Species of the thecate dinoflagellate genus Heterocapsa (Dinophyceae; Heterocapsaceae) from the tropical Mexican Pacific, with special reference to two new records in the area and the ultrastructure of Heterocapsa borneoensis

Background: The thecate dinoflagellate genus Heterocapsa encompasses marine planktonic species, which have been poorly studied in the Mexican Pacific coasts. Question: Is it possible to extend the knowledge of the genus Heterocapsa in the Mexican Pacific by means of different study techniques? Study sites: Acapulco Bay, Guerrero, Mexico During November 2021 and March 2024. Methods: Establishment of cultures, morphological analysis by light microscopy and molecular identification using the region D1-D2 (LSU rDNA) Results: Five strains belonging to species of the genus Heterocapsa were established. The strains were identified as H. borneoensis, H. iwatakii and Heterocapsa sp., respectively. Each strain was morphologically characterized by light microscopy, highlighting characters such as the overall shape of the cell, proportion of the episome and hyposome, shape and position of the nucleus and number and position of the pyrenoids. Additionally, the morphology of the body scales and the ultrastructure of H. borneoensis were studied. Molecular identification of two species was confirmed by analysing the D1-D2 region (LSU rDNA). The phylogeny showed that sequences obtained in this study clustered with previous sequences of H. borneoensis and H. iwatakii. Conclusion: This study confirmed the first record of H. borneoensis and H. iwatakii from the eastern Pacific Ocean.