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One hundred and sixty Formalin-preserved net samples were collected from different zones in the Mexican Pacific Ocean during 11 cruises and other surveys (1984--1999) to study the composition of silicoflagellates. A few bottle samples from a coastal zone containing living material were also analyzed. Observations were made by light and scanning electron microscopy. We describe five taxa of silicoflagellates: four species and one variety, including some morphological variants (traditionally considered as recognized species or varieties). Living specimens of Dictyocha californica were also studied, from which we consider it to be an extant species. Dictyocha calida may also be regarded as an extant species, as it is very characteristic and readily distinguished, with no intergrades with related taxa, but no living cells or cells with chloroplasts were found. A discussion of morphological variation of some species within the group and the characters of the siliceous skeleton as taxonomic criteria are given. The most ubiquitous species was Dictyocha fibula (and its related 'species' or varieties), found in most of the locations sampled, whereas D. californica and D. fibula var. robusta are regarded as neritic, warm water forms. [ABSTRACT FROM AUTHOR]

Species of the diatom genus Thalassiosira are an important component of the marine phytoplankton all over the world, and the Mexican Pacific Ocean planktonic flora is also rich in species of this genus. In this study 37 net plankton samples from the Gulf of Tehuantepec, Mexico were analyzed, yielding 23 taxa of the genus. Most of the species were studied using combined light and electron microscopy. New records for the Mexican Pacific Ocean are Thalassiosira hendeyi, T. intrannula, T. pseudonana, T. tenera and T. visurgis. The species of Thalassiosira recorded here seem to indicate that the Gulf of Tehuantepec represents a subtropical zone at least in one period of the year. [ABSTRACT FROM AUTHOR]