| Summary: | Pliocaenicus is a small, mainly fossil diatom genus that only rarely occurs in modern freshwater environments in eastern Russia. The only extant taxon of this genus, Pliocaenicus costatus , was studied from phytoplankton samples collected in Lake El'gygytgyn, a deep, ultraoligotrophic and low-conductivity crater lake in East Siberia. Lake El'gygytgyn is so far the northernmost site of observation for P. costatus , which is extant mainly in lakes in the Lake Baikal region. Pliocaenicus costatus was examined in detail by light and scanning electron microscopy. Typical features of P. costatus from Lake El'gygytgyn include the colliculate external valve surface, the absence of spines and, particularly, the high number of central area fultoportulae. Both structural and morphometric data indicate that our specimens do not perfectly match any of the three described varieties of P. costatus . However, we refrain from describing a new variety for P. costatus from Lake El'gygytgyn due to insufficient biogeographical, morphological and morphometric information on this taxon. © 2006 British Phycological Society.
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