Montana Field Guide

Morphological Category – Symmetric biraphid

Valves lanceolate with rounded, unprotracted apices. A relatively deep pseudoseptum occupies each apex. Axial Area narrowly lanceolate, widening gradually to the central area. Central Stauros nearly rectangular, expanding somewhat toward the valve margins. Raphe distinctly lateral with straight, weakly inflated proximal ends. Striae radiate throughout. Areolae relatively coarse, 12-16 in 10 µm.

Size Range
Length 81-124 µm. Width 16.3-23.5 µm. Striae in 10 µm: 15-18.

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

In comparison to Stauroneis thompsonii, S. americana has larger valves with lower stria and areola densities. S. finlandia and S. kishinena have strongly curved proximal raphe ends.

Mountain lake

Cool, alkaline water with low nutrients and conductivity (Bahls 2021).

Water Chemistry
At the type locality, water temperature measured 12.5 degrees C, pH was 8.83, and specific conductance was 107 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).

Stauroneis thompsonii is rated at high risk of extirpation from Montana due to climate change and regional warming (Bahls 2018).