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This volume summarizes the results of a ten-year survey of small-sized marine Achnanthales (Bacillariophyceae) in Central Polynesia (South Pacific), focusing on their valve ultrastructures using the Scanning Electron Microscope (SEM). The degree of colonization of benthic marine substrates, species richness and emergence of new diatom taxa appear to vary according to the geologic past, presence of coral reefs and degree of insularity of each Central Polynesia island. Several recently published taxa from French Polynesia, such as Cocconeis santandrea and Xenococconeis opunohusiensis, are presented herein, together with some unpublished observations and new illustrations on all of the 13 described taxa. A Venn diagram permits the first comparison with assemblages studied with the same methodologies from Central Polynesia, New Caledonia and Mascarenes (Indian Ocean). Potential endemism is briefly discussed. The taxonomy used in this volume is based on valve ultrastructure as seen in the SEM, which allows an easier comparison of the small marine tropical Achnanthales taxa, which can be difficult to distinguish by light microscopy. It questions the presence of forms or 'morphs' in several species complexes. The concept of endemism in marine eukaryotes, currently still controversial, opens up other perspectives on the biogeography of these organisms. Marine benthic and small-sized diatoms are poorly studied in contrast to freshwater diatoms, which are often used to develop diatom indices for determining water quality. This volume can help researchers working on the taxonomy of the order Achnanthales, but will also aid students beginning detailed studies of marine benthic diatoms.
