Ardissonea was resurrected from Synedra in 1986 and was included as a genus by Round, Crawford and Mann (“The Diatoms”) in its own Family and Order. They commented that there might be several genera involved since the type species of the genus possesses a double-walled structure and other taxa placed in Ardissonea have only a single-walled structure. Two other genera of “big sticks,” Toxarium and Climacosphenia, were placed in their own Families and Orders but share many characters with Ardissoneaceae, especially growth from a bifacial annulus. Eighteen taxa (11 new species) from Micronesia were compared with the literature and remnant material from Grunow’s Honduras Sargassum sample to address the concepts of Ardissonea and Ardissoneaceae. Phylogenetic and morphological analyses showed three clades within Ardissonea sensu lato: Ardissonea emend. for the double-walled taxa, Synedrosphenia emend. and Ardissoneopsisgen. nov. for single-walled taxa. New species include Ardissonea densistriatasp. nov.; Synedrosphenia bikarensissp. nov., S. licmophoropsissp. nov., S. parvasp. nov., and S. rectasp. nov.; Ardissoneopsis fulgicanssp. nov., A. appressatasp. nov., and A. gracilissp. nov. Transfers include Synedrosphenia crystallinacomb. nov. and S. fulgenscomb. nov. Synedra undosa, seen for the first time in SEM in Grunow’s material, is transferred to Ardissoneopsis undosacomb. nov. Three more genera have similar structure: Toxarium, Climacosphenia and Grunowagogen. nov., erected for Synedra bacillaris and a lanceolate species, G. pacificasp. nov. Morphological characters of Toxarium in our region support separation of Toxarium hennedyanum and T. undulatum and suggest additional species here and elsewhere. Climacosphenia moniligera was not found but we clarify its characters based on the literature and distinguish C. soulonalissp. nov. from it. Climacosphenia elongata and a very long, slender C. elegantissimasp. nov., previously identified as C. elongata, were present along with C. scimiter. Morphological and molecular phylogenetics strongly suggested that all these genera belong in one family and we propose to include them in the Ardissoneacae and to reinstate the Order Ardissoneales Round.
