Catherine Riaux Gobin Knihy

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Polynesia, such as Cocconeis santandrea and Xenococconeis opunohusiensis, are presented herein, together with some unpublished observations and new illustrations on all of the 13 described taxa.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.

Benthic marine diatoms from tropical environments are still poorly documented, despite their high diversity. Coral reef biotopes of the Mascarene Islands (Western Indian Ocean Archipelago: Réunion, Mauritius, Rodrigues Islands) provide the opportunity to study a large part of their diatom community: the abundant small-sized Achnanthales treated here. Diatoms of other environments of very variable nutrient status were studied in Réunion and Rodrigues Islands and include areas of both low and high anthropogenic impact. This monograph describes taxa of seven genera, upon which Cocconeis Ehrenberg was particularly diverse. Scanning and light microscope investigations document and illustrate a total of 59 taxa, eight of them are new descriptions from these islands. This study serves as a basis for further investigations in the Mascarenes, Réunion the Rodrigues Islands and other tropical areas.

Recent marine species of Cocconeis Ehrenberg and related taxa from sub-Antarctic Kerguelen Islands sediments were analyzed. The predominant macroalgae, particularly Macrocystis pyrifera and Durvillaea antarctica, favored epiphytism, allowing for a diverse proliferation of attached genera. Material collected during four austral summers between 1985 and 1992 led to the description of nearly 40 Cocconeis and related taxa, with several being new to these Austral localities, while others remain taxonomically uncertain. Diversity was notably high under the M. pyrifera canopy, with some species identified as epiphytes on larger diatoms and others as predominantly epipsammic. Taxa exhibited a wide range of dimensions and ornamentation variability. Communities varied by locality, influenced by tidal changes, macroalgal canopies, and mussel beds. While some Cocconeis species are ubiquitous, others appear to be endemic to these islands. Dominant species include Cocconeis scutellum and important components like C. fasciolata and C. stauroneiformis. Previous misidentifications and poor illustrations by earlier authors are clarified in this study. Scanning electron microscopy (SEM) was utilized for taxon illustration, with descriptions based on SEM and light microscopy provided where possible, emphasizing the uniqueness of the Austral diatom flora, particularly that of Kerguelen’s Land.