• Category
  • Length Range
    53-154 µm
  • Width Range
    8-15 µm
  • Striae in 10 µm
  • Synonyms
    Pleurosigma obtusatum W.S. Sullivant and Wormley 1859
  • Reported As
    Gyrosigma scalproides (Morales 2006)
    Gyrosigma scalproides (Bishop et al. 2017)



Valves are weakly sigmoid in outline. The central part of the valve has parallel margins. The apices are bluntly rounded and turned in opposite directions. The axial area is narrow. The central area is small and slightly elliptic longitudinally. The raphe is sigmoid. The proximal raphe ends project into the central area, while the terminal raphe fissures are turned in opposite directions. Transverse striae are gently curved and distinctly prominent in the central area. Longitudinal striae are not evident. Areolae are not visible under LM.

For many years, this taxon was recognized as Gyrosigma scalproides in North American surveys (Morales 2006).


In the southeast US, we observed large population in the rocky substrate of a stream with low nitrate (0.32 ± 0.01 mg/L) and phosphorus (<0.02 mg/L) in soft water environment. Valves were found in higher relative abundance in colder months.

This taxon has been documented in streams of the National Ecological Observatory Network (NEON) sites: Black Warrior River near Dead Lake (D08 Ozarks Complex), Newton, Georgia (D03), Adjuntas,  Puerto Rico (D04 Atlantic Neotropical) and Prairie Peninsula (D06 Manhattan, Kansas).

Original Description

PLEUROSIGMA OBTUSATUM, sp. nov. - Oblongo-lanceolatum, leviter sigmoideum, apicibus obtusis; long. .0025"; striis trans. et long. 56 in .001".
Fresh water: Columbus, Ohio: Gambier, O., Prof. H.L. Smith; Genessee river, Mr. C.A. Spencer.
A very small species remarkable for the obtuse ends. It may be a Collectonema, but we have not observed it in gelatinous envelopes.

  • Basionym
    Pleurosigma obtusatum
  • Author
    W.S. Sullivant and Wormley 1859

Original Images

Pleurosigma obtusatum orig desc

Cite This Page

Brown, S., Manoylov, K. (2023). Gyrosigma obtusatum. In Diatoms of North America. Retrieved July 15, 2024, from https://diatoms.org/species/gyrosigma-obtusatum


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