Stauroforma exiguiformis

(Lange-Bert.) Flower, Jones and Round 1996      Category: Araphid
BASIONYM: Fragilaria exiguiformis Lange-Bert. 1993

Spicaticribra kingstonii


Stauroneis absaroka

LM scalebar = 10 µm = 80 pixels.


Contributor: Eduardo A. Morales | Sarah Spaulding - April 2013
Length Range: 4-22 µm
Width Range: 3-4 µm
Striae in 10 µm: 16-20


Valves are lanceolate with convex central margins and broadly rounded ends. The central sternum is narrow and linear. Striae are distinct and composed of round areolae. Areolae tend to decrease in diameter from valve face/mantle junction to the axial area Striae are parallel at the valve center, becoming slightly radial toward the apices and extending continuously onto the valve mantle, where up to two rows of areolae may be present. Costae are wider than the striae. Spines are tubular and hollow, bifurcating toward the tips. Spines are present along the valve face edge, including the apices, positioned in line with the costae. A row of areolae is located on the apical mantle region, continuous with the mantle areolae from the rest of the valve. Apical pore fields and rimoportulae are absent. Copulae are open and lack ornamentation. Valvocopula is wider than the remaining copulae. In girdle view, frustules are rectangular. Cells form short ribbon-like colonies, joined by linking spines.

A brief summary of the nomenclature may help to understand the current name of this species. Fragilaria (virescens var.) exigua Grunow in Cleve and Möller no. 144 (Cleve and Möller, 1978) lacked a formal description, so it is invalid. Grunow in Van Heurck (1881) illustrated the taxon, again without valid description, and used the name Fragilaria virescens var.? exigua Grunow in Van Heurck. Lange-Bertalot (1993) validated the taxon, with the new name, Fragilaria exiguiformis. At the time, Lange-Bertalot (1993) referred to fig 125: 4 (SEM) and 126: 11-18 (LM) in Krammer and Lange-Bertalot (1991) to illustrate of the taxon. Although the North American specimens conform to the European LM illustrations, the SEM features of North American taxa are quite different, particularly in terms of spines and the mantle areolae. The spines and mantle areolae of the specimens illustrated here are more characteristic of Staurosira in Krammer and Lange-Bertalot´s illustration. The current placement in Stauroforma reflects Flower et al. (1996), who erected the genus Stauroforma as distinct from Fragilariforma, Fragilaria and Staurosira.

Original Description

Basionym: Fragilaria exiguiformis
Author: Lange-Bert. 1993
Length Range: µm
Striae in 10 µm:

Original Description

There is no description in Lange-Bertalot (1993), simply a change of name and status: Fragilaria exiguiformis Lange-Bertalot nov. nom. nov. stat.

Original Images

Cite This Page:
Morales, E., and Spaulding, S. (2013). Stauroforma exiguiformis. In Diatoms of the United States. Retrieved April 23, 2018, from

Species: Stauroforma exiguiformis

Contributor: Eduardo A. Morales | Sarah Spaulding

Reviewer: Rex Lowe


Cleve, P.T. and Möller, J.D. (1878). Collection of 324 diatom slides with accompanying analyses of A. Grunow. Parts 1-6 (1877-1882), Upsala.

Flower, R.J., Jones, V.J. and Round, F.E. (1996). The distribution and classification of problematic Fragilaria (virescens v.) exigua Grun./Fragilaria exiguiformis (Grun.) Lange-Bertalot: a new species or a new genus?. Diatom Research 11(1): 41-57. 10.1080/0269249X.1996.9705363

Lange-Bertalot, H. (1993). 85 Neue taxa und uber 100 weitere neu definierte Taxa erganzend zur Subwasserflora von Mittleuropa. Bibliotheca Diatomologica 27, 454 p. Cramer, Berlin, Stuttgart.

Van Heurck, H. (1881). Synopsis des Diatomées de Belgique. Atlas. Ducaju & Cie., Anvers. pls 31-77.

Links & ID's

Index Nominum Algarum (INA)

Transfer INA

California Academy of Sciences (CAS)

Stauroforma exiguiformis CAS

North American Diatom Ecological Database (NADED)

NADED ID: 193001

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

Stauroforma exiguiformis

EMAP Response Plots

Stauroforma exiguiformis

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.


- Correction to striae count

The striae count was originally reported as 9-10 in µm. The striae count was corrected to 16-20 in µm to match the specimens shown on the page. - S. Spaulding