Arcuatasigma marginale Lobban, 2018

Lobban, Christopher S. & Reid, Geraldine, 2018, New Arcuatasigma species (Bacillariophyta, Pleurosigmataceae) from Guam and Belize, and the taxonomic identities of Toxonidea challengeriensis and its variety, Phytotaxa 346 (2), pp. 169-179 : 170-172

Arcuatasigma marginale Lobban , sp. nov. Figs 1–6

Cells solitary, 174–184 μm long, 11.5–12.5 μm wide; transapical striae 21–22 in 10 μm. Plastids unknown. Longitudinal striae ca. 20 in 10 μm, more or less parallel to raphe except two marginal rows and slightly displaced around central area. External areola openings short slits, orientation varying across the valve. External central raphe endings approximate, non-overlapping.

Type: — GUAM. Outhouse Beach , Apra Harbor, 13.464 N, 144.656 E, in a collection of microalgal coating on calcareous sand, ca. 15 m depth. C.S. Lobban and M. Schefter, 13 Dec. 2010. Holotype CAS! frustule located 17.0 mm E and 7.4 mm S of the mark on slide 1696, from sample GU52O-4, slide 1696, accession # 627432, slide # 223050, shown in Figures 1 and 2 .

Etymology:— With reference to the marginal longitudinal striae.

Description:— The sigmoid frustules were broad in the middle in both valve and girdle view, narrowing and then expanding into spathulate apices both deflected in the same direction ( Figs 1, 2 ), achieved by the twisting of the frustule through one of the narrow parts ( Fig. 3 ), characteristic of the genus. Valves had very little mantle.Areolae in transapical and longitudinal striae ( Figs 1–5 ); the longitudinal striae were deflected slightly around the central area and there were two marginal rows parallel to the valve margin ( Fig. 4 ). On the side to which the apices curved, the longitudinal striae were exactly parallel to the raphe, but on the opposite side they curved slightly relative to the raphe ( Fig. 4 ). External openings of the areolae were slits, but their orientation varied across the valve face ( Figs 4, 5 ). In the two rows closest to the sternum, almost all slits were oriented apically; in rare cases where two areolae were joined by a continuous slit, the areolae were adjacent in the apical direction ( Fig. 4 , arrowheads). There were no V-shaped slits joining areolae transapically in these striae, in contrast to A. arenicolum and A. belizense . In the subsequent longitudinal striae many of the slits were oriented diagonally across the areolae ( Fig. 5 ), although this was not consistent (compare Figs 4 and 5 ). In the two marginal longitudinal striae the outer row had consistently square areolae with generally apically oriented slits, whereas the second row areolae were often transapically elongated and had transapically elongated slits that sometimes joined the marginal areolar slit in an irregular V-shape ( Fig. 5 arrows). A series of silica thickenings was evident between the longitudinal striae ( Figs 4, 5 ), which were not present in A. arenicolum , suggesting that the surface of this species was slightly ridged, but we do not have images from an angle to confirm this.

The axial area was very narrow and the raphe branches straight. There was a tiny elliptical central area (0.5 μm diam.), slightly displacing 4–5 transapical striae and making a wave in the longitudinal striae ( Fig. 4 ). The external central raphe endings were simple and approximate. Terminal raphe endings were hooked ( Fig. 6 ). Internal aspect was not observed in this species. Girdle bands appeared to be plain ( Fig. 4 ); they were broad in the middle of the cell but became very narrow elsewhere.

Figures

FIGURES 1–6. Arcuatasigma marginale Lobban, sp. nov. Figs 1, 2. Holotype in LM (differential interference contrast). Fig. 3. Twisted pole in SEM. The raphe on the valve face can be seen rolling around to the lower side, while the valve surface at the apex has rolled up from below. Fig. 4. Central portion of valve and valvocopula, oriented with poles curved to left (inset). Showing the non-overlapping raphe branches, areolae in the proximal rows joined apically (arrowheads) vs. those in the distal rows joined transapically (arrows). Also showing the longitudinal striae parallel to the sternum on the left, slightly angled on the right. Fig. 5. Detail of valve showing areola slit orientation trends in different rows (double-headed arrows) and joined areolae in the marginal rows (arrows). Fig. 6. Detail of frustule at apex. Scale bars: 10 μm (Figs 1–3), 2 μm Figs 4, 6), 1 μm (Fig. 5).