Montana Field Guide

Morphological Category - Asymmetric biraphid.

Valves large, attached to substrates by polysaccharide stalks. Dorsal margin strongly arched; ventral margin slightly concave with a tumid center. Apices broadly rounded. Axial Area narrow; Central Area small, rounded. One stigma present between the proximal raphe ends. Raphe branches lateral at their centers, narrowing towards the proximal and distal ends. Proximal raphe ends dilated. Terminal raphe fissures dorsally deflected nearly 90 degrees. Apical pore fields present at both apices. Striae radiate near the valve center, becoming parallel near the ends. Striae surrounding the central area alternately long and short. Areolae distinct, circular to squared, 9 to 12 in 10 µm. Areolae triangular adjacent to the axial and central areas.

Size Range
Length 63–200 µm. Width 23–45 µm. Striae in 10 µm 6–9. Stigmata 1.

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

Cymbella mexicana is distinguished from Cymbella janischii by its smaller size, more widely spaced striae and lack of strongly elongated areolae adjacent to the central area.

Rivers and creeks.

Cymbella mexicana is one of several large cymbelloid diatoms that form conspicuous growths in streams. This group includes Cymbella janischii and Didymosphenia geminata. Frustules extrude a mucous-like polysaccharide through an apical pore field at one pole, which quickly hardens to form a stalk by which frustules attach to a firm substrate and to the rest of the colony. Colonies of branched stalks with frustules at their ends may reach several cm in diameter, appear as “dirty cotton balls” in the water, cover large areas of substrate and generate complaints from the public (Bahls 2007).

Water Chemistry
Cool, alkaline waters with low to moderate nutrients and conductivity. Abundance-weighted mean values for selected water quality variables: conductivity = 1420 µS/cm; pH = 8.2; water temperature = 12.3 degrees C; total nitrogen = 0.500 mg/L; total phosphorus = 0.047 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).