Montana Field Guide

Morphological Category – Symmetric biraphid

Valves elliptic-lanceolate to linear-lanceolate with subcapitate apices. Axial Area linear, about three times wider than the raphe, and expands slightly near the central area. Central Area very wide at the valve margins and shaped like a bow tie. One to several short striae occur at each margin of the central area. Raphe branches lateral, becoming filiform towards the distal and proximal ends. Distal raphe ends hooked in the same direction. Proximal raphe ends nearly straight and tipped with unexpanded central pores. Striae somewhat wavy and strongly radiate throughout. Areolae easily distinguished in LM, 18-20 in 10 µm.

Size Range
Length 62-90 µm. Width 13.9-17.5 µm. Striae in 10 µm 18-20.

Useful Link:
Diatom Glossary [Diatoms of North America website]

Stauroneis pax is easily distinguished from S. fluminea (Patrick & Freese 1961) and S. gracilis by its exaggerated bow-tie shaped stauros, more strongly radiate striae and multiple short striae within the central area. Stauroneis pax may be distinguished from S. acidoclinata by its larger, especially wider, less linear valves and exaggerated stauros with multiple short striae.

Lakes, ponds, seeps and streams in the mountains.

Cool, alkaline waters with low nutrients and moderate conductivity.

Water Chemistry
In Montana and Alberta waters where S. pax is found, pH ranges from 8.0 to 8.2 and conductance ranges from 149 to 183 µS/cm.

Diatoms typically reproduce by cell division (mitosis) and occasionally by meiosis—sexual reproduction in which female and male gametes combine to form a specialized zygote called an auxospore. Repeated divisions result in cells of a population becoming progressively smaller and smaller. When cells reach a critically small size, sexual reproduction is initiated, resulting in an auxospore and initial cells that are the largest attainable for the species, after which cell division and size reduction resume (Amato 2010).

As a rare, cold-water stenothermal diatom in Montana and the contiguous United States, Bahls (2018) rated Stauroneis pax at moderate risk of extirpation due to climate change and regional warming.