• Category
  • Length Range
    30.2-55.8 µm
  • Width Range
    15.9-20.0 µm
  • Striae in 10 µm
    7-9 near the center, up to 10 near the apices
  • Synonyms
    Navicula gastrum (Ehrenb.) Kütz.



Valves are elliptic, with broadly rounded to rostrate apices. Valves are usually slightly asymmetric to the apical axis. The axial area is narrow and the central area is irregular or transverse due to striae of variable length and arrangement. The raphe is simple, filiform, and slightly curved toward the secondary side of the valve. The proximal raphe ends are slightly expanded. The distal raphe ends are abruptly hooked at the valve mantle. Striae are strongly radiate, curved and irregularly shortened near the central area. Striae are distinctly uniseriate, widely spaced from one another and number between 6-10 in 10 μm. Areolae number 24-27 in 10 μm.


Placoneis gastrum is generally epipelic in meso- to eutrophic and brackish freshwater lakes and streams (Cox 2003). Placoneis gastrum was a significant indicator of low total phosphorus and marginal total nitrogen in the Awash, Blue Nile, and Omo-Gibe river basins in Ethiopia (Beyene et al. 2013). It was originally described among marine algae from Vera Cruz, Mexico, but its presence there was not confirmed in a later study (Cox 2003). Populations of P. gastrum were found in the plankton in Lazy Lagoon and Beck’s Canal in West Lake Okoboji, Dickinson County, Iowa. It has also been found in Douglas Lake, Michigan, and many other sites around the world (Cox 2003).

Chloroplasts are single, usually positioned on one side of the cell opposite the nucleus, and consist of a lobed plate on each valve connected by a central bridge which contains a flat pyrenoid (Mann and Stickle 1995). The chloroplast and the pyrenoid are split during cytokinesis rather than dividing on their own before cell division, and these, along with the nucleus, are stationary during reproduction (Mann and Stickle 1995). The chloroplast structure was a defining part of Mereschkowsky’s original Placoneis description (Cox 2003), where he also stated that Placoneis was likely more closely related to Cymbella than to the naviculoids (Mann and Stickle 1995). Placoneis gastrum has been observed to occasionally form triplets for sexual reproduction connected by a single mucilage capsule (Mann and Stickle 1995).

Placoneis Gastrum1
Credit: USGS BioData
Distribution of Placoneis gastrum in rivers of the continental U.S. based on the National Water Quality Assessment program. Retrieved 03 June 2015.

Original Description

P. gastrum, testula minor ventre late lanceolato, apicibus constrictis obtusis, parem productis, papillaribus. Icon!

  • Basionym
    Pinnularia gastrum
  • Author
    Ehrenb. 1843

Original Images

P Gastrum Orig Ill 2
P Gastrum Orig Des

EMAP Assessment

EMAP Distribution

Placoneis Gastrum   Ehrenberg   Mereschkowsky 40
Map 40

Response Plots

Placoneis Gastrum   Ehrenberg   Mereschkowsky

Cite This Page

Woodell, J. (2015). Placoneis gastrum. In Diatoms of North America. Retrieved July 18, 2018, from https://diatoms.org/species/placoneis_gastrum


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