Cosmarium redimitum Borge (1899: 18)

Ramos, Geraldo José Peixoto, Oliveira, Ivania Batista De & Moura, Carlos Wallace Do Nascimento, 2021, Taxonomic updates and a new variety of Cosmarium redimitum from South America, Phytotaxa 514 (1), pp. 77-84 : 78-79

Cosmarium redimitum Borge (1899: 18)

Cosmarium redimitum was described by Borge (1899) based on material associated with Utricularia L. collected in Guyana. It is an endemic species from South America ( Förster 1982, Coesel 1996), with records until present for Guiana ( Borge 1899), Venezuela ( Salazar & Guarrera 2000), French Guiana ( Thérézien 1985, Oliveira 2000), Colombia (Coesel 1992, Duque et al. 2013), Argentina ( Couté & Tell 1981), and Brazil ( Borge 1925, Grönblad 1945, Förster 1964, 1969, Thomasson 1971, Sophia & Dias 1996, Bicudo et al. 2019, Saturtino et al. 2020). Thus, it has a known distribution between the 10th parallel north and 30th parallel south. This southern limit corresponds to the Corrientes region, Argentina, which according to Tell (1980) is the distribution limit for many tropical desmid species from American continent.

That species is characterized by having elongate cells, subcircular semicells with apices slightly truncate, cell wall with intumescences distributed all over the sides, surrounded by several punctae, and with a noteworthy apical crown of tubercles (ca. 6-7 in frontal view, and ca. 13 in apical view).

Nevertheless, there are different interpretations of some morphological features of this species, which naturally depends on the authors’ point of view and the tools used by them. For example, originally, Borge (1899) reported the presence of scrobicles on the face of semicells, surrounded by punctations. Such observation has been reported also by Förster (1982: Pl. 28, fig. 3), and Bicudo et al. (2019: 204, fig. 158). In LM, in fact those structures resemble somewhat large scrobicles. However, under SEM analyses, those “scrobicles” actually are intumescences similar to plates, which are approximately hexagonal and surrounded by punctations ( Figs. 1-2 ), as represented by Couté & Tell (1981: 91, Pl. XV, figs. 3- 4), and Duque et al. (2013, Appendix 1, Fig. C). There are some other interesting SEM photos provided by Oliveira (2000) based on material collected in French Guiana (228: figs. 38-39, not figs. 33-34). These photos presented by the last author are quite similar to that taxon illustrated by Borge (1899: Fig 18a) with a conspicuous crown of tubercles on the apical margin, but as revealed through SEM, there are no scrobicles on the cell wall. In apical view, the tubercles represented in Fig. 39 ( Oliveira 2000) also are rather similar to that illustrated by Borge (1899: fig 18c) because of the ellipsoid outline of those structures.

Analyzing all records of Cosmarium redimitum from South America, we have noticed that actually there are two morphotypes of this species, and they do not appear to be a polymorphism of the same taxon, as there are no reports of cells with intermediate characteristics of both morphotypes.

The first morphotype corresponds that originally described by Borge (1899) from Guyana, C. redmitum var. redimitum , with a remarkable apical crown of tubercles, well visible on the apical margin, but with no scrobicles on the cell wall. Among the analyzed records, some that correspond to this morphotype are reported by Thérézien (1985) and Oliveira (2000) to French Guiana, Coesel (1992) to Colombia, Grönblad (1945), Förster (1969), Thomasson (1971), Pereira (2011), Sophia & Dias (1996), Bicudo et al. (2019) and Saturtino et al. (2020) to Brazil. On the other hand, the second morphotype has scrobicles between the tubercles in the apical crown ( Couté & Tell 1981, Salazar & Guarrera 2000, Duque et al. 2013). In addition, those tubercles are most flattened than in the first morphotype, almost on the same level as other intumescences, and generally, in a number of 14-15.

Thus, we consider those differences sufficient to propose the second morphotype as a new variety of Cosmarium redimitum , as the first morphotype comprises the nominate variety.

Figures

FIGURE 1. Cosmarium redimitum var. acroscrobiculatum var. nov. (LM). A–D. Frontal view E.Apical view; arrowheads—scrobicles. F. Cell division G. Zygospore; arrowheads—elongated tubercles of the mother cell.

FIGURE 2. Cosmarium redimitum var. acroscrobiculatum var. nov. (SEM). A–B. Frontal view C. Cell division F. Cell wall details; white arrowheads—flattened tubercles; yellow arrowheads—scrobicles. E. Apical view.