Achnanthes obesa (Greville) Riaux-Gobin & Compère & Hinz & Ector, 2015

Riaux-Gobin, Catherine, Compère, Pierre, Hinz, Friedel & Ector, Luc, 2015, Achnanthes citronella, A. trachyderma comb. nov. (Bacillariophyta) and allied taxa pertaining to the same morphological group, Phytotaxa 227 (2), pp. 101-119 : 112-115

publication ID	https://doi.org/ 10.11646/phytotaxa.227.2.1
persistent identifier	https://treatment.plazi.org/id/712C87DA-0C4C-FF84-FF58-FA851ED54E95
treatment provided by	Felipe
scientific name	Achnanthes obesa (Greville)
status	comb. nov.

Treatment

Achnanthes obesa (Greville) comb. nov. ( Figs 4 View FIGURES 1–24 , 65–67 View FIGURES 58–67 , Table 2)

Basionym: Stauroneis (?) obesa Greville, Transactions of the Botanical Society of Edinburgh 8: 237, pl. 3, fig. 12. 1866 (figure reproduced here as Fig. 4 View FIGURES 1–24 ).

Synonym: Navicula obesa (Greville) A.Mann , pro parte typica.

Type:— AUSTRALIA. Queensland: Curteis Straits (slide BM 2736, cf. Williams 1988: 47, pl. 54, figs 5, 6).

Remarks:—From D.M. Williams’s type photographs ( Williams 1988): individual length 43 μm, SV with 13 striae in 10 μm, valve broad elliptic with three lateral gibbosities per hemivalve. Presence of a short stauros in fig. 5, while absent in fig. 6. The last remark implies that this taxon belongs to order Achnanthales and that the question mark by R.K. Greville probably denotes his hesitating about classifying this taxon.

Two individuals were observed in slide BM 2736 and one in BM 2593 (isotype slide also labelled as Stauroneis obesa ) ( Figs 65–67 View FIGURES 58–67 ). The individuals have undulated lateral margins, large width, narrow SV sternum, dense and parallel SV striae (13 in 10 μm), only slightly oblique at apices, RV finely striated (ca. 21 striae in 10 μm, Figs 66, 67 View FIGURES 58–67 ) and a high butterfly-like stauros ( Fig. 67 View FIGURES 58–67 , arrowheads). In view of his unique morphology and the specific epithet, we propose Achnanthes obesa to be restricted to the individuals with broad valves and lateral gibbosities.

In slide BM 2593 we observed an individual without lateral gibbosities (length 39 μm, width 20 μm, 11 SV striae in 10 μm, ca. 23 RV striae, not illustrated here) but its length/width is high (2.0 versus 1.7 for the individuals with gibbosities). The latter narrow individual can hardly be defined as ‘ obesa ’ and may probably pertain to Achnanthes trachyderma (see above).

Observations:— Stauroneis obesa was cited by Cleve (1895) as a synonym of Achnanthes mammalis . Mann (1925) recombined the first taxon as Navicula obesa (Greville) A.Mann , but on his illustrations (pl. 23, fig. 6 and pl. 24, fig. 1, op. cit.) the striae are strongly radiate and curved, while they are straight and only radiate at the apices in Stauroneis obesa ( Williams 1988: pl. 54, figs 5, 6, reproduced here as Figs 65–67 View FIGURES 58–67 ). The outline of Navicula obesa (see above) does not match the illustration by Greville (1866: pl. 3, fig. 12, reproduced in Fig. 4 View FIGURES 1–24 ); however, the type of Navicula obesa remains that of its basionym, Stauroneis (?) obesa (see McNeill et al. 2012, art. 7.3) and Navicula obesa is a synonym of Achnanthes obesa . On the other hand, the biraphid diatom observed and illustrated by A. Mann under this name belongs to an indeterminate taxon of the order Naviculales .

Achnanthes schmidtii Heiden in Heiden & Kolbe (1928: 581) ( Figs 21, 22 View FIGURES 1–24 , Table 2)

Type:—pl. 198, figs 37, 38 in Schmidt et al. (1895) (designated by Simonsen 1992: 31), reproduced here as Figs 21, 22 View FIGURES 1–24 .

Remarks:—The individuals are relatively small (25–46 μm long, 13–22 μm wide, with 8.5–10.0 SV striae in 10 μm). Following Simonsen (1992: pl. 30, figs 6–9; fig. 7 reproduced here as Fig. 23 View FIGURES 1–24 and fig. 6 reproduced here as Fig. 24 View FIGURES 1–24 with permission of E. Schweizerbart), Achnanthes schmidtii is a synonym of Achnanthes citronella . Nevertheless, the RV of Achnanthes schmidtii ( Fig. 24 View FIGURES 1–24 ) has coarse areolae and a very reduced stauros, therefore it may be considered a separated entity.

Achnanthes citrina (A.Schmidt) Peragallo (1897: 4) ( Figs 13, 14 View FIGURES 1–24 )

Basionym: Cocconeis citrina A.Schmidt in Schmidt et al. (1895: pl. 198, figs 28–30, reproduced here as Figs 13, 14 View FIGURES 1–24 ).

Remarks:—The outline of this taxon is almost discoid with apiculate apices (length 39–58 μm, width 24–49 μm). The RV and SV have a low density of striae (ca. 8 in 10 μm); the RV striae are composed of a limited number of areolae, and there is no stauros ( Fig. 14 View FIGURES 1–24 ). Mann (1925: 61) evoked a certain similarity of the latter taxon with that in Schmidt’s Atlas: pl. 198, figs 35, 36, 40, ‘except that the lower valve […] is very different’. A SV of this taxon has been illustrated by Desikachary (1988: vol. 5, pl. 402, fig. 7), but without a recent description or illustration of the RV, we prefer not to include this taxon in the key.

Cocconeis sabangi F. Meister (1932: 28 , pl. 9, fig. 63) ( Fig. 18 View FIGURES 1–24 )

Remarks:—Small taxon (23–33 μm long, 11–17 μm wide), with 12 SV striae in 10 μm and 30 RV striae in 10 μm. Meister (1932) did not illustrate the RV of his new taxon, but referred to figs 33–41 in Schmidt’s Atlas (pl. 198) with a remark about the incorrect striation in Schmidt’s figures where the fine striation of the RV is hidden by the rough structure of the SV. Hustedt in Schmidt et al. (1937) suggests Cocconeis sabangi to be a synonym of his new combination Achnanthes citronella . Foged (1984) also indicates Cocconeis sabangi to be a synonym of Achnanthes citronella , but he also proposed Cocconeis sabangi to be a synonym of Cocconeis apiculata . This doubtful taxon is not included in the identification key.

Cocconeis apiculata A.Schmidt in Schmidt et al. (1895: pl. 198, figs 31–32) ( Figs 2, 3 View FIGURES 1–24 )

Annotation in caption by A. Schmidt: ‘ Cocconeis apiculata A.Schmidt , following Grove = Cocconeis robusta Leud. Fortm. ’

Type:—Schmidt’s type material from the Cape of Good Hope has not been found, therefore Schmidt’s figures 31–32 are considered as types.

Description:—Following Schmidt’s figures: length 36–42 μm, width 19–23 μm, ca. 10–12 SV striae in 10 μm. No stauros. Cocconeis robusta Leuduger-Fortmorel (1879: 11 , pl. 1, fig. 1 reproduced here as Fig. 5 View FIGURES 1–24 ), cited by A. Schmidt as similar to his Cocconeis apiculata , shows apiculate apices that may be similar to C. apiculata , while all other features are different: in C. robusta the striae are short and robust, radiate but not curved, the axial area is wideelliptical, devoid of structures, the valve shape is elongate. Cocconeis robusta may be close to Stauroneis australis Greville (1863: 579 , pl. 15, fig. 13). Stauroneis australis was typified by Williams (1988: BM 2678, holotype, pl. 54, fig. 7, individual length 110 μm). From D.M. Williams’s photograph, SV has 9 striae in 10 μm, with striae coarser near the margin and irregularly spaced, and presents a short, well-identifiable stauros.

Remarks:—A. Schmidt in Schmidt et al. (1895) did not refer directly or indirectly to Stauroneis apiculata ( Fig. 1 View FIGURES 1–24 ); therefore, this name cannot be the basionym of his Cocconeis apiculata which thus is clearly the name of a new species. In fact, the stauros present in Stauroneis apiculata ( Williams 1988, pl. 54, figs 2, 3; and our Figs 1 View FIGURES 1–24 , 58, 60, 61 View FIGURES 58–67 ) is not illustrated in Cocconeis apiculata A.Schmidt in Schmidt et al. ( Figs 2, 3 View FIGURES 1–24 ). Following these remarks, we can note that Mann (1925: 61, 62) probably improperly stated that ‘Schmidt’…‘makes Cocconeis apiculata (Greville) A.Schmidt and illustrates in the latter’s Atlas, plate 198, figures 31, 32’. Mann (1925: 60) also proposed Schmidt Atlas’s fig. 38 (unnamed in pl. 198) to be the same Cocconeis apiculata as fig. 31, with the following remark: ‘it may be synonymous with Raphoneis mammalis Castracane’ [here reproduced as Fig. 6 View FIGURES 1–24 ] […] and if it is the case, Castracane’s name would have priority against Schmidt’s one’. Finally, A. Mann concluded ‘see a discussion of these forms under C. citronella , new species’. It can be noted that Mann (1925: 62) also states that ‘it may be here added that Castracane’s Rhaphoneis mammalis is probably a synonym of Cocconeis robustus Leuduger-Fortmorel’ (see above the note about Cocconeis robusta ).

Foged (1984: 28) states that ‘ Cocconeis apiculata and Achnanthes citronella are difficult to separate, they may belong to the same species’.

From A. Schmidt’s illustrations, ascription of this species to Achnanthes rather than to Cocconeis is not possible. An examination of the original material from the Cape of Good Hope would be necessary to ascertain its taxonomical position. As it differs from most of the taxa here treated by the small rounded central area, not prolonged transversally in a stauros, we prefer not to include this doubtful taxon in the identification key.

Tentative identification key based on LM observations

1. Protracted apices, neither discontinuity nor shoulders near apices....................................................................................................2

- Striae biseriate, radiate, RV with a stauros.................................................................................. Achnanthes mammalis sensu Cleve

2. SV striae uniseriate, strongly radiate, RV with a stauros, fine SV areolae, long apices with presence of SV areolae......................... ........................................................................................................................................................................... Achnanthes apiculata

- Protracted apices, shoulders, SV striae uniseriate only radiate on apices, coarse SV areolae, reticulum-like...................................3

3. Length> 45 μm, SV stria density <7 in 10 μm, short RV stauros ................................................................... Achnanthes citronella

- Length <45 μm, SV stria density> 7 in 10 μm, RV stauros more or less wide................................................................................4

4. Undulated margins, high L/W ratio......................................................................................................................... Achnanthes obesa - Oblong to linear valve shape, wide RV stauros............................................................................................ Achnanthes trachyderma - Short RV stauros, coarse RV striation ............................................................................................................... Achnanthes schmidtii