Montana Field Guide

Morphological Category – Asymmetric biraphid

A smaller, more compact form of C. mexicana. Valves broad, cymbelloid, semi-elliptic. Apices bluntly rounded and weakly protracted. Dorsal margin strongly arched; ventral margin slightly convex to nearly flat. Axial Area linear, a bit wider than the raphe. Central Area small and circular to elliptic. A single large stigma is present between the proximal raphe ends. Raphe branches lateral, becoming narrower near the proximal and distal ends. Proximal raphe ends terminate in bulbous central pores that are deflected slightly towards the ventral margin. Terminal raphe fissures deflected dorsally at about 45º. Well-developed pore fields present at both apices. Striae radiate throughout. Dorsal and ventral striae variable in length in the central portion of the valve are. Areolae very distinct, number 10-12 in 10 µm. Areolae on the ventral side of the central area elongate and triangular in shape.

Size Range
Length 63–102 µm. Width 26–31 µm. Striae in 10mm 6–9. Stigmata 1.

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

Individuals that represent variety kamtschatica exhibit a smaller, more compact form of C. mexicana with bluntly rounded and weakly protracted apices.

Mountain streams and small rivers. May occur in association with the nominate variety, but often the only variety of the species present.

Cool, alkaline waters with low to moderate nutrients and conductivity.

Water Chemistry
Abundance-weighted mean values for selected water quality variables: conductivity = 1731 µS/cm;
pH = 8.0; water temperature = 20.1 degrees C; total nitrogen = 0.190 mg/L; total phosphorus = 0.018 mg/L.

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).

Climate change and regional warming