New geographical and taxonomical observations on two rare Gonatozygon species (Zygnematophyceae, Desmidiales) from the upper Okavango Delta, Botswana

Abstract

Two extremely rare and morphologically similar Gonatozygon species, namely G. dicranoaculeatum and G. chadefaudii, were previously recorded from only two geographical locations in the world. G. dicranoaculeatum was reported from Zambia and Australian billabongs, while G. chadefaudii was known from French Guiana and Australian billabongs. During June 2023, a Gonatozygon species was discovered in the upper reaches of the Okavango Delta, Botswana. This species had characteristics corresponding to both G. dicranoaculeatum and G. chadefaudii and can, therefore, be regarded as an intermediate between these two species. The main distinguishing characteristics between G. dicranoaculeatum and G. chadefaudii include the structure of the apices and chloroplasts, the presence of girdle bands and a slight median constriction, the cell length-to-width ratio, and the length of the spines on the surface. Gonatozygon cells from the Okavango Delta corresponded to G. dicranoaculeatum with regards to the chloroplast structure and the absence of a median constriction and girdle bands but were closer to G. chadefaudii in terms of their dilated apices and dimensions. Due to the discovery of cells with intermediate characteristics, the authors propose the merger of G. dicranoaculeatum and G. chadefaudii into a single taxon, namely G. dicranoaculeatum, as this name has priority. G. chadefaudii should then be transferred to a synonym of G. dicranoaculeatum.