Further studies on the species diversity of the marine planktonic diatom genus Skeletonema (Bacillariophyta) in the Mexican Pacific

Following recommendations on species level identification in the diatom genus Skeletonema, some papers have been published describing new species. A combination of different observation techniques (LM, SEM and TEM) and/or the use of molecular tools for an accurate identification is now required, because semicryptic species have been found. In this study, cultures were established from one water sample collected from a location in the Mexican Pacific to provide a morphological characterization of four species: Skeletonema ardens, S. grevillei, S. pseudocostatum and S. tropicum. The selected morphological features for analysis were chains, cells, chloroplasts, valves, terminal or intercalary fultoportulae and rimoportulae, and cingular bands. Two of the described species, Skeletonema ardens and S. grevillei, are new records for the Mexican Pacific. Skeletonema costatum, that has traditionally been considered as abundant, frequent, and widely spread all over the region, was not recognized in this study. Together with the four species discussed here, there are six Skeletonema species represented in the Mexican Pacific, counting with Skeletonema japonicum and S. marinoi, previously reported for the region. The coexistence of four Skeletonema species in a same water sample is remarkable, considering that only Skeletonema costatum had historically been the typical species recorded, which was additionally producing blooms in coastal waters.