• Category
  • Length Range
    52.7-82.9 µm
  • Width Range
    9.7-12.0 µm
  • Striae in 10 µm



Valves are linear, with a slightly gibbous center and broadly rounded, slightly subcapitate apices. The axial area is linear and about one-third as wide as the valve. The central area is asymmetric and only a bit wider than the axial area. The surface of the valve of both axial and central areas are mottled. Outer fissure of the raphe is straight, while the inner fissure is curved forming a lateral raphe. The features of the raphe may be difficult to discern against the mottled surface of the valve. Proximal raphe ends are weakly expanded, deflected to one side, and closely spaced. Terminal raphe fissures are deflected to the primary valve side. The terminal raphe fissures are comma- or sickle-shaped. Striae are weakly radiate at the valve center and parallel throughout the rest of the valve.

Krammer (2000) recognizes several varieties of P. acrosphaeria and the nominate variety is treated here. The varieties are described from outside continental North America: P. acrosphaeria var. sandvicensis A. Schmidt is described from Hawaii, P. acrosphaeria var. parma Krammer is described from Java and P. acrosphaeria var. tumidula Krammer is described from Germany.


The specimens of P. acrosphaeria pictured on this web page are from a spring, a small lake, and a pond in western Montana. Kootenai Pond (photo below) has a pH of 7.78 and a specific conductance of 139 µS/cm. Krammer (2002) reports this species as mainly tropical in distribution, but also widespread in temperate regions of the northern hemisphere, where it is epipelic in waters with average electrolyte content and circumneutral pH.

Kootenai  Pond And  Citadel  Peaks
Credit: Loren Bahls
Kootenai Pond, Glacier National Park, Montana: home of Encyonema vulgare
Mud  Lake  Bob  Marshall  Wilderness 1
Credit: John Pierce
Mud Lake, Bob Marshall Wilderness: home of Encyonema droseraphilum

Original Description

  • Author
    W.Sm. 1853

Original Images

Pacro  Or Ill001
Pacro  Or Descr001

Cite This Page

Bahls, L. (2014). Pinnularia acrosphaeria. In Diatoms of North America. Retrieved August 18, 2018, from https://diatoms.org/species/pinnularia_acrosphaeria


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