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Fragilaria socia

(Wallace) Lange-Bert. 1980      Category: Araphid
BASIONYM: Synedra socia Wallace 1955

Fragilaria recapitellata


Fragilaria synegrotesca

LM scalebar = 10 µm = 80 pixels.


Contributor: Gina LaLiberte | Melissa Vaccarino - November 2015
Length Range: 14-43 µm
Width Range: 3.4-4.1 µm
Striae in 10 µm: 16.4-18.5


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.

Original Description

Basionym: Synedra socia
Author: Wallace 1955
Length Range: 16-42 µm
Width Range: 3.5-4 µm
Striae in 10 µm: 17

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).

Original Images

Cite This Page:
LaLiberte, G., and Vaccarino, M. (2015). Fragilaria socia. In Diatoms of the United States. Retrieved May 20, 2018, from

Species: Fragilaria socia

Contributor: Gina LaLiberte | Melissa Vaccarino

Reviewer: Eduardo A. Morales


Lange-Bertalot, H. (1980). Zur systematischen Bewertung der bandförmigen Kolonien bei Navicula und Fragilaria. Kriterien für die Vereinigung von Synedra (subgen. Synedra) Ehrenberg mit Fragilaria Lyngbye. Nova Hedwigia 33: 723-787.

Patrick, R.M. and Reimer, C.W. (1966). The Diatoms of the United States exclusive of Alaska and Hawaii, V. 1. Monographs of the Academy of Natural Sciences of Philadelphia 13.

Wallace, J. (1960). New and variable diatoms. Notulae Naturae 331: 1-8.

Links & ID's

Index Nominum Algarum (INA)

Transfer INA
Original INA

California Academy of Sciences (CAS)

Fragilaria socia CAS
Fragilaria socia CAS

North American Diatom Ecological Database (NADED)

NADED ID: 66020

Autecology Discussion

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


Fascicles of Fragilaria socia

Credit/Source: Holotype slide, Academy of Natural Sciences GC 4036a. Gina LaLiberte