• Category
  • Length Range
    41.0-82.0 µm
  • Width Range
    11.0-17.0 µm
  • Striae in 10 µm
    17-19 midway between valve center and apex



Valves are broadly linear, with parallel or weakly concave margins. Apices are obtusely rounded. The axial area is narrow and undulate, bordered on each side by a narrow conopeum. The conopea are continuous through the central area, where they expand laterally. Near the valve apex, the conopea are somewhat inflated before they terminate in a point. The central area is wide and transversely rectangular. The central nodule, defined by the inner margins of the conopea, is small, elliptic, and somewhat asymmetric. The raphe is filiform and follows the undulate margins of adjacent conopea. Proximal raphe ends are deflected slightly to one side and tipped with small pores. Distal raphe ends are comma-shaped. Striae surrounding the central area are parallel, straight, shorter, and more widely spaced than more distal striae, which are curved and radiate.


Specimens of S. alastos pictured and described on this page were collected from five ponds and small lakes in the Rocky Mountains of Montana and Colorado and the North Cascade Mountains of Washington. pH ranged from 6.73 to 6.98 and specific conductance measured 22 to 71 µS/cm. Sellaphora alastos was described from a river in eastern Canada (Hohn and Hellerman 1963) and also reported from an oligo-dystrophic lake in Finland (Lange-Bertalot and Metzeltin 1996).

Lake Caroline3
Credit: Ryan Davis, Adventurers and Scientists for Conservation
Lake Caroline, Chelan County, Washington: home of Sellaphora alastos.

Original Description

  • Basionym
    Navicula americana var. alastos
  • Author
    M.H.Hohn and Hellerman 1963
  • Length Range
    56.7 µm
  • Width
    12.5 µm
  • Striae in 10µm
    12 at valve center, 16 near the apices

Original Images

Sellaphora Alastos Origimag1001
Sellaphora Alastos Origdesc001

Cite This Page

Bahls, L. (2014). Sellaphora alastos. In Diatoms of North America. Retrieved April 14, 2024, from https://diatoms.org/species/sellaphora_alastos


The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Sellaphora alastos from all the stream reaches where it was present. Note that the relative abundance scale is the same on each plot. Explanation of each environmental variable and units are as follows:

ELEVATION = stream reach elevation (meters)
STRAHLER = distribution plot of the Strahler Stream Order
SLOPE = stream reach gradient (degrees)
W1_HALL = an index that is a measure of streamside (riparian) human activity that ranges from 0 - 10, with a value of 0 indicating of minimal disturbance to a value of 10 indicating severe disturbance.
PHSTVL = pH measured in a sealed syringe sample (pH units)
log_COND = log concentration of specific conductivity (µS/cm)
log_PTL = log concentration of total phosphorus (µg/L)
log_NO3 = log concentration of nitrate (µeq/L)
log_DOC = log concentration of dissolved organic carbon (mg/L)
log_SIO2 = log concentration of silicon (mg/L)
log_NA = log concentration of sodium (µeq/L)
log_HCO3 = log concentration of the bicarbonate ion (µeq/L)
EMBED = percent of the stream substrate that is embedded by sand and fine sediment
log_TURBIDITY = log of turbidity, a measure of cloudiness of water, in nephelometric turbidity units (NTU).
DISTOT = an index of total human disturbance in the watershed that ranges from 1 - 100, with a value of 0 indicating of minimal disturbance to a value of 100 indicating severe disturbance.