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Achnanthidium deflexum

(Reimer) Kingston 2003      Category: Monoraphid
BASIONYM: Achnanthes deflexa Reimer 1966

Achnanthidium atomus


Achnanthidium druartii

LM scalebar = 10 µm = 80 pixels.


Contributor: Marina Potapova - May 2009
Length Range: 7-28 µm
Width Range: 3.1-5.3 µm
Striae in 10 µm: 15-28 in the middle of raphe valve


Valves are linear-elliptica to elliptic, with subrostrate ends. The raphe valve is concave. The raphe valve possesses a linear axial area, which widens slightly in the middle portion of the valve. The terminal raphe fissures are hooked towards the same side of the valve and are slightly expanded at the ends. Externally, the proximal raphe ends are teardrop-shaped. Internally, the proximal raphe ends curve to opposite sides. The rapheless valve is convex. It has a narrow, linear axial area widening slightly in the middle. Striae are parallel or very slightly radiate throughout both valves. Stria density of the raphe valve is 15-28 in 10 µm in the middle and 35-50 in 10 µm at the apices. Stria density of the rapheless valve is 15-26 in 10 µm in the middle and 30-35 in 10 µm at the apices. The external openings of the areolae are transapically elongated, appearing as narrow slits. Each stria in the middle part of the valve is composed of 4-5 areolae. Very short marginal striae are often present in the middle portion of the valve.

Note that the great range in stria density within these observations, as well as within the original description, is due to the strong gradient of change in stria density particularly at the ends of the valves. Furthermore, the density depends on the length measured. For example, the number of striae in 10 µm, 5 µm or 2 µm (measured in SEM images) will each result in a different measure.

Original Description

Basionym: Achnanthes deflexa
Author: Reimer 1966
Length Range: 7-25 µm µm
Width Range: 3.8-4.5 µm
Striae in 10 µm: 20-22 in the center valve, 28-30 at the ends

Original Description

Valve linear-elliptical to elliptical with extremities varying from subtlely protracted subrostrate to obtusely rounded, not protracted. Raphe valve with narrow, linear axial area curving at the ends; central area small, elongate-elliptical or lacking. Raphe filiform; proximal raphe ends close; distal ends curved or deflected in the same direction, subterminal. Striae parallel or very slightly radiate. Indistinctly punctate; more distant at the center of the valve than at the ends. Pseudoraphe valve with narrow, linear pseudoraphe; no distinct central area. Striae parallel or slightly radiate; indistinctly punctate. Striae, 20-22 in 10 µm at the center, becoming about 28-30 in 10 µm near the ends on both valves. Length, 7-25 µm. Breadth, 3.8-4.5 µm.

Original Images

Original text and images reproduced with permission by Academy of Natural Sciences of Philadelphia.

Cite This Page:
Potapova, M. (2009). Achnanthidium deflexum. In Diatoms of the United States. Retrieved May 20, 2018, from

Species: Achnanthidium deflexum

Contributor: Marina Potapova

Reviewer: Sarah Spaulding


Kingston, J.C. (2000). New combinations in the freshwater Fragilariaceae and Achnanthidiaceae. Diatom Research 15(2): 409-411. 10.1080/0269249X.2000.9705504

Kingston, J.C. (2003). Araphid and Monoraphid Diatoms. In: Freshwater Algae of North America. Ecology and Classification (J.D. Wehr and R.G. Sheath, eds). Elsevier Science pp. 595-636.

Patrick, R.M. and Reimer, C.W. (1966). The Diatoms of the United States exclusive of Alaska and Hawaii, V. 1. Monographs of the Academy of Natural Sciences of Philadelphia 13.

Potapova, M. and Ponader, K.C. (2004). Two common North American diatoms, Achnanthidium rivulare sp. nov. and A. deflexum (Reimer) Kingston: morphology, ecology and comparison with related species. Diatom Research 19(1): 33-57.

Links & ID's

Index Nominum Algarum (INA)

Original INA
Transfer INA

California Academy of Sciences (CAS)

Achnanthidium deflexum CAS

North American Diatom Ecological Database (NADED)

NADED ID: 1038

Autecology Discussion

Although formation of mucilagenous stalks was never observed in A. deflexum, this species might be able to produce stalk as other Achnanthidium species.

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 deflexum

EMAP Response Plots

Achnanthidium deflexum

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.