P. australis is a toxic diatom species which produces Domoic acid. This species occurs mainly in the southern hemisphere, however, there have been some reports from the northern hemisphere. In the girdle view, the cells overlap one fourth of the cell length. In the valve view the cell shape is linear-lanceolate with rounded tips, asymmetrical along the longitudinal axis. They have more or less parallel margins, which are sometimes less distinct in smaller organisms.