• Category
  • Length Range
    14.1-42.9 µm
  • Width Range
    3.4-4.1 µm
  • Striae in 10 µm



Valves range from narrow and linear at the large end of the size range to fusiform at the small end of the size range. The ends are rostrate and protracted. The central area is expanded, usually to both sides of the valve but in smaller specimens the central area may be expanded unilaterally. The central area is distinctly thickened, forming a fascia, and may appear to be slightly raised. In addition, the central area may be "set off" from the rest of the valve by slight constrictions on each side. Within the central area itself, ornamentation is variable within a population. Striae may be absent, shortened striae may be present on one side of the valve, or ghost striae may be present. Striae are alternate and slightly radiate. Cells may be attached to the substrate by a mucilage pad, with a number of cells together forming small colonies.

At the small end of the size range, valves with an asymmetric central area may be indistinguishable from F. vaucheriae.


Frustules are joined in colonies, attached to a benthic substrate by a mucilage pad.

Fragilaria Socia  E2 39 7 117 3  Acker Nikon 4036A Crop
Credit: Holotype slide, Academy of Natural Sciences GC 4036a. Gina LaLiberte
Fascicles of Fragilaria socia

Original Description

Synedra socia nov. sp.

Valvis anguste lanceolatis, 16-42 µ longis, 3.5-4 µ latis, apicibus attenuato-rostratis, pseudo-nodule exentrico vel saepe marginem attingente, pseudoraphe angusta, striis 17 in 10 µ.

In outline, the longest specimen resembles that of Synedra rumpens var. familaris Grun. It differs from this in being somewhat wider (3.5-4 µ instead of 2-3 µ) which makes the diatom appear stouter. The striae have a morphology very similar to that found in Syndera acus var. angustissima Grun., i.e., more distinct, heavier. The central area generally extends to the margins, but in the smaller forms the area may become slightly unilateral. The shortened straie on the side of the central area are suggestive of Synedra vaucheriae Kütz. I have examined two different Kützing Decas #24, and the Synedra vaucheriae striae are coarser, and the striate side exhibits no swelling, i.e., is straight. Synedra socia forms the typical Synedra fascicles. It has been found in the Savannah River, a typical enriched eutrophic river. It reached maximum development in the later winter and early spring.

Type - Savannah River, Screven County, Georgia, Coll. R. Patrick, January 18, 1952 (A-G.C. 4036a).

  • Basionym
    Synedra socia
  • Author
    Wallace 1955
  • Length Range
    16-42 µm
  • Width
    3.5-4 µm
  • Striae in 10µm

Original Images

Synedra Socia 1 A  E Edit
Synedra Socia  Wallace Text1 Edit

Cite This Page

LaLiberte, G, and Vaccarino, M. (2015). Fragilaria socia. In Diatoms of North America. Retrieved December 13, 2018, from https://diatoms.org/species/fragilaria_socia


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