Arcuatasigma arenicolum 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 : 172

Arcuatasigma arenicolum Lobban , sp. nov. Figs 7–18

Cells solitary, length 164–185 μm, width 8–11 μm; two lobed plastids along the valve margin. Transapical striae 28–29 in 10 μm, longitudinal striae ca 36 in 10 μm, parallel to the raphe, no distinction of marginal rows. External areola openings all apically oriented slits, except V-shaped slits commonly joining two areolae. External central pores overlapping, internal central area with a curved bar on each side.

Type:— GUAM. Outhouse Beach , Apra Harbor, 13.464 N, 144.656 E, in a collection of microalgal coating calcareous sand, ca. 15 m depth. C.S. Lobban and M. Schefter, 13 Dec. 2010. Holotype CAS! frustule located 13.8 mm E and 7.3 mm S of the mark on slide 1696, from sample GU52O-4, slide 1696, accession # 627432, slide # 223050, shown in Fig. 9 .

Etymology:— Latin, arenicola , -um, sand dwelling.

Additional materials examined:— GU52V-4

Description:— This species had sigmoid frustules with spathulate apices both deflected in the same direction ( Figs 7, 9, 10 ). Frustules were nearly square in transapical section, because the copulae together were of similar width as the valve, and they tapered similarly toward the poles ( Fig. 13 ). Thus frustules may lie in valve view, girdle view, or in between, and the appearance of the apices varied accordingly. In valve view the apices were very narrow; the spathulate expansion was apparent only when the apex was in girdle view ( Figs 12, 13 ) and it was then compounded by the width of the copulae ( Fig. 12 ); the striae across it had ca. 8 areolae. The frustule was twisted at one end, characteristic of the genus ( Fig. 10 ).

Two plate-like plastids lay along the valve faces, which may look ribbon-like in girdle view and there was a small plastid or extension at the apices ( Figs 7, 8 ). Valve finely striated with very little mantle ( Fig. 11, 13 ). Areolae formed transapical and longitudinal striae ( Figs 11, 13, 14, 17 ); the external openings of the areolae were apically oriented slits, except for the frequent occurrence of transapically joined pairs of areolae in which the external opening was a V-shaped slit ( Fig. 15 ).

The axial area was very narrow and there was a tiny elliptical central area (4 μm diam.), that slightly displaced the longitudinal striae ( Fig. 14 ). The external central raphe fissures are ended in small, overlapping pores ( Fig. 14 ); the terminal fissures were hooked ( Fig. 16 ). Internally, the raphe slit ran along the side of a rib ( Fig. 17 ), the central nodule was flanked by central bars of silica ( Fig. 17 ), and the termini ended in a helictoglossa ( Fig. 18 ). The cingulum was plain, broad in the middle of the cell, becoming first very narrow and then broader toward the apices ( Figs 12, 13 ). Arcuatasigma arenicolum was more common in the sample than A. marginale .

Figures

FIGURES 7–18. Arcuatasigma arenicolum Lobban, sp. nov. Figs 7–12 DIC, Figs 14–18 SEM. Figs 7, 8. Live cell in valve and girdle view, showing plastids. Fig. 9. Holotype. Fig. 10. Another cell; arrowhead indicates position of twist in the frustule. Fig. 11. Detail of central part of cell in valve view. Fig. 12. Detail of apex in girdle view. Fig. 13. Portion of frustule showing exterior and interior of valve and the non-rotated apex. Fig. 14. Detail of central area showing overlapping raphe pores. Fig. 15. Detail of valve surface showing smooth surface and V-shaped openings of paired areolae (arrows). Fig.16.Apex of frustule. Fig. 17. Detail of internal central area. Fig. 18. Internal aspect of apex showing helictoglossa. Scale bars: 10 μm (Figs 7–13), 2 μm (Figs 16–18), 500 nm (Figs 14, 15).