• Category
  • Length Range
    6.3-19.9 µm
  • Width Range
    6.1-11.7 µm
  • Striae in 10 µm
    34 in center to 40 ends
  • Reported As
    Fragilaria spec. aus dem Yellowstone Nationalpark/USA (Krammer and Lange-Bertalot 1991)



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.


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

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.

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

Original Images

Fraghorstii Origimag1
Fraghorstii Origimag2
Fraghorstii Origdesc

Cite This Page

Bahls, L. (2012). Fragilariforma horstii. In Diatoms of North America. Retrieved June 23, 2018, from https://diatoms.org/species/fragilariforma_horstii


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