Caloneis mendosina, Frenguelli, 1934

Sunesen, Ines, Tardivo Kubis, Jonas A. & Sar, Eugenia A., 2017, A re-investigation of three Frenguelli’s Caloneis taxa (Pinnulariaceae, Bacillariophyta) from Argentina, Phytotaxa 305 (3), pp. 165-178 : 176-177

Comparison of Caloneis mendosina with morphologically related species ( Table 2)

Caloneis mendosina is similar to C. silicula and associated varieties based on the observations of Cleve (1894). Krammer & Lange-Bertalot (1986) revealed that the most similar taxon to Caloneis mendosina is C. silicula f. peisonis (Grunow) Krammer in Krammer & Lange-Bertalot (1985: 22; 1986: 88, pl. 172, fig. 8). The comparison between the LM figures of both taxa shows that they have similar valve forms, e.g., linear-elliptic to linear-panduriform with triundulated outline, and apices are cuneate; nevertheless, the central undulation is narrower than distal undulations in C. mendosina and wider in C. silicula f. peisonis ( Table 2). SEM illustrations of C. mendosina and C. silicula f. peisonis ( Cocquyt 1999: 427, figs 1–8, 24, 26–29) provide more information on similarities and differences between the two taxa. Both taxa are similar in having a deep valve mantle, robust and narrower toward the poles and two girdle bands, an opened valvocopula and a segmental copula ( Cocquyt 1999: figs 26–29). Nevertheless, in C. mendosina the valve surface is narrow and flat with a curved proximal region and a vertical distal region merging gently into a deep mantle, while in C. silicula f. peisonis the valve surface is wide and flat curving abruptly into a deep mantle. This causes the frustules of C. mendosina to orientate in girdle view, while frustules of C. silicula f. peisonis are commonly lying in valve view. Also the terminal and central raphe fissures are slightly curved, deflected in the same direction in C. mendosina , whereas in opposite directions in C. silicula f. peisonis .

Other two freshwater species that show similarities with Caloneis mendosina are C. columbiensis Cleve (1894: 51 , pl. 3, fig. 4) described from Columbia River and C. nipponica Skvortzow (1936: 267 , pl. 2, fig. 7, pl. 3, fig. 9, pl. 4, fig. 15) described from diatom clay of Biwa Lake. They are similar in the linear-panduriform, triundulated valve outline. C. columbiensis pictured by Cleve (1894) and Metzeltin et al. (2005, figs 12–14) differs from C. mendosina in having the central undulation wider than the distal undulations; denser striae, straight toward the distal undulations becoming radial at the apices; and relatively narrower fascia barely longer than the central nodule ( Table 2). Caloneis nipponica pictured by Skvortzow (1936) and lectotypified by Ohtsuka & Tuji [2002: 244, figs 1, (Lectotype, slide 30000136), 2] also agree with C. mendosina in having a central undulation narrower than distal undulations, broad fascia, striae radiate throughout the valve and terminal external raphe fissures deflected toward one side of the apex in the same direction as the central external raphe fissure ( Table 2). Both taxa show subtle differences in stria density, 17–18 in 10 μm in C. nipponica vs 10–16 in 10 μm for C. mendosina . Unfortunately, LM and SEM descriptions of the frustule in girdle view are not available for Caloneis nipponica , thus a comparison cannot be completed. However, considering that C. mendosina was erected earlier than C. nipponica the ultrastructural analysis of the later was not imperative for determining the correct epithet of the taxon and was beyond the scope of this study.

Despite the fact that not many species in the genus Caloneis have been analyzed with SEM, it is possible to detect two groups of species with triundulated outline based on the depth of the valve mantle. One group including the species C. fasciata (Lagerstedt) Cleve (pictured by Antoniades et al. 2009, figs 4–6), C. fusus Hamilton & Antoniades in Antoniades et al. (2009, figs 50–58), C. schumanniana (Grunow) Cleve and C. lewisii Patrick (pictured by Stancheva et al. 2009, figs 39–42 and 43, 44 respectively) is characterized by a shallow valve mantle almost regular in depth around the valve. The second group includes C. australis Zidarova, Kopalová & Van de Vijver (2016: 40 , figs 13–17), C. mendosina (this study), C. silicula f. peisonis (pictured by Cocquyt 1999, figs 26–29) and is characterized by having a deep valve mantle, slightly or abruptly shallower at the apices. Among the mentioned species C. mendosina is the only one that shows a mantle with curved proximal region and vertical distal region.

Caloneis laticingulata Metzeltin, Lange-Bertalot & García-Rodríguez (2005: 29 , pl. 156, figs 1–8) was described based on LM. Metzeltin et al. (2005, fig. 8) shows that this species clearly belongs to the second group and is similar with C. mendosina having a broad girdle, reinforced angles, and a clepsydra shape due to the overlapping of epivalve and hypovalve mantles and valvocopulae. However, C. laticingulata can be distinguished from C. mendosina by the valve outline, linear lanceolate with a central undulation and cuneate apices, and by the central and terminal external raphe fissures bent in opposite directions ( Table 2).