Auxosporulation in <i>Chaetoceros acadianus</i> sp. nov. (Bacillariophyceae), a new member of the Section Compressa

We document the fine structure of auxospores in a <i>Chaetoceros</i> species isolated from the Acadian coast of New Brunswick, Canada. Auxospore development in this species occurs in a terminal rather than lateral position, a characteristic never before observed in this genus. Our observations suggest that auxosporulation was uniparental, probably an extreme form of autogamy with sister nuclei fusing following meiosis II. Mature auxospores were adze-shaped to sub-globular and contained both scales and transverse perizonia in their walls. The transverse perizonial band structure was similar to longitudinal perizonial bands found in other species of <i>Chaetoceros</i> and differed from the pinnate bands of pennate transverse perizonia, which consisted of a central rib and bilateral fimbria. Instead, the band structure in <i>C. acadianus</i> was more similar to unilateral fimbriate bands in cymatosiroids. We also propose that our diatom represents a species new to science and is a member of the <i>Chaetoceros</i> Section Compressa. We provide its morphological, molecular and reproductive characterization.