• Category
  • Length Range
    11.7-28.1 µm
  • Width Range
    2.5-4.5 µm
  • Striae in 10 µm
  • Synonyms
    Eunotia hemicyclus (Ehrenb.) Ralfs
    Ceratoneis hemicyclus (Ehrenb.) Grunow
    Pseudo-eunotia hemicyclus (Ehrenb.) Grunow



Valves are arcuate, with dorsal margin inflations. Valves at the smaller end of the size range have an opposing ventral inflation near the apices. Apices are sub-rostrate, with a rounded end. The ends are weakly reflexed in larger valves. Transapical costae are perpendicular to the ventral valve margin and terminate in short spines. The stria are composed of uniseriate areolae positioned between the transapical costae, features not visible in LM. A short raphe is present on the ventral mantle, near each valve end.


The population shown here was found within a lake in Nunavut Canada, from sediments at a water depth of 3.6m. The water column specific conductivity was 52 μS and pH was 5.2. Other taxa found in abundance within the sample include Psammothidium marginulatum, Stauroforma exiguiformis, Frustulia spp., Encyonema spp., Aulacoseira nivalis, Brachysira brebissonii, and Chamaepinnularia mediocris. Semiorbis hemicyclus was found at 3.5% relative abundance (Edlund et al. 2021).

Patrick and Reimer (1966) reported S. hemicyclus from mid-Atlantic to New England states occurring in cool, low mineral waters, often rich in organic matter. Semiorbis hemicyclus has been recorded in acidic waters of northern Europe (Stabell 1993), low mineral content (Grönlund and Kauppila 2000), and oligotrophy (Lange-Bertalot and Metzeltin 1996).

Original Description

S. testula parva lineari semicirulari obtusa transversim striata. Diamet. longit. 1/96-1/72'''. Fossilis ad Degernfors Sueciae. An fragmenta corpusculorum aliorum?

  • Basionym
    Synedra hemicyclus
  • Author
    Ehrenb. 1840

Original Images

Semiorbis hemicyclus original Illus
Semiorbis hemicyclus orig desc

Citations & Links



  • North American Diatom Ecological Database
    NADED ID: 9104001

Cite This Page

Edlund, M., Stone, J., Burge, D. (2023). Semiorbis hemicyclus. In Diatoms of North America. Retrieved May 24, 2024, from https://diatoms.org/species/semiorbis-hemicyclus


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