• Category
  • Length Range
    32-70 µm
  • Width Range
    9.4-15 µm
  • Striae in 10 µm
    11 on the raphe valve, 9-10 on the rapheless valve



Frustules are heterovalvar. The valve margins are biundulate. The valve apices are rostrate, to only slightly expanded. The raphe valve face is concave about the transapical axis. On the raphe valve, the axial area is centered on the apical axis and may be gently curved. The central area is a distinct transverse fascia, that may be asymmetric. The proximal raphe ends are straight and slightly expanded. The striae are distinct, uniseriate and punctate. The striae are nearly parallel near the central valve becoming radiate near the ends. The rapheless valve is convex about the transapical axis and lacks a fascia. The narrow sternum is eccentric and positioned on the margin, except at the widest points of the valve. The striae are distinct, uniseriate and punctate. The striae are nearly parallel near the central part of the valve becoming radiate and curved near the ends.

As a result of the concave and convex valve faces, the frustule of A. coarctata is recurved in girdle view, where the raphe valve valve fascia is on concave side of the frustule.


An aerophytic species, frequently found as an epiphyte on wet mosses or rocks. Abundant in many samples from subaerial habitats (Tofilovska et al. 2014).

Original Description

Achnanthidium coarctatum, Bréb.

Valve linear-elliptical, constricted at the centre, attenuate, and constricted towards the rounded extremities. Striæ moniligorm, 24 in ·001”. Length of frustule ·0013” to ·0018”. v.v. Bréb in Kütz. Sp. Alg. p. 54 ad specim. authen. quæ dedit am. De Brébisson.

Fresh water. Cave near Royat ; Grassmere, Westmoreland, Aug. 1853, W. Sm.

  • Basionym
    Achnanthidium coarctatum
  • Author
    Bréb. ex W.Sm. 1855

Original Images

Achnanthes coarctata orig illus
Achnanthes coarctata orig descr

EMAP Assessment

EMAP Distribution

Achnanthes Coarctata 40 Copy
Map 40

Response Plots

Achnanthes Coarctata   Br Bisson   Grunow

Cite This Page

Lowe, R., Spaulding, S. (2011). Achnanthes coarctata. In Diatoms of North America. Retrieved September 18, 2019, from https://diatoms.org/species/achnanthes_coarctata


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