Pleurotaenium subcoronulatum var. rectum Borge 1903: 82

Levanets, Anatoliy & Vuuren, Sanet Janse Van, 2024, New insights into the morphology, taxonomy and geographical distribution of Pleurotaenium subcoronulatum (Zygnematophyceae, Desmidiales), Phytotaxa 655 (2), pp. 144-158 : 152

8. Pleurotaenium subcoronulatum var. rectum Borge 1903: 82 ; pl. 2, fig. 3

Type locality: Brazil, Rio Grande do Sul, Cachoeira, under Characeae , 22 February 1893.

Description: Semicell margins are parallel and straight ( Fig. 4G ) and not undulated like that of the type. There is a crown of 8–10 slightly swollen tubercles around the apex. Cells are 13–15 times longer than wide. The width at the isthmus is 24.5–25 µm, at the basal inflation 30 μm, and at the apex 27–27.5 μm ( Borge 1903). Some cells correspond to P. subcoronulatum var. detum , having a constriction below the apex; however, the constriction is never as prominent as that of P. subcoronulatum var. detum . In addition, in some individuals such a constriction is present on one semicell, but not in the other.

Distribution:

South America: Brazil ( Borge 1903; Bicudo & Martau 1974).

Note: Krieger (1937) included this variety as synonym in Pleurotaenium subcoronulatum var. africanum . In our opinion Borge’s variety is morphologically different due to the smaller size of the cells (0.8-0.9 times), wider basal inflation and apex, and a half number of the tubercles. We believe that this alga deserves to remain in the rank of variety.

From the map ( Fig. 3 ), together with paragraphs above, it is clear that P. subcoronulatum (including infraspecific taxa) is widely distributed, being present on most continents. Surprisingly, it was not recorded from any European countries. The Russian Federation, where P. subcoronulatum var. detum was recorded ( Fig. 3A ), forms part of both Europe and Asia, but the sampling site where this taxon was found was located close to the Chinese border, in the Asian part.

A new alga, Docidium subcoronulatum var. major De Wildeman (1897: 91) was described, from the freshwaters of Indonesia (Java, Tjampea [now Ciampea] near Bogor). De Wildeman (1897) noted that this variety was very similar to the type described by Turner (1893), except for its size which was considerably larger. Although the cells were approximately the same length than the type ( Table 1 ), they were noticeably wider. Dimensions were indicated as follows: Isthmus 44 μm in diameter, basal inflation 56 μm, apex 52 μm. These measurements are almost double the size indicated for the type by Turner (1893) ( Table 1 ).

In the following paragraphs taxonomical and nomenclatural recommendation is proposed based on morphological characteristics and their taxonomical value in the distinguishing of infraspecific taxon of Pleurotaenium subcoronulatum .

As previously noted, Docidium subcoronulatum W.B.Turner was transferred to the genus Pleurotaenium Nägeli (1849: 104) , as P. subcoronulatum (W.B.Turner) West & G.S. West (1895), and as a result, the transfer of D. subcoronulatum var. major De Wildeman is also necessary. As this variety is morphologically related to Pleurotaenium subcoronulatum (W.B.Turner) West & G.S.West, D. subcoronulatum var. major should be included in it. However, a comparison of the characteristics of Pleurotaenium subcoronulatum f. majus (discussed in the paragraphs above) and D. subcoronulatum var. major are compared, it is clear that these two taxa are morphologically almost identical. Similar characteristics include various aspects of morphology and cell dimensions. The morphology of these two taxa furthermore corresponds to the morphology of the cells found in the Okavango Delta. Based on similar characteristics and dimensions, it is proposed to combine P. subcoronulatum f. majus and D. subcoronulatum var. major and to raise the combined taxon to the level of variety ( P. subcoronulatum var. majus ). P. subcoronulatum f. majus and D. subcoronulatum var. major should be regarded as a basionym and synonym, respectively. The width of the cells and the basal swelling are distinct morphological characteristics which differentiate the proposed variety from the type and other infraspecific taxa.

Figures

FIGURE 3. Global distribution of all infraspecific taxa of Pleurotaenium subcoronulatum. A) Comprehensive depiction sourced from previous publications, B) Close-up view of the southern African region, highlighting the distribution of P. subcoronulatum along with its infraspecific taxa.

FIGURE 4. Comparison of the morphology of P. subcoronulatum with closely related infraspecific taxa. A) P. subcoronulatum (from Turner 1893), B) P. subcoronulatum (from West & West 1895, C) P. subcoronulatum f. elongatum (from Prescott et al. 1975), D) P. subcoronulatum var. africanum (from Krieger 1937), E) P. subcoronulatum var. detum (from West & West 1896), F) P. subcoronulatum var. maximum (from Van Oye 1953), G) P. subcoronulatum var. rectum (from Borge 1903) and H) P. subcoronulatum var. spinulosum (from Opute 2000). No scale bars; dimensions indicated in descriptions.