Bunodeopsis pelagica is one of the few species of sea anemones known to be found in the floating algae of Sargassum spp. It has been reported in the North Atlantic, particularly in the Canary Islands and the coast of Jamaica. The present study is the first to report the occurrence of B. pelagica in the Mexican Atlantic, found as epiphyte on Sargassum natans off Alacranes reef (southern Gulf of Mexico) and in Puerto Morelos reef (Mexican Caribbean), revealing that this species is much more widespread than has been reported. Bunodeopsis pelagica is mainly distinguished from other species of Bunodeopsis by having simple minute protuberances on its column, while its congeners have larger and much more developed vesicular projections. With the record of B. pelagica, the number of known species of actiniarian sea anemones from the Mexican Atlantic increases to 23.

There have been recent attempts to improve our knowledge about dinoflagellates of the order Dinophysales either in Mexican marine waters or worldwide, and although new records and even new species have reliable illustrations and descriptions, this group is so diverse that it needs to be studied in more detail. This paper is the product of the analysis of net phytoplankton material collected from coasts of the tropical Mexican Pacific (Gulf of California, Central Mexican Pacific and Gulf of Tehuantepec), and the southern Gulf of Mexico. Material was studied using light microscopy and scanning electron microscopy (SEM). Eleven taxa (10 species and one variety) of the section Hastata were fully identified, with nine species studied for the first time by electron microscopy (only Dinophysis hastata and D. schuettii had been previously studied by SEM), and including seven new records for the Mexican Pacific (Dinophysis monacantha, D. nias, D. phalacromoides, D. pusilla, D. swezyae, D. uracantha var. mediterranea, and D. uracanthoides), and one new record for the Gulf of Mexico (Dinophysis uracanthoides). Two species remain as unidentified, as only one specimen of each was found. Additionally, we studied in detail the new species, Dinophysis conjuncta sp. nov., for which a full description is provided. Descriptions, measurements, illustrations and local distribution data are provided for each species.