Fragilariforma horstii

E.Morales, Manoylov and L.L.Bahls 2012      Category: Araphid

REPORTED AS: Fragilaria spec. aus dem Yellowstone Nationalpark/USA (Krammer and Lange-Bertalot 1991) 

Fragilariforma constricta


Fragilariforma marylandica

LM scalebar = 10 µm = 80 pixels.


Contributor: Loren Bahls - March 2012
Length Range: 6-20 µm
Width Range: 6.1-11.7 µm
Striae in 10 µm: 34 in center to 40 ends


Valves are cruciform with narrowly rostrate apices. The axial area is very narrow, straight and faint in LM. Striae on each side of the valve are opposite one another, curved and mostly radiate throughout. Alternate striae near the valve center are shorter and do not reach the axial area. Frustules may be joined forming colonies in short chains.

This taxon should be compared with Staurosira inflatissima (Hustedt) Metzeltin and Lange-Bertalot 2007. Future examination may show the taxa to be conspecific.

Original Description

Author: E.Morales, Manoylov and L.L.Bahls 2012
Length Range: 4-15 µm
Width Range: 4-12 µm
Striae in 10 µm: 30-38

Original Description

Frustules rectangular in girdle view and only partially seen due to prominent central swelling, forming chains with the aid of spines (Figs 11, 16, 17). Valves isopolar, cruciform with narrowly rostrate, tapering apices (Figs 1– 5, 7–10, 12, 13). Length 4 –15 μm, breadth 4 –12 μm, striae density 30 – 38 in 10 μm (n = 79). Sternum very narrow and straight, faint in LM (Figs 1– 5,7–10, 12, 13 and 14). Valve mantle steep (Figs 14 – 21) with edge parallel to the valve face/valve mantle border (Figs 14, 15) and devoid of ornamentation at the valve apices (Figs 14, 15). Striae on each side of valve axial area aligned opposite to one another, curved on valve face and mostly radial throughout the valve (Fig. 14). Striae uniseriate, some uninterrupted from valve face to valve mantle (Figs 14, 16), but some others interrupted by a spine located on the valve face/valve mantle edge (Fig 14). Some striae shorter and not ending at the valve axial area, especially those located in the valve central expansion (Figs 14, 15, 17, 19). Volae not seen. Spines solid, conical becoming spatulate at the ends, and surrounded at the base by a ring of silica with projections (Figs 14, 16, 17). Spines mostly on costae, but sometimes along a stria at the valve face/valve mantle edge (Figs 14, 16, 17). Costae raised only on the valve interior making striae appear as troughs (Figs 18 – 21). Apical pore fields present, with several round poroids irregularly disposed at the valve mantle (Figs 14 –16, 18, 21). One portula per valve and represented by a round hole, isolated, and located in the place where a striae would have developed (Figs 15, 18, 21). Girdle bands wide, without perforations (Fig. 19). Plastids not observed.

Original Images

Cite This Page:
Bahls, L. (2012). Fragilariforma horstii. In Diatoms of the United States. Retrieved April 20, 2018, from

Species: Fragilariforma horstii

Contributor: Loren Bahls

Reviewer: Sarah Spaulding


Hustedt, F. (1952). Neue und wenig bekannte Diatomeen. IV. Botaniska Notiser 1952:366-410.

Krammer, K. and Lange-Bertalot, H. (1991). Bacillariophyceae. 3. Teil: Centrales, Fragilariaceae, Eunotiaceae. In Ettl, H., Gerloff, J., Heynig, H. & Mollenhauer, D. (Eds.). Süsswasserflora von Mitteleuropa. 2(3): 1-576. Gustav Fisher Verlag, Stuttgart, Germany.

Metzeltin, D. and Lange-Bertalot, H. (2007). Tropical Diatoms of South America II. Iconographia Diatomologica 18, 1-877.

Morales, E.A., Manoylov, K.M. and Bahls, L.L. (2012). Fragilariforma horstii sp. nov. (Bacillariophyceae) a new araphid species from the northern United States of America. Nova Hedwigia, Beiheft 141: 141-154.

Links & ID's

Index Nominum Algarum (INA)

California Academy of Sciences (CAS)

North American Diatom Ecological Database (NADED)


Autecology Discussion

There are six records of Fragilariforma horstii in the Montana Diatom Collection from ponds and low gradient streams in western Montana. Here the mean pH is 7.8 and the mean specific conductance is 49 µS/cm. The type locality for this species is in Alaska.


Paradise Spring Brook, Glacier National Park, Montana: home of Fragilariforma horstii.

Credit/Source: Loren Bahls