Intraspecific diversity in Scrippsiella trochoidea (Dinopbyceae): evidence for cryptic species

Scrippsiella trochoidea is a widely distributed neritic dinoflagellate that produces calcareous resting cysts. We assessed the level of intraspecific diversity at the molecular, morphological and physiological levels among 15 strains identified as S. trochoidea and isolated from the Gulf of Naples (Italy, Mediterranean Sea), and an additional isolate from the Faeroe Islands. We investigated the morphology of motile cells and cysts, mating modality, encystment success, and growth rates at different light irradiances. The ribosomal DNA internal transcribed spacer (ITS) region was sequenced to infer phylogenetic relationships among the S. trochoidea strains and closely related species. The molecular analysis revealed a well-supported lineage comprising strains with a Scrippsiella plate pattern. Within this clade, a number of distinct ITS haplotypes were recorded but the relationships among them were only partially resolved. The 16 S. trochoidea isolates grouped into five single-strain clades and three multi-strain clades. The grouping of haplotypes in a series of distinct clades suggests the existence of cryptic species within what has previously been considered a single species, based on the morphological features of the motile cells and cysts. Some of the ITS haplotypes were distinguishable visually, based on minor morphological features of the motile cells and cysts, but in two cases morphologically almost identical strains fell into different clades. Our results showed that the majority of the strains are homothallic; only S. trochoidea v. aciculifera from the Faeroe Islands is heterothallic. Cyst production rates were notable for their diversity, even among strains grouping with the same ITS haplotype, as were growth rates at different light irradiances. Based on phylogenetic results, two new combinations are proposed: S. operosa (Deflandre) Montresor comb. nov. and S. infula (Deflandre) Montresor comb. nov.