Valves are broadly lanceolate to slightly rhombic, and moderately sigmoid. The valve face is broadly arched. Apices are acutely rounded. The raphe is sigmoid and eccentric near the apices. The distal raphe ends are bilaterally deflected. Proximal raphe ends are variable, and may extend into the central area. The axial area is narrow and opens to a distinct, circular central area. Striae are decussate. At the apices, striae are more numerous and curve slightly as they continue from the valve face to the valve mantle. The valve length to width ratio is 3.8-4.9.

Species in the genus Pleurosigma may be distinguished by measurement of the angle of divergence of the oblique striae from the raphe, termed "raphe angle" (Sterrenburg 2002). Sterrenburg (2001) reported +8° to +13° raphe angles for P. inflatum, compared to +17° to +21° for P. diversestriatum. The specimens imaged here show large variability in raphe angle within the population. Raphe angle measurements in this observed population (Edisto Marina, South Carolina) range from 10-17°.


Pleurosigma inflatum is a marine taxon found in warm and tropical waters. However, it has also been observed in the North Sea and Normandy (Peragallo 1889), and the Newfoundland Banks (Lohman 1942). The specimens posted on this page are from estuarine waters of the Edisto Marina, near the coast of South Carolina.

E3 Kmjpleurosigma Inflatum  P Diverse Striatum Dfb
Credit: Photo by: Katherine M. Johnson
Pleurosigma inflatum (left) and P. diversestriatum (right) in dark field.

Original Description

The genus Pleurosigma has no less than five species, two being quite new, and both having the markings arranged diagonally, that is, with (what the Rev. W. Smith considers) cells placed alternately in contiguous rows. The outline of the largest (fig. 8) is very clumsy and the ends obtuse, and the median line but slightly flexed—this I call validum; the other, P.inflatum, on the contrary, is of a graceful outline, the apices acute, the flexure of the median line considerable, and is broad in proportion to its length (fig. 9).

  • Author
    Shadbolt 1853

Original Images

Plinflatum Originalimage
Plinflatum Originaldes

Cite This Page

Johnson, K. (2016). Pleurosigma inflatum. In Diatoms of North America. Retrieved June 18, 2018, from


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