Abstract
A new centric diatom, Aulacoseira principissa sp. nov., is described from the sub-Antarctic region. The morphology of the species is illustrated using detailed light and scanning electron microscopy observations. Its main discriminating features include the presence of typical bifurcating spines, a single rimoportula, a typical valve heterovalvy with spineless separation valves and a variable striation pattern on the discus. The new species is compared to related species belonging to the complex of Aulacoseira distans. Aulacoseira principissa has a broad geographical distribution ranging from the sub-Antarctic islands in the southern Indian Ocean to South Georgia in the southern Atlantic Ocean. It is commonly found in slightly acidic, oligotrophic pools with low specific conductance values.