• Category
  • Diameter
    2.5-4 µm
  • Width Range
    8-20 µm
  • Striae in 10 µm
  • Synonyms
    Melosira granulata subsp. angustissima (O.Müll.) A.Cleve



Frustules are cylindrical and form colonies. Valves are 2.5-4 µm in diameter, with a mantle height of 8-20 µm. The ratio of the mantle height to valve diameter is greater than 3.2. The mantle has straight sides and the valve face is flat. The mantle areolae are square. The rows of areolae on the mantle are curved to the right (dextrorse), but often are almost straight and parallel to pervalvar axis in separation valves. There are 10-20 rows in 10 µm. The valve faces of most valves do not have areolae, but a single ring of areolae might be found on the valve face of separation valves. Linking spines are located at the end of each pervalvar costa. They are short and bifurcated. Separation spines originate from two pervalvar costae. Most separation spines are 2-6 µm long and broadly triangular, but 1 or 2 spines per valve are very long, almost equal in length to the valve mantle. The ringleiste is solid and moderately shallow.

Kilham and Kilham (1975) reported that A. granulata var. angustissima represents a morphological variation within the life cycle of A. granulata. We, however, observed a break in the distribution of valve diameters and mantle height/valve diameter ratios in natural populations of A. granulata sensu lato from the US. Because of the discontinuity in size, we recommend separating the variety angustissima from the nominate variety until additional studies clarify the taxonomy of this species complex. We suggest that specimens with a valve diameter 4 µm or less and mantle height/valve diameter ratio greater than 3.2 are be identified as A. granulata var. angustissima. We suggest that specimens with either valve diameter greater than 4 µm, or mantle height/valve diameter ratio lower than 3.2, or both, are placed in the nominate variety, A. granulata.

Note that it is not clear, whether A. granulata var. angustissima from North America represents the same biological species as the type of this taxon from Africa, but the name has been used by North American workers in the past and is here recommended to be used to maintain consistency.


Original Description

Discus kreisrund, ohne Dornen, mantel cylindrisch; Sulcus schmal und wenig tief, Ansatzring schwach trichterförmig. Porenreihen in steilen Sprialen angordnet, Poren grob. Membran mittelstark. Porenreihen 8-9 auf 10 μ, Poren in pervalvarer Richtung 10-12 auf 10 μ. Lg. 60 μ, lat. 3 μ.

  • Author
    O.Müll. 1900
  • Length Range
    3 µm
  • Width
    30 µm
  • Striae in 10µm

Original Images

Melosira granulata var. angustissima orig illus
Melosira granulata var. angustissima orig illus

Cite This Page

English, J., Potapova, M. (2010). Aulacoseira granulata var. angustissima. In Diatoms of North America. Retrieved July 15, 2024, from https://diatoms.org/species/aulacoseira_granulata_angustissima


The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Aulacoseira granulata var. angustissima 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.