• Category
  • Diameter
    19.7-46.9 µm
  • Striae in 10 µm



Frustules are heavily silicified, especially on the valve face. Valves are disc-shaped. Valve face is flat with 2–4 (usually 3) carinoportulae located at the center. The carinoportulae are surrounded by a ring or collar. In smaller cells, large areolae are scattered across the valve face, forming coarse striae near the margin. In larger cells, large areolae are arranged into pronounced, radially oriented striae, which extend closer to the middle of the valve face.

There are 4.1–5.3 striae in 10 µm at the margin. Siliceous spinules may be present across the valve face. Striae continue from valve face to valve mantle. Ridges made of solid silica are present at the margin of the valve face, which these continue onto the mantle. Ridges are similar to spokes or struts and form a right angle with the mantle. Linking spines may protrude from ridges. When viewed using LM, striae appear to continue between ridges and on the mantle. When viewed using SEM,a pattern of striation is apparent beginning at the ridge feature on the valve face and continuing on the mantle. The pattern is composed of small areolae arranged into two parallel rows of striae found in furrows between each ridge feature. Furrows form a rounded junction with the mantle, creating a gap between ridge features. The pattern of ornamentation continues onto the mantle and terminates at a mantle step.

The valve mantle may be heavily silicified. Orthoseira as a genus is described as having “deep” mantles (Round et al. 1990). There has not been a formal description of what “deep” implies, and many species of Orthoseira lack any description of mantle height. Due to the subjective nature of this feature, the height of the mantle is not remarked upon for this species. The authors have encountered what could be considered both short and deep mantles in Orthoseira taxa.

Cells are cylindrical in girdle view. Girdle bands are open and ligulate. Many small poroids are present on each girdle band. The poroids are arranged into parallel rows of striae perpendicular to the long axis. Poroids are also present on the ligula. Narrow bands lacking poroids or ornamentation are observable on the advalvar and abvalvar margins of each cingulum.

Cells form chains via linking spines. Chains are formed when the spines projecting from the ridges of one valve slot into the gap between ridges of the next valve. Mantle striae are covered, and not observable, when cells are joined in chains.

Orthoseira oregoniana is similar to taxa within the O. roeseana complex (Houk 1993). There is some uncertainty regarding whether Houk’s description reflects the original type material. Until the uncertainty regarding the type material of O. roeseana is addressed, we can only compare this taxon with Houk’s 6 groups within the O. roeseana complex. Orthoseira oregoniana could only be confused with 3 groups: “tatrica”, “epidendron” and “dendrophila”, as these groups all have ridges near the margin (termed ‘ribs’ by Houk). However, group “tatrica” has spathulate spines while O. oregoniana has simple spines. Group “epidendron” has many more carinoportulae (2–7), and group “dendrophila” is smaller (8–29 µm in diameter).


Orthoseira oregoniana has only been reported along Horsetail Trail, in the Columbia River Gorge Scenic Area near Cascade Locks, Oregon. It was found growing near waterfalls (along the splash zone of a fall, attached to surfaces wetted by rock seeps, among bryophytes in cool/moistened/shaded areas). This taxon likely has a wider distribution than currently reported, with reported observations limited by sparse sampling.

Orthoseira taxa are generally considered rare, and only occur in similarly specific habitats, often those that are highly oxygenated. Orthoseira oregoniana co-occurred with two other taxa of Orthoseira that were not named or described in Danz and Kociolek (2022).

Original Description

LM: Valves disc-shaped, 23–45μm in diameter (Figs 59–63) (n = 16). Valve face flat, faintly rounded at the margin, forming a right angle with the mantle (Figs 59–64). Small, central, hyaline area may be present (Figs 60–62). Carinoportulae 2–4 (usually 3). Each carinoportula with a ring or collar surrounding it (Figs 60–63). No additional structures located between the carinoportulae. Valve coarsely striated, striae radiate from central area (Figs 59–63). Areolae more dispersed near the centre, become more regular near the margin (Figs 59–63). When aligned into striae, areolae 4–8 in 10μm. Large, regular, ridges at the margin (Figs 59–64). The ends of some ridges with additional spines projecting from them (Figs 60, 63–64). Striae continue between the ridges and onto the mantle (Figs 59–64). Ridges continue onto the mantle (Fig. 64). Cells cylindrical in girdle view, thickened beyond the mantle edges (Fig. 64). Some large poroids in rows on the girdle (Fig. 64).

SEM (Figs 65–72): Externally the valve face is flat with ridged edges (Figs 65, 67, 68). Very small pore-free central area is present (Figs 65–66). Carinoportulae present at the centre, surrounded by an inset ring with silica nodules (Figs 65–66). Carinoportulae unoccluded internally. No additional structures between carinoportula. A high density of siliceous spinules occur in the centre of the valve obscuring the areolae (Fig. 65). Spinule density lower near the margin, and areolae in radiate striae continuing between marginal ridges (Figs 65, 67). Ridges formed of solid silica, without perforations, some with linking spines (Figs 67–68). Two rows of striae between each ridge, continuing onto the mantle and terminating at the mantle step (Fig. 68). Cells cylindrical in girdle view (Figs 68–69). Girdle bands open and ligulate (Fig. 69). Poroids present at the centre of each girdle, lacking at the edges (Fig. 69). Where present, poroids of each girdle band form striae in rows perpendicular to the long axis (Fig. 69). Poroids small, simple (Fig. 69). Chains of cells formed via linking spines (Fig. 69). Spines from one cell slot between the ridges of the next cell (Fig. 69). Internally, valve face flat, rounding at the edges (Figs 70, 72). A small central hyaline area present (Fig. 70). Carinoportulae located centrally, circular, with a ring located within (Figs 70–71). No additional central structure or etched slits present. Striae composed of large areolae radiate from the centre (Figs 70, 72). Near the margin, slight ridge features, corresponding to those seen externally, present (Fig. 72).

FIB (Figs 73–75): Valve face flat, rounded both internally and externally near the margins (Fig. 73). Areolae pass all the way from exterior to interior (Fig. 73). Ridges composed of solid silica, not perforated (Fig. 73). Girdle bands with slight thickenings (Figs 74–75).

  • Author
    Danz and Kociolek 2022
  • Length Range
    23-45 µm

Original Images

Orthoseira oregoniana orig illus 1
Orthoseira oregoniana orig illus 2
Orthoseira oregoniana orig illus 3
Orthoseira oregoniana orig desc

Cite This Page

Danz, A., Kociolek, P. (2022). Orthoseira oregoniana. In Diatoms of North America. Retrieved March 31, 2023, from https://diatoms.org/species/orthoseira-oregoniana


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