. pulchrum) nested as basal to morphologically similar species (C. eminens and C. intermedium) and as a sister group of a former Murrayella species, C. punctatum. C. elongatum differs from C. eminens and C. intermedium in the chain formation, second apical (2') plate not being divided, horns with coarse poroid ornamentation, and missing prominent distal spinules. The taxonomy of Centrodinium sensu stricto is revised, with a discussion in the identities of C. complanatum, C. eminens, and C. maximum. The name C. deflexum is restored as a senior synonym of C. intermedium and C. ovale.