Cocconeis skvortzowii (Sheshukova) Sheshukova 1951 in

Gogorev, Rinat, Yurchak, Maria, Sokolova, Irina, Glushchenko, Anton & Kulikovskiy, Maxim, 2024, Nomenclature and systematics of two Cocconeis species (Bacillariophyta) from Lake Baikal: Cocconeis baicalensis and Cocconeis skvortzowii, PhytoKeys 247, pp. 219-248 : 219-248

Cocconeis skvortzowii (Sheshukova) Sheshukova 1951 in Proshkina-Lavrenko (ed.) Opredelitel presnovodnykh vodoroslei SSSR 4: 193, figs 104 a – c (with indirect reference; with authorship of basionym “ Skv. ”) emend. Gogorev & Yurchak

Cocconeis placentula Ehrenberg var. skvortzowii Skabichevskij ex Sheshukova 1950 in Proshkina-Lavrenko (ed.) Diatomovyi analiz 3: 86, pl. 30, figs 10 a, b (with indirect reference; as combination with authorship “ (Skv.) Skabitsch. ”). Basionym.

Cocconeis placentula Ehrenberg var. baikalensis Skvortzov 1937 in Philippine Journal of Science, C 62 (3): 310, pl. 5, figs 5, 7, 8, illegitimate name, later homonym of C. placentula var. baicalensis Skvortzov & K. I. Meyer 1928 . Replaced synonym.

≡ Cocconeis placentula Ehrenberg var. sibirica Skabichevskij 1952 in Botanicheskie materialy otdela sporovykh rasteniy Botanicheskogo instituta imeni V. L. Komarova 8: 36 (as combination with authorship of basionym “ Skv. ”), illegitimate superfluous name. ≡ Cocconeis placentula Ehrenberg subsp. sibirica Skabichevskij 1977 in Prirodnye kompleksy nizshikh rastenii Zapadnoi Sibiri: 127, fig. 2, 9–11 (as combination with authorship of basionym “ Skv. ”). Synonyms.

Type materials.

Lectotype • (designated here): Skvortzov, 1937, Philippine Journal of Science, Section C 62 (3): 310, Pl. 5, figs 7, 8 .

Epitype • (designated here): figures here represented by Fig. 18 C ( Baikal , specimen BL 15645 , leg. A. P. Skabichevsky, 20 July 1965) .

Type locality.

Russia, Siberia, Lake Baikal, Olhon Gate, Boguchanskaya Gulf, Cape Elokhin, Cape Kotelnikovskiy .

Description.

LM (n = 17) (Fig. 16 A – Q ). Valves lanceolate-elliptical, 8.9–16.1 μm in length (mean 12.7), 5.6–9.0 μm in breadth (mean 7.1). Length to breadth ratio 1.5–2.0: 1 (mean 1.8: 1). RV with straight raphe, SV with broad lanceolate axial area. On RV 20–24 striae in 10 μm (mean 22), on SV – 18–23 in 10 μm (mean 21.4). Often SV with “ ghost raphe ”, so such valves similar to RV in view. Valvocopula closed or open, with sparsely spaced fimbriae.

SEM (n = 17). Valves from elliptical to linear-elliptical, 11.6–27.0 μm in length (mean 14.7–16.7), 6.3–17.7 in breadth (mean 9.1–10.5). Length to breadth ratio 1.3–1.9: 1 (mean 1.6: 1).

RV (n = 9). Valves concave, with slightly convex middle part. Axial area narrowly linear or indistinguishable. Striae uniseriate, 21–24 in 10 μm (mean 22.7). Areolae rounded or unevenly rounded, 22–33 in 10 μm of stria (mean 29.1), absent at valve apices. Valve mantle (contra mantle) slightly reversed.

RV, external view (Figs 17 A – C , 19 A – C ). Central area absent. Raphe straight, proximal endings slightly widened, distal endings more widened. Striae weakly radial, sometimes interrupted by clear or weakly defined narrow submarginal hyaline strips. Areolae increased in size and more densely located toward valve edge. Marginal ridge wider, 0.4–1.0 μm in breadth, and more distinct than submarginal hyaline strips.

RV, internal view (Fig. 19 D ). Central area small, round. Proximal raphe endings directed opposite, distal endings form small helictoglossae. Areolae equal in size.

RVVC. Valvocopula closed, up to 1.3 μm in breadth, with straight or weakly curved fimbriae, up to 1.3–1.6 μm high, 7–8 in 10 μm.

SV (n = 8). Valves convex, with concave middle part. Axial area usually wide, lanceolate, rarely narrowly or broadly lanceolate, with often ghost raphe. Central area absent. Striae uniseriate, 13–20 in 10 μm (mean 17.6), consisted of 2–4 elongated areolae.

SV, external view (Figs 17 D, E , 18 A, B ). Openings of areolae slit-like, usually located in depression. Axial area often with ghost areolae (small depressions) located in “ striae ” or randomly.

SV, internal view (Figs 17 F , 18 C, D ). At edge, striae consisted of 1–2 rounded or elongated areolae, divided into 1–2 parts by thin or thickened baffle. Areolae rounded or irregularly rounded near axial area, in 2–3 (up to 5) curving apical rows (distance between areolae in stria different). Adjacent “ postmacroareolae ” delimited by small ribs and sometimes extended up to axial area.

There are no data on copulae, except valvocopulae.

Comments.

Our data differ from the data of Skvortsov and Skabichevsky (Table 4 ) in the following: the range of valve length and width has been increased, namely, the minimal size of the studied valves is less than those noted in the literature; the stria density on the RV corresponds to the previous data, but density on the SV has been increased. Data in the literature available on areola density in stria most likely refer to a SV; our data obtained are significantly higher and refer to a RV.

Valves shown in Figs 17 D, E , 18 A, B are somewhat similar to Cocconeis margaritata Riaux-Gobin & Al-Handal and Cocconeis spp. ( Riaux-Gobin et al. 2010) according to the areolae pattern on external surface of the SV, namely, the openings of areolae are slit-like and located in a depression.

A comparison of Cocconeis skvortzowii with related species (Table 4 ) showed that the species is externally similar to small valves of C. pseudothumensis (including the ratio of valve length to breadth), but differs greatly in the stria density on the RV. The same results can be seen for C. neothumensis as well as the C. diminuta / C. neodiminuta complex.

The comparative analysis shows similarity / overlapping in stria density of Cocconeis skvortzowii with C. lineata and C. pseudolineata . But two last species have the larger size of valve compared to C. skvortzowii , and, accordingly, a larger length-to-breadth ratio. Data on the stria density on the SV in C. lineata are quite contradictory from different references, which most likely indicates a complex of several species. If we take into account only data on the epitype of C. lineata , then their stria density is lower than that of C. skvortzowii .

Two species, Cocconeis disculus and C. euglypta , differ from C. skvortzowii in both valve shape and SV structure, namely, in the number of apical rows of areolae.

A direct reference to the location of Cocconeis placentula Ehrenb. var. baicalensis is absent in Skvortzow and Meyer (1928): “ The list is published without reference to the place where a certain species had been found ”. However, the locations of 36 samples are given in their publication, of which later Meyer (1930, 341) indicated 8 typical locations for this variety (Table 1 ).

The three permanent slides with the date given in the protologue, and one more with a later date are deposited in the LE diatom collection. Slides No. 15 and 16 (according to the numbering of Skvortzow and Meyer 1928, 2) are labelled as the village of Merkusheva (“ village Merkutov ”), on the label of third one it is given “ d. Merkushevo [Merkusheva]. ” The exact location (with coordinates) was established using a 1912 map (http://www.etomesto.ru/map-chita_zabaykalskaya-oblast-1912). Reliable and abundant findings of Cocconeis baicalensis were not identified when studying these slides in LM.

Also, in the Diatom collection LE there is authentic material collected by K. I. Meyer in 1921–1928 (35 samples, Table 1 ), including 9 samples (data June 30 1926, or no data), the locations of which are indicated in Skvortzow and Meyer (1928) and Meyer (1930). The last ones are used to make permanent slides stored in LE, in two of them ( LE A 0004242 – LE A 0004245) clear and numerous valves of Cocconeis baicalensis were found.

Skabichevsky’s material (Table 1 ) was studied in LM and SEM to clarify the morphology and taxonomy of Cocconeis baicalensis , since many morphological characters and elements of the valve are not satisfactory when studied in LM. We choose SEM photograph Fig. 9 C , as the epitype of C. baicalensis , which clearly show the morphological features of the RV in the internal view, namely its 3 D-shape, density and arrangement of striae, shape and size of hyaline areas, poorly represented in the type material. Additionally, figs 8 A, 10 A, 12 C show the same of the RV in the external view and SV in the external and internal view.

Figures

Figure 9. Cocconeis baicalensis A – D raphe valve, internal view B valve with valvocopula. SEM. Specimen No. BL 15645, Island Bolshoy Ushkaniy. Scale bar: 5 µm.

Figure 16. Cocconeis skvortzowii A – C raphe valve D – Q sternum valve D – O, Q valve with ghost raphe. LM. Slides No. BL 18566 – BL 18675, Island Bolshoy Ushkaniy. Scale bar: 10 μm.

Figure 17. Cocconeis skvortzowii A – C raphe valve, external view C Lower valve (arrowhead) D – F sternum valve D, E external view F internal view. SEM A, C specimen No. BL 15646, Island Bolshoy Ushkaniy B, D – F specimen No. BL 15645, Island Bolshoy Ushkaniy. Scale bars: 3 µm (A, B, D); 5 µm (C); 6 µm (E); 8 µm (F).

Figure 18. Cocconeis skvortzowii A – F sternum valve A, B external view B sternum with ghost raphe and ghost areolae C, D internal view. SEM. Specimen No. BL 15645, Island Bolshoy Ushkaniy. Scale bars: 4 µm (A, B, D); 5 µm (C).

Figure 19. Cocconeis skvortzowii A – D raphe valve A – C external view D valve with valvocopula, internal view. SEM A, B specimen No. 336, Boguchanskaya Gulf C specimen No. 318, Kotelnikovskiy Mayak D Cape Elokhin. Scale bars: 2 µm (A – C); 5 µm (D).