Species of the planktonic diatom genus Skeletonema (Bacillariophyta) from the Mexican Pacific Ocean

After the recent revisions of the diatom genus Skeletonema, our knowledge on its diversity has greatly improved, although its taxonomy has become more complicated, especially with the descriptions of cryptic or semicryptic species, which make species recognition difficult. The combination of different observation techniques (LM, SEM and TEM) and/or the use of molecular tools are now required for an accurate identification. In this paper, we studied numerous phytoplankton samples collected with nets or bottles from locations along the Mexican Pacific, and provided observations on the morphospecies of Skeletonema. Four taxa were found using LM, SEM and TEM: Skeletonema sp. aff. japonicum and Skeletonema cf. marinoi, S. pseudocostatum and S. tropicum. Some features of the chains, cells, valves, terminal or intercalary fultoportulae and rimoportulae, and cingular bands, are given for each species. The taxon named here Skeletonema cf. marinoi showed particular characters (ribs-like structures), not previously described, on the fultoportulae of terminal valves, which arise suspects of the presence of new species within the species complex. We annotate Skeletonema pseudocostatum as a new record from the Mexican Pacific, where only three species have been previously recorded. The species Skeletonema costatum, traditionally considered as abundant, frequent and widely spread all over the region, was not recognized in our samples.