Because of the absence of MEB data, this study has not been submitted for publication.
Because these three taxa have one plastid lying against one valve and partly against the girdle (see chloroplastes and below, fig. 2), they would belong to the genus Achnanthidium rather than to the genus Achnanthes.
Some scarce specimens of Achnanthes brockmanniiSimonsen (1962) have been found in 8 samples among 46 samples collected around the
"Ile aux moutons" (southern Finistère) and around the "Ile de
Groix" (Morbihan), between 5 and 23 meters deep (middle water
Two taxa presenting some similarities with Achnanthes brockmanniiwere also observed. We named them Achnanthes sp. cf brockmannii and A. sp. aff brockmannii.
The first one is characterized by a central area of the raphe valve forming as a
bi-sigmoid transapically oriented fascia. The central area of the raphe valve of A. sp. aff. brockmannii is similar to that of A. brockmannii, i.e., it forms as a sigmoid transapically oriented fascia, but this fascia does not reach the valve margins.
Specimens of A. sp. cf. brockmannii
were found in 26 samples collected between 5 and 23 meters deep. In
most of these samples only some specimens were observed, excepted in
three samples collected at 5, 9 and 10 meters deep where they were more
Some scarce specimens of A. sp. aff. brockmannii were found in 8 samples collected between 9 and 20 meters.
The transapical striae of the rapheless valve are irregularly lineate.
Figs. 1 and 3 : A. sp. aff. brockmannii.
Fig. 1 :Raphe valves (b,c,e), frustules (d,f) and rapheless valve (a), with lengths between 27 and 40 µm.
Fig. 3 : Two Rapheless valves (L = 27 and 31 µm).
Fig. 2 : A. sp. cf. brockmannii : Two rapheless valves (L = 32 and 39 µm).
Variation of the length/width ratio as a function of the length of the frustules/valves for the 3 taxa :
The length/width ratio as a function of the length, increases more rapidly for A. brockmannii than for A. sp. aff. brockmannii.
This add an element in favour of the hypothesis that, although the
central area of the raphe valves of these two taxa both form as a
sigmoid fascia and that the striae of their rapheless valves are
lineate, they would be different species. The length/width ratio as a function of the length varies similarly for A. sp. cf. brockmannii and for A. sp. aff. brockmannii. However, the different shape of the central area of the raphe valves of these two taxa and the fact that the
striae of the rapheless valves have a different structure in the two
taxa, suggest that they would be also different species.
All the characteristics of A. brockmannii and of A. sp. cf. brockmannii point out that these two taxa are also different species.
So, we propose that A. sp. cf. brockmannii andA. sp. aff. brockmannii are two new species.