• Category
  • Length Range
    5.6-22.5 µm
  • Width Range
    3.5-5.2 µm
  • Striae in 10 µm
    20-26 in the center, up to 35 at the ends of raphe valve; 18-22 in the center, up to 25 at the ends of rapheless valve



Valves are linear-elliptic to elliptic. Apices are variable, and may be rounded to blunt and irregular. Valves are often slightly asymmetric about the median apical plane. The raphe valve is concave. The axial area is narrow, and widens slightly in the middle portion of the valve. A rectangular to trapezoidal central area that reaches valve margin (fascia) is often asymmetric. Externally, the terminal raphe fissures are hooked toward the same side of the valve. The proximal raphe ends are teardrop-shaped. In the central part of the valve the striae are slightly radiate, 20-25 in 10 µm. Near the apices the striae are parallel and number up to 35 in 10 µm.

The rapheless valve is convex, with a narrow, linear axial area. In some specimens, the axial area is slightly widened in the center of the valve. The striae are parallel throughout and number 20-28 in 10 µm in the center. Striae number up to 25 in 10 µm at the apices. External openings of the areolae are small, circular and indistinguishable under LM.

The specimens presented here are from the Bluestone River, Virginia. Most specimens have 'typical' linear-elliptic valves, but we also encountered populations with lanceolate valves and more apiculate ends. These valves are similar in shape to Achnanthidium pyrenaicum, but possess the ornamentation characteristic of A. delmontii. Thus, this taxon appears to not only be variable in its morphology, but it may co-occur with A. pyrenaicum. As a result, it may be difficult to separate these two taxa.


Achnanthidium delmontii was first reported from central Appalachian rivers in 2007. This was the first report in North America and it corresponded to the first report in France. This taxon is appears to be expanding its range throughout eastern US.

The specimens illustrated here in the Autecology section are from Lewis Creek, Virginia. They have lanceolate valves with apices as in Achnanthidium pyrenaicum, but the striae pattern is that of A. delmontii. That is, with slightly curved and radiate striae around a relatively large central area on raphe valve. It is not clear whether these specimens belong to A. delmontii, A. pyrenaicum or represent yet another species.

Adelmontii Va667 1
Adelmontii Va667 2 Jpg

Original Description

LM: valves are linear with rounded apices becoming elliptical in small individuals (Figs 1–68). On the raphe valve, the axial area is narrow. The central area is irregular forming generally a rectangular fascia but a shortened stria can be present on one of the margin side. The raphe is filiform, straight, with distinct central pores. Striae are slightly radial. On the rapheless valve the axial area is acicular. Striae are parallel to slightly radiate at the apices; in most cases, two striae are slightly more spaced apart in the middle part of the valve.

SEM: externally, the raphe is straight with droplet–like proximal endings spaced about 0.8 μm apart (Figs 69, 70, 74). The terminal fissures are deflected to the same side and terminate on the valve face near the valve margin (Figs 69, 70, 75). Internally, the proximal raphe endings are curved in opposite directions (Figs 71, 72, 77) and the distal raphe endings terminate in small helictoglossae (Figs 71, 72). The central area is generally irregular, forming a more or less rectangular fascia but can present on one side a shortened stria consisting of one to three areolae (Figs 74, 77). On the raphe valve, the striae are composed of 5 to 7 (45 to 57 in 10 μm) more or less rounded areolae (Figs 69, 70). Sometimes, the Voigt fault is marked by a shortened stria (Figs 70, 72). On the rapheless valve, the striae consist of 6 to 7 areolae (55 à 61 in 10 μm) and two of them are more distant in the middle part of the valve (Figs 73, 76). On both valves, the areolae are internally occluded by hymenes perforated by delicate slits (Figs 79–82). There is a row of elongated areolae on the mantle (Figs 75, 76, 78) with a gap at the poles; the mantle elongated areolae are separated from those of the valve face by a hyaline area along the edge of the valve face (Figs 75, 76).

  • Author
    Pérès, Le Cohu and Barthès 2012

Original Images

Achnan Delmontii Orig Ill1
Achn Delmontii Orig Ill2

Cite This Page

Ciugulea, I., Potapova, M. (2020). Achnanthidium delmontii. In Diatoms of North America. Retrieved September 25, 2021, from https://diatoms.org/species/achnanthidium-delmontii


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