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Abstract
Cocconeis costata var. subantarctica var. no v., a rare taxon from the Kerguelen Archipelago (Southern Ocean, Indian sector), is described using light and scanning electron microscope observations and was compared to related taxa. Apart from the wide-elliptical central area devoid of striae, the raphe valve features highly resemble those of the C. costata complex: the open crista marginalis is similar to the one occurring in other varieties of C. costata, whereas the slightly dissymmetric and hook-like central raphe endings are a distinctive feature. This Subantarctic diatom has a two-layered sternum valve, with alveolate striae consisting of elongated chambers internally open through marginal elliptical wide and regular foramina. On the external side, striae of the sternum valve vary from biseriate to triseriate near the margin, and are not restricted to the foramina location. The resemblance of our new taxon with C. costata var. paciflca and C. barleyi, which also possess more or less extended SV chambers, is discussed. The epizoic diatom genera Bennettella and Epipellis, previously assigned to the genus Cocconeis Ehrenberg, also show such alveolate-chambered striae, but are associated with several other very specific features. Several small sized Cocconeis taxa from Antarctic and Subantarctic environments, such as C. costata var. antarctica and C. melchiori, show a certain morphological similarity to Cocconeis costata var. subantarctica var. nov., but they have marginal short striae on the sternum valve and lack a two-layered sternum valve or chambers.