Achnanthaceaea Kützing

Al-Handal, Adil Y., Compère, Pierre & Criobe-Usr, Catherine Riaux-Gobin, 2016, Marine benthic diatoms in the coral reefs of Reunion and Rodrigues Islands, West Indian Ocean, Micronesica 2016 (3), pp. 1-78 : 17-18

Family Achnanthaceaea Kützing

Achnanthes brevipes C.A. Agardh (Pl. 7, Fig. 20) References: Witkowski et al. 2000, pl. 45, figs 11-12; Cox 2006, figs 1, 3, 4, 6-9; Lobban et al. 2012, pl. 38, figs 1-4.

Features: Apical axis: 42-76 µm, transapical axis: 8-18 µm, striae: 10-11 in 10 µm. This species can be distinguished by the eccentric pseudoraphe on the sternum valve and the raphe on the other valve is enclosed in a stout rib.

Remarks: The specimens examined are rather elongate and relatively similar to A. angustata Greville but valves are shorter and striae are denser (7-8 in 10 µm for A. angustata ). The variability of the valve outline of this species makes it rather uneasy to identify, it also share similarities with other taxa such as A. brevipes var intermedia (Kützing) Cleve and Achnanthes bongarinii (M. Peragallo) A. Mann. A. brevipes is cosmopolitan species on marine and brackish coasts. Occurrence: Frequent in Reunion and Rodrigues Islands. Previously reported from Seychelles (G).

Achnanthes cf. brevipes var intermedia (Kützing) Cleve (Pl. 7, Fig. 24) References: Krammer & Lange-Bertalot 1991, fig. 1:4-8; Snoeijs 1993, p. 16, fig. 2)

Features: Apical axis: 20-26 µm, transapical axis: 8-12 µm, striae 14 in 10 µm. Valve lanceolate and slightly constricted in the middle, with broadly rounded apices. Raphe sinuous, axial area narrowly linear, central area transversely extending into widening stauros. Sternum narrow, linear and exentric.

Remarks: Very few specimens were observed. Valves are rather shorter and wider than those reported in the literature and striae are denser. Our specimens, however, look very similar to the figures of A. brevipes var intermedia shown by Snoeijs 1993 although puncta appears rather coarser. A. brevipes var intermedia can also be easily confused with Achnanthes taylorensis Kellogg, Stuiver, Kellogg et Denton , where both appear to be conspecific under LM. The major difference is in stria density where the later possess denser striae.

Occurrence: Frequent in Reunion and rare in Rodrigues Islands. Previously reported from Seychelles (G).

Achnanthes kuwaitensis Hendey (Pl. 7, Fig. 21) References: Hendey 1970, fig. 6: 63, 64, McIntire & Reimer 1974,, pl. II, fig. 6a-c, pl. III, fig. 4ab; Witkowski et al. 2000, pl. 43: 13-15; Al-Handal 2009, fig. 58.

Features: Apical axis: 40-48 µm, transapical axis: 8-9 µm, striae: 10-11 in 10 µm. The species is distinguished from A. brevipes by possessing subtrangular pseudo-ocelli on both apices of the sternum valve.

Remarks: Only raphe valves were observed, valve structure matches descriptions by Hendey

(1970) but specimens were slightly shorter than those described from other regions with striae slightly denser. When SV is observed, A. kuwaitensis can be easily recognized and differentiated from other Achnanthes species with linear valves by the terminal orbiculi on the ends of the SV. We differentiated our specimens from A. brevipes in that the observed raphe valves have very narrow dilated central area and more slender valves with round apices, features not shared by A. brevipes . It is widely distributed in warm and temperate waters around the world.

Occurrence: Rare in Reunion and Rodrigues Islands.

Achnanthes pulchella Heiden (Pl. 7, Fig. 29) References: Simonsen 1992, pl. 21, figs 1-5.

Features: Apical axis: 28-32 µm, transapical axis: 10-12 µm (middle part), striae (SV) 11-13 in 10 µm, (RV) 16-17 in 10 µm, areolae 18-19 in 10 µm. Valves slightly constricted in mid-valve with concave margins and obtusely rounded apices. Sternum valve with very narrow axial area and coarsely areolated striae, areolae rectangular. Raphe valve with very narrow axial area and straight raphe. Central stria are more spaced. Areolae on RV are more dense and smaller in size.

Remarks: This species has not been widely reported and was not described in detail in the literature (see illustrations in Riaux-Gobin et al. 2011c).

Occurrence: Frequent in Reunion and rare in Rodrigues Island. Previously reported from Seychelles (G).

Achnanthes trachyderma (F.Meister) Riaux-Gobin et al. (Pl. 7, Fig. 9) Basionoym: Cocconeis trachyderma F.Meister.

References: Meister 1935, p. 99, figs 63–64; Riaux-Gobin et al. 2015, p. 108, figs 27-32 (LM), figs 45-57 (SEM).

Features: Apical axis: 29-32 µm, transapical axis: 14-15 µm, 9-11 SV striae in 10 µm, 26 RV striae in 10 µm. Valves elliptical with produced capitate apices. Raphe valve slightly concave, raphe filiform and straight, axial area narrow, proximal raphe endings straight and close to each other, terminal raphe fissures slightly curved and continuing on the mantle. Extended and narrow stauros delineated on the margins by short striae. SV strongly convex, one row of large areolae lies on each side of the narrow sternum.

Remarks: Achnanthes trachyderma is relatively small (<45 µm long, see the tentative identification key in Riaux-Gobin et al. 2015) with an oblong-elliptic shape, and a SV striation denser than in Achnanthes citronella (A.Mann) Hustedt in Schmidt et al. (1874–1959: pl. 415, figs 3–8). On the largest A. trachyderma individuals, only the RV examination permits a precise determination (in A. citronella the RV central area is small and the stauros very reduced, while extended in A. trachyderma ). For these reasons, this taxon has probably been often misidentified as Achnanthes citronella in several publications (see references and comments in Riaux-Gobin et al. 2015, p. 115 and table 3, p. 116)

Occurrence: Rare in Reunion Island

Achnanthes sp. (Pl. 7, Figs 22, 23) Features: Valves linear, with almost parallel margins, slightly constricted in the middle with round apices, 15-30 µm long, 4-6 µm wide. Transapical striae 12-14 in 10 µm. Raphe straight, axial area narrow, central area expanded transversely forming a narrow fascia reaching the valve margins. Remarks: The taxon appears similar to A. kuwaitensis but is much smaller, with a denser striation and fewer areolae per stria.

Occurrence: Rare in Rodrigues Island.