Dinoflagellates (Dinophyta) of orders Dinophysiales and Prorocentrales of the Veracruz Reef System, Mexico. Dinoflagellates are a major taxonomic group in marine phytoplankton communities in terms of diversity and biomass. Some species are also important because they form blooms and/or produce toxins that may cause diverse problems. The composition of planktonic dinoflagellates of the orders Prorocentrales and Dinophysiales, in the Veracruz Reef System, were obtained during the period of October 2006 to January 2007. For this, samples were taken from the surface at 10 stations with net of 30µm mesh, and were analyzed by light and scanning electron microscopy. Each species was described and illustrated, measured and their distribution and ecological data is also given. A total of nine species were found and identified, belonging to four genera: Dinophysis was represented by three species; Prorocentrum by three, Phalacroma by two, and only one species of Ornithocercus was detected. From the samples, four potentially toxin-producer species were found: Dinophysis caudata, D. rapa, Phalacroma rotundata and Prorocentrum micans. The number of species found in this study is low, especially considering the higher numbers observed in other areas of the Gulf of Mexico, where some reports have recorded up to 53 species of the order Dinophysiales and 14 for Prorocentrales. Identification keys for orders, genera and species for the study area are provided with this study.

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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.

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