• Category
  • Length Range
    141-183 µm
  • Width Range
    15-18 µm
  • Striae in 10 µm
    19-23 transverse; 16-21 diagonal
  • Synonyms
    Gyrosigma delicatulum (W.Sm.) J.W.Griff. and Henfr. 1856
    Pleurosigma angulatum var. delicatulum (W.Sm.) Van Heurck 1885



Valves are large and narrow. Valves are lightly silicified. The valve outline is sigmoid, with acutely rounded apices. The valve face is slightly convex across the apical axis. The central area is small and elliptic. The axial area is narrow. Areolae are arranged to form a decussate pattern. Length to breadth ratio ranges from 8-11.8.

The raphe is sigmoid, running along the center of the valve, becoming slightly eccentric near the apices. The angle at which diagonal stria intersect each other was quantified as the stria intersection angle (58 to 65 degrees) in the manner prescribed by Sterrenburg, (2002). The curvature of the raphe sternum was quantified as the raphe angle (6-7 degrees).

In SEM, the areolae are elongated along the apical axis. Areolae are occluded by a transapical silica bar. Helictoglossae are present.

Valves fluoresce cobalt blue at 100x and 200x magnification under dark field microscopy. The color becomes paler at higher magnification (400x). The color displayed under dark field is somewhat indicative of stria intersection angle (Sterrenburg 2002).

This species has been reported with a variable range in North America: 130-280 µm (Patrick and Reimer 1966), 128-190 µm (Czarnecki and Blinn 1978), and 154-170 µm (Lawson and Rushforth 1975).


Pleurosigma delicatulum was observed in an epilithic sample from a rock located about 20 cm deep at the edge of Lake Granbury, Hood County, Texas. In Lake Granbury, P. delicatulum co-occurred with Gyrosigma acuminatum, Ctenophora pulchella, and Surirella brebissonii. The conductivity of surface waters in Lake Granbury has been documented to fluctuate between 881 and 1960 µS/cm from 2016-2021. Over the same period of time the pH of the lakes surface water was documented to range between 7.7 and 8.7, and the temperature from 8.4 - 30.7 C. During a coring expedition in the summer of 2019, a pH of 9.4 was recorded in the lake surface waters.

Specimens from the Iowa Lakeside Lab Reimer Herbarium were present in epiphytic samples from Dugout Creek, Dickinson County, Iowa, and the Missouri River, upstream of the Niobrara River confluence in Knox County, Nebraska.

Pleurosigma delicatulum occurs on soft mud, rock and plant substrates. Pleurosigma delicatulum is known to occur in estuaries, rivers and lakes with hard water. This species appears to be limited by low conductivity. Pleurosigma delicatulum does not occur in great abundance and is easily overlooked in brightfield LM because its valves are very lightly silicified.

Pleurosigma delicatulum is widely distributed (Patrick and Reimer 1966). The type material was collected from brackish water in Lewes, United Kingdom (Smith 1852). According to AlgaeBase (Guiry 2019), there are reports of this taxon from every continent with the exception of Australia and Antarctica.

Pleurosigma Delicatulum Dark Field
Credit: Lane Allen
One valve of P. delicatulum in dark field at 400x
Pleurosigma Delicatulum Dark Field2
Credit: Lane Allen
Two valves of P. delicatulum in dark field at 200x
Pleurosigma delicatulum LM4 RA
Credit: Lane Allen
The raphe angle is quantified as the angle formed between a line placed on top of the raphe at the center of the valve and a line placed on top of the raphe at the apice (Sterrenburg, 2002).
Pleurosigma delicatulum LM4 SIA
Credit: Lane Allen
The striae intersection angle is quantified as the angle formed by the intersection of the oblique striae along the apical axis (Sterrenburg, 2002).

Original Description

Pleurosigma delicatulum, n. sp. Valve narrow lanceolate, flexure slight, extremities acute: colour very pale straw with a slight tinge of pink. Average length of valve 1/112 of an inch, greatest breadth of ditto 1/1500 of an inch. Stria 1/30000 of an inch apart (v. v.).

  • Author
    W.Sm. 1852
  • Length Range
    227 µm
  • Width
    17 µm
  • Striae in 10µm

Original Images

Pleurosigma Delicatulum Orig Illus
Pleurosigma Delicatulum Orig Desc

Cite This Page

Allen, L., Lee, S. (2019). Pleurosigma delicatulum. In Diatoms of North America. Retrieved May 20, 2024, from https://diatoms.org/species/pleurosigma-delicatulum


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