The genera Cocconeis Ehrenberg and Amphicocconeis gen. nov.

Taxa present on the sandy sediments in the sublittoral zone of South Brittany


In samples (39) collected during the may-august period (2003-2009), on sandy bottoms between 5 and 23 meters deep, around the "Ile de Groix" and the "Ile aux moutons", 45 taxa belonging to the 2 genera Cocconeis and Amphicocconeis were encountered.

Thirty five taxa are small forms (< 20-30 µm length). They belong to the nanophytobenthos. Several of them are already known to attach either only to mineral substrata (epilithic or epipsammic species) or also to organic substrata (epiphytic species) (Riaux-Gobin, 1991 a, b). So, likely most of these 35 taxa develop on the surface of sand grains, in depressions and crevices where they are protected from the abrasion that occurs when the sand is disturbed (Round et al., 1990). This could explain that we have obtained mostly rapheless valves of the small forms, although the samples have been shaken deeply to separate the diatoms from the sand. It is to note also that several small taxa have measurements lower than that indicated in the diagnoses.

Eight taxa are larger forms and belong to the microphytobenthos. At least 4 of them are designated epiphytic or also epipsammic but "not obviously attached to sand grains" (Riaux-Gobin, 1991 a, b). Likely, these 8 large taxa would live preferentially on small detrital fragments of macrophytes present on the surface of the sand.

Although, for most of the 45 taxa only rapheless valves were observed, identification was tentatively proposed for 26 of them.
They are presented in :

Genera Cocconeis and Amphicocconeis 1

Genera Cocconeis and Amphicocconeis 2

For most of the other taxa, the observed characterisitics differ conspicuously from those given in the diagnoses. So their identification remains to be determined.  They are presented in :

Genera Cocconeis and Amphicocconeis 3

Genera Cocconeis and Amphicocconeis 4

Retour à infralittoral de Bretagne
Return to sublittoral zone of South Brittany