Achnanthidium standeri (Cholnoky) comb. nov. and Achnanthidium taiaense (J. R. Carter et Denny) comb. nov., two new combinations of morphologically similar Achnanthidium species from Africa
Taylor, Jonathan C.; Morales, Eduardo A.; Ector, Luc
Two African Achnanthes Bory species, namely A. standeri Cholnoky and A. taiaensis J. R. Carter et Denny, were studied under light (LM) and scanning electron microscopy (SEM) from type material and newly found populations. Both species are best placed in Achnanthidium Kütz. based on type of striae, structure of the areolae and raphe. We therefore propose the new combinations Achnanthidium standeri (Cholnoky) comb. nov. et emend. and Achnanthidium taiaense (J. R. Carter et Denny) comb. nov. Both species stand out from others in the genus due to presence of a special type of areola that has a slit-like aperture toward the valve exterior and are round to oval at the internal opening. These special areolae, originally referred to as "Schattenlinien" (translated here as "shadow lines"), are interspersed with regular areolae along the valve margins and on the valve mantle in both raphe and rapheless valves. Although both species have these distinctive shadow lines, they can be differentiated from each other based on a number of features of the raphe valves, including the density of striae toward the apices, the size of areolae toward the apex and presence of a prominence at the central nodule. Also the degree of bending of the frustules in girdle view is another distinguishing feature. Additionally, A. standeri has small denticulations around the inner areolar perimeter, a characteristic that also makes it unique among all Achnanthidium species reported in the literature to date. The ecology of both taxa is discussed based on literature reports and our own findings.