• Category
  • Length Range
    65-149 µm
  • Width Range
    6.5-6.8 µm
  • Striae in 10 µm
  • Synonyms
    Synedra ulna var. oxyrhynchus f. contracta Hust. 1930
    Fragilaria ulna var. contracta (Østrup) Main 1988



Valves are elongate and linear, with a variably constricted central margin. In larger specimens, the central margins are slightly constricted to nearly straight. The valve tapers slightly in the middle 3/4 of the valve, and narrows to approximately 1/3 the maximum width near the valve ends. Valves are 6.5 to 6.8 µm in width at the center, and away from the central area are 6.4 to 8.1 µm in width. Apices are rostrate to sub-capitate, and rounded to truncated. Striae are nearly parallel and arranged opposite one another; however, near the poles the striae begin to curve and become alternate to one another. The axial area is narrow and linear, expanding slightly near the central area. The central area is rectangular and contains ghost striae throughout. The central area may be surrounded by shortened, marginal striae. Individual areolae not discernible in LM. Each valve has two rimoportula, one at each apex.

Note that the specimens presented here differ from the original type illustration (below). The type material should be examined to resolve the circumscription of this taxon.


Original Description

Var. contracta m. Fig. 660/1. L. 0,112 mm., Br., greatest, 0,075 mm., in the middle 0,06 mm. Striae 9 in 0,01 mm. Slightly constricted. Striae absent from the centre. This variety must doubtless be regarded as a transitional form of Synedra Goulardii Bréb. in litt. (Cl. & Grun. arc. Diat. tab. VI, fig. 119), which is, however, shorter and broader. S. Goulardii has been found in Buenos Ayres, Porto Rico, Kamchatka and near Jenisei. Found in a gathering from the Faeroes labelled: >Green Algae from a swamp on Nolso<.

NOTE: the above reported width range was changed to correct an original publication error. Østrup reported original widths as 75 μm and 60 μm.

  • Basionym
    Synedra ulna var. contracta
  • Author
    Østrup 1901
  • Length Range
    112 µm
  • Width
    7.5 at widest, 6 in the middle µm
  • Striae in 10µm

Original Images

Synedra Ulna Var Contracta Orig Plate And Text

EMAP Assessment

EMAP Distribution

Synedra Ulna Var  Contracta Øestrup 40
Map 40

Response Plots

Synedra Ulna Var  Contracta

Cite This Page

Bishop, I. (2015). Ulnaria contracta. In Diatoms of North America. Retrieved July 21, 2018, from https://diatoms.org/species/synedra_ulna_var._contracta


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