Freshwater Diatoms
- data somewhat restricted at present whilst we populate site
In the preface of Foged (1977) he states that many of the species he investigated in Ireland have a much greater range of variation as regards the size and shape of valve than he had observed previously in areas of similar size. He also observes that the diatom flora does not seem to be richer in species than in other areas with similar ecological conditions, and that several genera and species shown in the temperate zone of Europe and America were absent in the investigated material.
Frustules are monoraphid and heterovalvar, with one valve (the R-valve, or RV) bearing the raphe-sternum, and one valve (the P-valve, or PV) bearing a pseudoraphe, or raphe-less sternum. On the RV, the raphe-sternum is generally central, with a median fascia. On the PV, there is no fascia, and the narrower pseudoraphe is more or less excentric. Valves are slightly bent around the transapical axis, the RV being concave and the PV convex, giving the frustule a characteristic V-shape when seen in girdle view. Striae are uniseriate or multiseriate, varying with the species. Cells are either solitary or form short chains, in the former case with the RV appressed to a substrate, in the latter case most frequently attached to a substrate by a mucilage stalk produced from the RV.
Originally conceived as a very broad, inclusive group, intensive study over the past two decades has resulted in the separation of most species into newly created genera. Those remaining are almost exclusively marine, with only a few exceptions; e.g., Achnanthes coarctata (Brébisson) Grunow, a quite common aerophil. Unfortunately, some species originally described in Achnanthes are now taxonomically "stranded": they clearly do not belong to the genus sensu stricto, but have not been formally moved to other genera.
Valves are linear to elliptical with the apical axis flexed in girdle view. Cells are heterovalvar, with one raphid and one rapheless valve. Usually epiphytic, adhering by one valve face or by a mucilage stalk or pad from one apex. Ends are variable, rounded, rostrate, capitate or apiculate. Axial areas of both valves variable in form and may be dissimilar on the same frustule. Threadlike raphe present only on valve on inside of flexed girdle.
Frustules are monoraphid and heterovalvar, with one valve (the R-valve, or RV) bearing the raphe-sternum, and one valve (the P-valve, or PV) bearing a pseudoraphe, or raphe-less sternum. Both the raphe-sternum and pseudoraphe are centrally located. Valve outlines are quite variable with the species, ranging from ovoid to linear-lanceolate to elongate-linear, and they may show rostrate or capitate ends. Like the marine Achnanthes, members of this genus are bent around the transapical axis, with the resultant V-shape in girdle view. The RV and PV often differ in shape of the central area, and may also differ in the density of striation. It is frequently the case that both valves must be seen to produce a reliable species identification. Cells are attached to a substrate by a mucilage stalk, and many are rheophils, abundant in rapids, strong current, and the surf zone of larger lakes.
Achnanthes and Achnanthidium show many differences at the ultrastructural level, and it is possible that their more obvious morphological similarities may be due to evolutionary convergence.
EUCOCCONEIS Cleve ex Meister 1912
Frustules are monoraphid and heterovalvar, with one valve (the R-valve, or RV) bearing the raphe-sternum, and one valve (the P-valve, or PV) bearing a pseudoraphe, or raphe-less sternum. The general structure is achnanthoid, like Achnanthes, Achnanthidium, etc., but differing in that the valves are somewhat twisted around the apical axis, so that each valve exhibits a sternum that is sigmoid in shape. The commonest species, E. flexella (Kützing) Meister, is most frequently encountered in the periphyton of oligotrophic lakes.
Cymbella C. A. Agardh 1830.
Seventeen species are described in Agardh 1830 and no type is designated.
LEMNICOLA Round & Basson 1997
A monotypic genus created for the common L. hungarica (Grunow) Round & Basson, a species adapted to life attached to the floating macrophytes of the family Lemnaceae (Duckweeds). The general structure is typically achnanthoid, and cells are solitary. Sterna are prominent on both valves, and expanded on the RV into a highly characteristic, asymmetrical stauros. Striae are slightly radiate, and biseriate.
A guide to the morphology of the Diatom Frustule, Barber & Haworth 1981
The Diatom:Biology and Morphology of the Genera. Round et al.(1990)
Cells solitary, naviculoid, lying in valve or girdle view. Usually 4 girdle-appresssed plastids, one in each quadrant of the cell: occasionally 2 plastids lying against the epivalve. An exclusively freshwater, epipelic genus of very wide distribution but rarely abundant.
Elliptic to lanceolate, occasionally linear, outline sometimes gibbous or triundulate, isopolar and isobilateral. Ends rounded, apiculate, rostrate or capitate. Raphe central, usually fine and threadlike with often a slightly oblique slit. The polar raphe endings are characteristically forked externally into two long, straight terminal fissures, while the central external endings are frequently curved or deflected in opposite directions. Stria uniseriate, appearing as lines of round or transapically elongate pores and interrupted peripherally by a longitudinal line or series of lines. Axial area narrow, central area rounded or elliptic, occasionally an oblique fascia.
A very distinctive genus set apart by the combination of valve, plastid and raphe structure.
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