• Category
  • Length Range
    9-68 µm
  • Width Range
    7-32 µm
  • Striae in 10 µm
    20-24 on raphe valve, 15-25 on rapheless valve



Valves are elliptic to linear-elliptic and relatively flat. The raphe (R) valve has a narrow axial area and a small circular or oval central area. The raphe is straight and filiform. The distal raphe ends are straight and expanded externally. The proximal raphe ends are straight externally and slightly expanded. Internally, the proximal raphe ends are deflected to opposite sides. Striae are radiate and interrupted by a hyaline ring positioned close to the valve margin. The areolae are usually well distinguished under LM, 15-25 in 10 µm along a stria. The valvocopula attached to raphe valve has conspicuous fimbriae. The rapheless (RL) valve has a linear to linear-lanceolate axial area and radiate striae. The external openings of areolae are narrow, long transapical slits. The internal openings are comparatively shortened and are oval, or nearly round. There are 8-20 areolae in 10 µm along a stria in the RL valve.

The concept of Cocconeis placentula presented here is in the broad sense (sensu lato). We group several varieties (and species), including the nominate variety C. placentula var. placentula, C. euglypta and C. lineata. The reason for presenting such a wide concept of this species is the lack of clearly established differentiating criteria between these taxa in monitoring programs. There has been a great deal of confusion because the RL valve is needed to determine the variety or species; the R valves of the varieties appear to be indistinguishable. In our experience, neither original descriptions of these taxa, nor the descriptions available in the floras are sufficient to separate them with any degree of certainty. Jahn et al. (2009) initiated a study of type materials of Cocconeis, including that of the nominate variety of C. placentula. Recent work (Romero and Jahn 2013) provided lectotypification of C. lineata and C. euglypta, yet further studies are necessary to determine whether and how individual species within C. placentula sensu lato may be distinguished using morphological characters.


10  Cocconeis
Credit: Mark Edlund
Light micrograph of living cells of Cocconeis placentula attached to a glass slide. Cells of Achnanthidium are also present. Chloroplasts are visible within the silica cell walls.
Gsed 002
Credit: Sarah Spaulding
Scannning electron micrograph of living cells of Cocconeis placentula attached to a rock. Note the mucilage pads present around a number frustules. The raphe valve is fixed to the substrate, while the rapheless valve faces up. Scale bar is equal to 50 µm.

Original Description

  1. Cocconeïs Placentula, Kuchen-Schildchen. C. testula elliptica plana, margine abrupto, extus et intus laevis.Cocconéide Gâteau, à carapace elliptique plate, escarpée au bord, extérieurement et intérieurement lisse.Aufenthalt: Bei Berlin. Auf Vaucherien und Lemna-Wurzeln findet sica diese Form zuweilen sehr zahlreich bei Berlin. Ich sah sie auch am Byssus des Mytilus polymorplius. — Länge 1/120 Linie; Breite mehr als 2/3 der Länge; Höhe 1/4 bis 1/5 der Länge. Eine Abbildung konnte nicht mehr gegeben werden. (Vergl. Conferva pennatula, VAHL, Flora danica, T. 945. 1792.)
  • Author
    Ehrenb. 1838

Original Images

Cocconeis placentula orig descr

EMAP Assessment

EMAP Distribution

Cocconeis Placentula  Ehrenberg 3
Map 3

Response Plots

Cocconeis Placentula  Ehrenberg

Cite This Page

Potapova, M, and Spaulding, S. (2013). Cocconeis placentula sensu lato. In Diatoms of North America. Retrieved July 21, 2018, from https://diatoms.org/species/cocconeis_placentula


The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Cocconeis placentula sensu lato 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.