Volume 068: Riaux-Gobin, Catherine, Andrzej Witkowski, Richard W. Jordan and Michel Coste: Small marine Achnanthales (Bacillariophyceae) from coral reefs of Polynesia (South Pacific). 2023. 20 figs. 6 tabs. 96 p. gr8vo. Paper bd.

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 r ecently 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opunohusiensis, are presented herein, together with some unpublished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.