Cocconeis scutellum var. parva

Riaux-Gobin, Catherine, Witkowski, Andrzej & Igersheim, Anton, 2018, Cocconeis scutellum var. parva (Bacillariophyceae) re-examination and typification, Phytotaxa 343 (1), pp. 20-34 : 28-30

Cocconeis scutellum var. parva history and ambiguities

Cocconeis scutellum var. parva was first illustrated by line drawings ( Van Heurck 1880 –1885: pl. 29, figs 8–9 reproduced in Figs 4–5 ; Peragallo & Peragallo 1897 –1908: pl. 4, figs 3, reproduced in Figs 6–7 ; Hustedt 1933 –1959:

fig. 791). The latter drawings highlight the small size and high stria density of C. scutellum var. parva (11 in 10 μm in Cleve 1895, 11–14 in 10 μm in Hustedt 1933 –1959, 10–11 in 10 μm in Frenguelli 1930: pl. 2, fig. 5). It can be noted that a RV hyaline rim is clearly present in Fig. 5 and Fig. 7 , and that, in Peragallo & Peragallo (loc. cit.), the marginal SV areolae are grouped in a kind of cluster ( Fig. 6 ). Giffen (1963: pl. 3, fig. 41) illustrated a SV with 12 striae in 10 μm, with a very similar pattern to that of the nominate variety (op. cit.: fig. 40) except for a coarser striation in the latter. Several authors have since illustrated C. scutellum var. parva with LM (e.g., Poulin et al. 1984: figs 49, 52, 53, 60; Krammer & Lange-Bertalot 1991: pl. 57, figs 5–7; Witkowski et al. 2000: pl. 38, RV fig. 10; Hein et al. 2008: pl. 17, figs 5–7; Álvarez-Blanco & Blanco 2014: pl 22, figs 12–16). None of these more recent publications represent the type material, and they do not give clear information about characteristic features of the variety.

Several authors described taxa via EM, as similar -or close- to C. scutellum var. parva ( Okuno 1957, Poulin et al. 1984, Suzuki & Kobayashi 2002, Riaux-Gobin & Romero 2003, Suzuki et al. 2005, De Stefano et al. 2008, Riaux-Gobin et al. 2011b, Zupo et al. 2014, see Table 3). Here again, none of the latter descriptions refer to the type material. De Stefano et al. (2008) show specimens where the RV rim is missing (op. cit.: figs 99, 101), with RV marginal areolae apically elongate (op. cit.: figs 97, 100), which do not match Grunow’s original type line drawings. We can remark that the images provided by Suzuki & Kobayashi (2002: pl. 4, figs 4–5) can be compared to those of C. scutellum var. ornata in Mizuno (1982). A reinvestigation of the Kerguelen material ( Riaux-Gobin & Romero 2003) allowed for a better definition of the taxon that is now published as Cocconeis subantarctica Riaux-Gobin & Witkowski (2017) . The taxon described by Okuno (1957) has dissimilar stria densities on its SV and RV, therefore it cannot be compared with the taxon illustrated by De Stefano et al. (2008). The images from Zupo et al. (2014), are very similar to those published by Sar et al. (2003: SEM figs 10–12) under the name Cocconeis sp. Sar et al. (2003: 83, 102) compared their specimens with others identified as ‘ C. diminuta Pantocsek’ by Montgomery (1978: SEM, pl. 67 A). Nevertheless, the taxon illustrated by Montgomery (1978) does not show a marginal row of axially elongate areolae and looks different from that illustrated by Sar et al. (2003). C. diminuta [ Cocconeis disculus var. diminuta (Pantocsek) Scheschukova in Zabelina et al. (1951: 194, fig. 105,4a,b)] also has a complex history [e.g., the description by Pantocsek (1901) is invalid and the type lost, see Krammer (1990)]. C. diminuta would merit a re-investigation as here tentatively proposed for C. scutellum var. parva .

authors. See comments in the text.

In conclusion, all of the descriptions concerning Cocconeis scutellum var. parva (see above references and Table 3) are puzzling. They apparently refer to different taxa. Nevertheless, and following Albert Grunow’s original drawings, and LM observation of Slide no 1035 ( Figs 9–16 ), C. scutellum var. parva has similar stria densities on the RV and SV and simple areola occlusion (no loculi), suggesting it was inaccurately described in several of the previously cited works ( Table 3), particularly for those describing SV loculi ( Okuno 1957, Suzuki & Kobayashi 2002, Suzuki et al. 2005).

Figures

FIGURES 1–7. Published drawings of: Cocconeis scutellum Ehrenberg (1838: 194, pl. 14: fig. 8/1–4, reproduced in Fig. 1; Van Heurck 1880–1885, pl. 29, figs 1–2, reproduced in Figs 2–3); Cocconeis scutellum f. parva Grunow in Van Heurck 1880–1885 (pl. 29, figs 8–9, reproduced in Figs 4–5: Reproduction of original sketches drawn by Grunow (Grunow’s ‘Bilder-Sammlung’, Acqu. 1901/3604); C. scutellum var. parva Grunow (illustrated in Peragallo & Peragallo 1897–1908, pl. 4, figs 3, reproduced in Figs 6–7). Scale bars = 20 μm.

FIGURES 8–16. Albert Grunow collection, slide no. 1035 housed in W (8); Cocconeis scutellum f. parva RV (9–12), note the marginal rim and the RVVC fimbriae (10, 12); C. scutellum f. parva SV (13–16). Scale bars = 10 μm.