Ehrenberg 1838 Category: Monoraphid
BASIONYM: Cocconeis placentula Ehrenberg 1838
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.
Basionym: Cocconeis placentula
Author: Ehrenberg 1838
Length Range: µm
Striae in 10 µm:
Cite This Page:
Potapova, M., and Spaulding, S. (2013). Cocconeis placentula. In Diatoms of the United States. Retrieved July 28, 2016, from http://westerndiatoms.colorado.edu/taxa/species/cocconeis_placentula
Species: Cocconeis placentula
Reviewer: Kalina Manoylov
Ehrenberg, C.G. (1838). Die Infusionsthierchen als vollkommene Organismen. Ein Blick in das tiefere organische Leben de Natur. erlag von Leopold Voss, Leipzig. pp. 1-xvii, 1-548, pls. 1-64.
Geitler, L. (1927). Somatische Teilung, Reduktionsteilung, Copulation und Parthenogenese bei Cocconeis placentula. Archiv für Protistenkunde 59: 506–549.
Geitler, L. (1932). Der Formwechsel der pennaten Diatomeen (Kieselalgen). Archiv für Protistenkunde 78: 1–226.
Geitler, L. (1958). Fortpflanzungsbiologische Eigentümlichkeiten von Cocconeis und Vorarbeiten zu einer systematischen Gliederung von Cocconeis placentula nebst Beobachtung an Bastarden. Österreichische Botanische Zeitschrift 105: 350–379.
Geitler, L. (1982). Die infraspezifischen Sippen von Cocconeis placentula des Lunzer Seebachs. . Algological Studies 30: 1–11.
Jahn, R., Kusber, W.H. and Romero, O.E. (2009). Cocconeis pediculus Ehrenberg and C. placentula Ehrenberg var. placentula (Bacillariophyta): Typification and taxonomy. Fottea 9(2): 275–288.
Romero, O. and Jahn, R. (2013). Typification of Cocconeis lineata and Cocconeis euglypta (Bacillariophyta). Diatom Research 28(2): 175-184. 10.1080/0269249X.2013.770801
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.
Credit/Source: Mark Edlund
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.
Credit/Source: Sarah Spaulding