• Category
  • Length Range
    5-17 µm
  • Width Range
    4.5-6.2 µm
  • Striae in 10 µm
    24-34 in center of raphe valve, 20-25 in center of rapheless valve, up to 45 at apices
  • Reported As
    Achnanthes exigua var. heterovalvata (Krasske 1923)
    Achnanthidium exiguum var. heterovalvum ( (Krasske) Czarnecki 1994)



Valves are linear-elliptical to elliptical-lanceolate with narrowly capitate, subcapitate, rostrate, or subrostrate apices. Larger valves are sometimes slightly constricted in the middle. Both raphe and rapheless valves have a narrow slightly sigmoid axial area. The raphe valve has a distinct fascia that is often slightly wider on one side. The rapheless valve has a small, transapically rectangular, often asymmetric, central area. The raphe is straight, but deflected to opposite sides near the apices, with terminal raphe fissures strongly curved to opposite sides. The external proximal raphe ends are simple, located in slight “pin-hole” depressions, giving the appearance that they are expanded. Internally, the central raphe ends curve toward opposite sides. The striae are radiate on both valves, but almost parallel at the apices. The raphe valve has 24-34 striae in 10 µm in mid-valve, up to 40-45 in 10 µm at the apices. The raphless valve has 20-25 striae in 10 µm at mid-valve and up to 35-40 in 10 µm at the apices. A few very small areolae may be present on the mantle of both valves. Areolae are round or transapically elongated externally, apically elongated internally, ca. 55-70 in 10 µm within the striae.

The variety Achnanthes exiguum var. heterovalvata was described as having a stria density of 34 in 10 µm. Our observations of material from the US, however, show that the nominate variety includes a gradient of stria density from 24 to 34 in 10 µm. We see that the variation of A. exiguum is quite broad, and therefore, consider Achnanthes exiguum var. heterovalvata as a subjective synonym.


Original Description

St. minutissima, elliptico-lanceolata, breviter rostellata, Long. 1/200´´´

  • Basionym
    Achnanthes exigua
  • Author
    Grunow in Cleve and Grunow 1880
  • Length Range
    11.3 µm

Original Images

Orig Ill  Ahexig
Orig Descr002

EMAP Assessment

EMAP Distribution

Achanthidium Exiguum 40 Copy
Map 40

Response Plots

Achnanthidium Exiguum Copy

Cite This Page

Potapova, M. (2010). Achnanthidium exiguum. In Diatoms of North America. Retrieved June 23, 2018, from https://diatoms.org/species/achnanthidium_exiguum


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