Thecal tabulation, body scale morphology and phylogeny of Heterocapsa philippinensis sp. nov. (Peridiniales, Dinophyceae) from the Philippines

•Based on the first survey of Heterocapsa in the Philippines, H. philippinensis sp. nov. is described.•H. philippinensis possesses the sausage-shaped nucleus and circular pattern on the 2a plate.•Body scale ultrastructure is variable in the genus but stable in the clade of H. steinii.•Occurrence of H. pygmaea in Philippines shows the wide distribution of the species.•DNA sequences of H. lanceolata and H. orientalis will help their molecular identification. The thecal tabulation and body scale structure of the marine armoured dinoflagellate Heterocapsa, isolated from Philippines, were examined using LM, SEM and TEM, and its phylogenetic position was inferred from ITS and LSU rDNA sequences. Cells were ovoid and the plate tabulation (Po, cp, X, 5′, 3a, 7′′, 6c, 5s, 5′′′, 2′′′′) was consistent with most Heterocapsa species. The second anterior intercalary plate (2a) had a circular pattern with a thick marginal border free of pores. The nucleus was longitudinally elongated and curved, and located at the dorsal side of the cell. Discoid lobes of brownish chloroplast were peripherally distributed, and a pyrenoid was positioned at the centre. The triradiate body scales, measuring 250–300 nm in diameter, consisted of a roundish basal plate with six radiating ridges, nine peripheral uprights/spines, and three radiating spines. These components were identical to those of H. pseudotriquetra and H. steinii, except for the roundish outline of basal plate. Molecular phylogeny showed that the species clustered with H. pseudotriquetra and H. steinii. This species was differentiated from all other Heterocapsa species in the sausage-shaped nucleus and circular pattern on the 2a plate. This study proposed a novel species Heterocapsa philippinensis sp. nov. for the isolate.