Xanthidium trilobum Nordstedt (1870: 230)

Santos, Maria Aparecida Dos, Ramos, Geraldo José Peixoto & Moura, Carlos Wallace Do Nascimento, 2022, Taxonomic notes on genera Bourrellyodesmus and Xanthidium (Zygnematophyceae) from Chapada Diamantina, Bahia, Brazil, Phytotaxa 536 (3), pp. 228-242 : 238-239

Xanthidium trilobum Nordstedt (1870: 230) ( Figs 6 , 48–54 , 67 )

Synonym:

Xanthidum triblobum var. laeve Lemmermann (1914: 216 , fig. 14)

Cell dimensions:—length 48–62 μm (without spines), 61–89 μm (including spines), width 43–60 μm (without spines), 52–85 μm (including spines), thickness 18–25 µm, isthmus 10–18 μm, spines 5–13 μm long. Zygospore 110 µm in diameter (including spines), globular in shape and provided with long, apically furcated spines.

Habitat:—This species was common in study area, and found associated to C. caroliniana , N. amazonum and U. foliosa ; EC 0.02 mS. cm-1 (±0.01); DO 6.9 (±1.2); TDS 0.02 (±0.01); pH 7.1 (±1.1); T 30°C (±2.3); WT 68.7 (±14) cm.

Material examined:— BRAZIL. Bahia: Andaraí, Pantanal dos Marimbus, Lagoa do Baiano. Samples: HUEFS 253741, HUEFS 253746, HUEFS 253754, HUEFS 253756, HUEFS 253765, 253766, HUEFS 253768, HUEFS 253781, HUEFS 253796, HUEFS 253802, HUEFS 253811 .

Taxonomic notes:— Xanthidium trilobum was described by Nordstedt (1870: 230) based on specimens from Lagoa Santa (Minas Gerais). This species is characterized by the presence of trilobate semicells angles with forked spines, and a central protrusion on the face of semicells ornamented by granules.

This taxon has been reported in several Brazilian regions: South ( Paula et al. 2014), Southeast ( Børgesen 1891; Lemmermann 1914, as X. trilobum var. laeve Lemmermann ; Borge 1899, 1918; Bicudo et al. 2018) Midwest ( Borge 1925; Camargo et al. 2009; De-Lamonica-Freire 1992), Northeast ( Oliveira et al. 2011), and North ( Grönblad 1945; Förster 1964, 1969, 1974, Thomasson 1971, Bittencourt-Oliveira 1993, Lopes & Bicudo 2003; Souza & Melo 2011), in which morphological variation can be seen on the margins between the basal and polar lobes, right to rounded angles; apical margin, retuse to rounded; protrusion on the face of the semicell from shallow to projected, and ornamented by obtuse granules or granules with spiniform ends.

Some specimens identified in the present study are similar to those illustrated by Thomasson (1971, Pl. 16, fig.14) and Grönblad (1945, Pl. 8, fig. 148) from specimens collected in the Amazon region.

Morphologically, Xanthidium trilobum resembles X. subtrilobum West & G.S. West (1897: 88) , especially because of the shape of the cell, although the latter differs by having semicells with a spine not forked at the angles of the lobes, and the presence of a spine in the middle of the outer margin of each lateral lobe in apical view.

The zygospore with long, apically furcated spines analyzed in the Marimbus do Baiano were similar to reported by Coesel (1988) for the material collected in a shallow pool in Eskol, municipality of Roura, south of Cayenne, French Guiana.

The SEM images of X. trilobum cells observed ( Fig. 67 ) resembles to material studied to the Argentina by Couté & Tell (1981: 214, Pl.10, fig. 3), showing the facial semicells ornamented by a circle of nine granules surrounding a central one, and semicells with projected angles in short cylindrical processes, with 2 spines at the end.

Coesel (1996), analyzing the biogeography of desmids, commented the possibility that Xanthidium trilobum and X. subtrilobum have been sister taxa, the former reported from South and Central America, whereas the latter known from Africa, S. E. Asia, and N. Australia; and that the differences in morphology between both taxa occurred by supposed allopatric speciation by the dispersal model (colonization) rather than by the vicariance model (fragmentation).

Xanthidium trilobum was the second most common taxon of the genus in the study area and registered in approximately 37% of all analyzed samples.

Figures

FIGURES 1–6. Line drawings. 1. Bourrelyodesmus comperei sp. nov. 2. Xanthidium concinnum var. longispinum var. nov. 3. Xanthidium mamillosum var. mediolaeve. 4. Xanthidium nordstedtii. 5. Xanthidium regulare var. asteptum. 6. Xanthidium trilobum.

FIGURES 48–65. Xanthidium trilobum (48–54): frontal view (48, 53), lateral view (49), apical view (50), detail of central ornamentation circled on semicell of Fig. 48 (51), cell–division (52), zygospore (54). Xanthidium verrucosum sp. nov. (55–65): frontal view (55, 59, 62–65), lateral view (56, 61), apical view (57–58, 60), morphological variation of the ornamentation (55, 65) and spines size (63–65). Cell formation (65), showing a tiny spine on the basal lobe (arrows). Scale Bar: 40 μm, except Fig. 54 (50 μm, horizontal bar).

FIGURES 66–67. Xanthidium nordstedtii (66): frontal view (SEM). Xanthidium trilobum (67): frontal view (SEM). Scale Bar: 20 μm.