Achnanthidium exiguum

(Grunow) Czarn. 1994      Category: Monoraphid
BASIONYM: Achnanthes exigua Grunow in Cleve and Grunow 1880

REPORTED AS: Achnanthes exigua var. heterovalvata (Krasske 1923) | Achnanthidium exiguum var. heterovalvum ( (Krasske) Czarnecki 1994) 

Achnanthidium eutrophilum


Achnanthidium gracillimum

LM scalebar = 10 µm = 80 pixels.


Contributor: Marina Potapova - December 2010
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


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

Basionym: Achnanthes exigua
Author: Grunow in Cleve and Grunow 1880
Length Range: 11.3 µm
Striae in 10 µm:

Original Description

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

Original Images

Cite This Page:
Potapova, M. (2010). Achnanthidium exiguum. In Diatoms of the United States. Retrieved April 23, 2018, from

Species: Achnanthidium exiguum

Contributor: Marina Potapova

Reviewer: Sarah Spaulding


Cleve, P.T. and Grunow, A. (1880). Beiträge zur Kenntniss der Arktischen Diatomeen. Kongliga Svenska-Vetenskaps Akademiens Handlingar, 17(2): 121 pp., 7 pls.

Czarnecki, D.B. (1994). The Freshwater Diatom Culture Collection at Loras College, Dubuque, Iowa. Kociolek, J.P. (ed.) Proceedings of the 11th International Diatom Symposium, California Academy of Sciences, San Francisco, CA, p. 155-173.

Links & ID's

Index Nominum Algarum (INA)

Transfer INA

California Academy of Sciences (CAS)

Achnanthidium exiguum CAS

NCBI Genbank Taxonomy

Achnanthes exigua NCBI

North American Diatom Ecological Database (NADED)

NADED ID: 1024

Autecology Discussion

EMAP Assessment

The Environmental Protection Agency (EPA) western Environmental Monitoring and Assessment Program (EMAP) study was completed during the years 2000-2004 (see citations at bottom of this page). Over 1200 streams and rivers in 12 western states (Arizona, California, Colorado, Idaho, Montana, Nevada, North Dakota, Oregon, South Dakota, Utah, Washington and Wyoming) were selected for sampling based on a stratified randomized design. This type of design insures that ecological resources are sampled in proportion to their actual geographical presence. Stratified randomized design also allows for estimates of stream length with a known confidence in several “condition classes” (good or least-disturbed, intermediately-disturbed, and poor or most-disturbed) for biotic condition, chemistry and habitat.

EMAP Distribution

Achnanthidium exiguum

EMAP Response Plots

Achnanthidium exiguum

EMAP citations

Results are published in:

Johnson, T., Hermann, K., Spaulding, S., Beyea, B., Theel, C., Sada, R., Bollman, W., Bowman, J., Larsen, A., Vining, K., Ostermiller, J., Petersen, D. Hargett, E. and Zumberge, J. (2009). An ecological assessment of USEPA Region 8 streams and rivers. U.S. Environmental Protection Agency Region 8 Report, 178 p.

Stoddard, J. L., Peck, D. V., Olsen, A. R., Larsen, D. P., Van Sickle, J., Hawkins, C. P., Hughes, R. M., Whittier, T. R., Lomnicky, G. A., Herlihy, A. T., Kaufman, P. R., Peterson, S. A., Ringold, P. L., Paulsen, S. G., and Blair, R. (2005). Environmental Monitoring and Assessment Program (EMAP) western streams and rivers statistical summary. U.S. Environmental Protection Agency Report 620/R-05/006, 1,762 p.

Stoddard, J. L., Peck, D. V., Paulsen, S. G., Van Sickle, J., Hawkins, C. P., Herlihy, A. T., Hughes, R. M., Kaufman, P. R., Larsen, D. P., Lomnicky, G. A., Olsen, A. R., Peterson, S. A., Ringold, P. L., and Whittier, T. R. (2005). An ecological assessment of western streams and rivers. U.S. Environmental Protection Agency Report 620/R-05/005, 49 p.