• Category
  • Length Range
    51.8–142.1 µm
  • Width Range
    6.4–9.9, at the central swelling: 7.8–15.8 µm
  • Striae in 10 µm
    6–10 in the center, 10–16 near the apices



Valves are weakly arcuate. The dorsal margins are convex and the ventral margins are concave. Most specimens have gibbous swellings observed in the valve center. Apices are weakly capitate, with acutely rounded to cuneate ends. Helictoglossae are present near the apices. Terminal raphe fissures are visible on the valve face, around the apices to the dorsal margin. Striae are perpendicular to the valve margins. Striae are typically regularly spaced, although irregular spacing is common. A sternum is visible near the ventral margin. Areolae are distinct and number 23–28 in 10 µm.

The concept of E. formica presented conforms to the taxon in a broad sense (sensu lato). Lange-Bertalot et al. (2011) draw distinction within European specimens of E. formica, and recognize the species E. formicina and E. myrmica. These species were distinguished based on differences in apicies; rounded in E. formicina and acuminate in E. myrmica. Our specimens overlap in areola density described for E. formica sensu stricto, E. formica sensu lato, E. formicina, and E. myrmica, and show a range of apices from rounded to acuminate. Because we see such notable variation in a single population, we consider the variation to be attributable to phenotypic plasticity within E. formica.


Burge (2014) found E. formica occurring among the epiphyton and benthos of cypress-tupelo wetlands with: 22.2–27.4 C, 5.6–7.4 pH, specific conductivity 71.2–834.0 µS/cm, 8.1–321.0 NTU, DO 0.1–7.7 mg/L, NO3-N 0.01–0.3 mg/L, NO2-N <0.01– 0.05 mg/L, orthophosphate 0.02–2.08 mg/L, and TP 0.11–3.93 mg/L. Other common species were Eunotia bidens, E. bilunaris, E. metamonodon, E. pectinalis, and E. superbidens as well as species of Gomphonema, Navicula, Nitzschia, and Pinnularia.

Credit: Credit/Source: USGS BioData
Distribution of Eunotia formica in the continental U.S. Retrieved 08 May 2015.
Credit: Credit/Source: USGS BioData
Distribution of Eunotia formica in the continental U.S. Retrieved 08 May 2015.

Original Description

E. formica, striata linearis, media parte utrinque et utroque apice turgidis. E. nodosa apicibus non reflexis, sed inflatis rectis.

  • Author
    Ehrenb. 1843

Original Images

E  Formica  Ehrenberg 1843

EMAP Assessment

EMAP Distribution

Eunotia Formica  Ehrenberg 40
Map 40

Response Plots

Eunotia Formica  Ehrenberg

Cite This Page

Burge, D, and Edlund, M. (2015). Eunotia formica. In Diatoms of North America. Retrieved July 17, 2018, from https://diatoms.org/species/eunotia_formica


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