• Category
  • Length Range
    118-143 µm
  • Width Range
    11-12 µm
  • Striae in 10 µm
  • Synonyms
    Scalptrum obscurum (W.Sm.) Kuntze



Valves are linear-lanceolate and slightly sigmoid, with acutely rounded apices. The central area and proximal raphe are set at an oblique angle to the valve margin. The central area is small and elliptic. The raphe is strongly sigmoid and eccentric toward the convex valve margin, especially near the apices. The proximal raphe ends almost meet in the central area and are straight. The distal raphe ends deflect slightly toward the opposite valve margin. The transverse striae are fine, evenly spaced throughout and parallel, though can appear undulate due to curvature of the valve face. Longitudinal striae are not evident.

This taxon has been variously included in Pleurosigma (W. Smith 1856; Sterrenberg 1993) and Gyrosigma (Reid 2011). It is unusual in that transverse striae are evident, but longitudinal striae are not.


According to Patrick and Reimer (1966), G. obscurum is euryhaline, growing in slightly to highly brackish water. This species is widely distributed across the Eastern and Southern US, but relatively rarely encountered.

Smiths  Mill  Pond,  Maine
Credit: Google Maps September 2011.
Smiths Mill Pond, ME
Gyrosigma obscurum distribution map
Credit: USGS BioData
Presence of Gyrosigma obscurum in river surveys in the USGS NAWQA program.

Original Description

Pleurosigma obscurum, n. sp. Valve linear, slightly attenuated towards the obtuse extremities; median line not central: colour a very pale pine, striae obscure. Average length of valve 1/193 of an inch, greatest breadth of ditto 1/2000 of an inch (v.v). β. much smaller, more gradually attenuated. In brackish water, Poole Bay, 1849. Lewes, 1851. β. Poole Bay, covering a sponge. No power I have applied has brought the striae into view either in this or the last species, but the colour of the dry valves indicated their presence in both cases.

  • Basionym
    Pleurosigma obscurum
  • Author
    W.Sm. 1852
  • Length Range
    132 µm
  • Width
    13 µm

Original Images

Pleurosigma Obscurum Orig Fig
Pleurosigma Obscurum Orig Descr

Cite This Page

Polaskey, M., Ripple, H. (2018). Gyrosigma obscurum. In Diatoms of North America. Retrieved May 30, 2024, from https://diatoms.org/species/gyrosigma_obscurum


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