• Category
  • Length Range
    4-22 µm
  • Width Range
    3-4 µm
  • Striae in 10 µm



Valves are lanceolate with convex central margins and broadly rounded ends. The central sternum is narrow and linear. Striae are distinct and composed of round areolae. Areolae tend to decrease in diameter from valve face/mantle junction to the axial area Striae are parallel at the valve center, becoming slightly radial toward the apices and extending continuously onto the valve mantle, where up to two rows of areolae may be present. Costae are wider than the striae. Spines are tubular and hollow, bifurcating toward the tips. Spines are present along the valve face edge, including the apices, positioned in line with the costae. A row of areolae is located on the apical mantle region, continuous with the mantle areolae from the rest of the valve. Apical pore fields and rimoportulae are absent. Copulae are open and lack ornamentation. Valvocopula is wider than the remaining copulae. In girdle view, frustules are rectangular. Cells form short ribbon-like colonies, joined by linking spines.

A brief summary of the nomenclature may help to understand the current name of this species. Fragilaria (virescens var.) exigua Grunow in Cleve and Möller no. 144 (Cleve and Möller, 1978) lacked a formal description, so it is invalid. Grunow in Van Heurck (1881) illustrated the taxon, again without valid description, and used the name Fragilaria virescens var.? exigua Grunow in Van Heurck. Lange-Bertalot (1993) validated the taxon, with the new name, Fragilaria exiguiformis. At the time, Lange-Bertalot (1993) referred to fig 125: 4 (SEM) and 126: 11-18 (LM) in Krammer and Lange-Bertalot (1991) to illustrate of the taxon. Although the North American specimens conform to the European LM illustrations, the SEM features of North American taxa are quite different, particularly in terms of spines and the mantle areolae. The spines and mantle areolae of the specimens illustrated here are more characteristic of Staurosira in Krammer and Lange-Bertalot´s illustration. The current placement in Stauroforma reflects Flower et al. (1996), who erected the genus Stauroforma as distinct from Fragilariforma, Fragilaria and Staurosira.


Original Description

There is no description in Lange-Bertalot (1993), simply a change of name and status: Fragilaria exiguiformis Lange-Bertalot nov. nom. nov. stat.

  • Basionym
    Fragilaria exiguiformis
  • Author
    Lange-Bert. 1993

EMAP Assessment

EMAP Distribution

Stauroforma Exiguiformis  Flower   Jones Et  Round 10
Map 10

Response Plots

Stauroforma Exiguiformis  Flower   Jones Et  Round


Jul 21, 2018 - Correction to striae count

The striae count was originally reported as 9-10 in µm. The striae count was corrected to 16-20 in µm to match the specimens shown on the page. - S. Spaulding

Cite This Page

Morales, E, and Spaulding, S. (2013). Stauroforma exiguiformis. In Diatoms of North America. Retrieved July 21, 2018, from https://diatoms.org/species/stauroforma_exiguiformis


The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Stauroforma exiguiformis 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.