Achnanthidium lucectorii E.Morales & Manoylov, 2023

Morales, Eduardo A. & Manoylov, Kalina M., 2023, Achnanthidium lucectorii sp. nov. and A. enigmaticum sp. nov., novel diatoms (Bacillariophyta, Achnanthidiaceae) from Texas, U. S. A., Phytotaxa 592 (1), pp. 1-20 : 3-6

Achnanthidium lucectorii E.Morales & Manoylov sp. nov. Figs 1–36 (LM), 37–42 (SEM)

Description:— Frustules apically sigmoidally arched (in an open V-shaped girdle view), producing a raphe valve with a concave valve center ( Figs 22, 23 ). Valves elliptic with swollen central area and subcapitate apices ( Figs 1–21, 24–36), less noticeable in smaller valves. Length: 10.2–35.7 µm, width: 3.1–4.3 µm, stria density 26–28 (up to 30 toward apices) in 10 µm in RV, and 26–27 (up to 32 toward apices) in 10 µm in SV, 1–4 areolae per stria on both valves. Hymenes, girdle bands and plastids not determined.

RV with narrow, lanceolate, raised axial area (externally [ Figs 37–39 ] and internally [ Fig. 40 ]), tapering into a pointy end at valve apex, becoming enlarged in central area where it connects with the central stauros ( Figs 37–40 ). Central stauros with bow-tie shaped fascia (rarely apically rhomboid in some larger valves) ( Figs 1–21 ), with raised central nodule in external view ( Figs 37–39, 41 ), flat in internal view ( Fig. 40 ). Virgae wider than striae except at valve ends where they become slender and rod-shaped, straight or zig-zagged due to polygonal shape of areolae ( Figs 38, 39 ). Virgae lower with respect to axial area and at same level as striae in internal and external views ( Figs 37–40 ). Raphe uniformly filiform and straight, with proximal ends opposite to each other ( Figs 37–39 ). Distal fissures straight and extending over striation regions at both poles ( Fig. 39 , white arrows). Internally, proximal raphe ends with a small capitate terminus ( Fig. 40 , white arrows) and distal ends terminating in low helictoglossae fused with a low thickening of valve apex ( Fig. 40 , black arrow). Striae varying from convex toward valve central area (striae frequently lacking or being shorter on central stauros) becoming extremely convex and noticeably denser toward apices ( Figs 37–39, 41 ). Areolae round or elliptic toward the central sternum, becoming transapically elongated toward valve face/mantle transition, and densely packed and polygonal at valve extremes ( Fig. 38 , open arrows), usually with two large areolae on both sides of distal raphe end ( Figs 37, 39 ). A single row of slit-like areolae present on valve mantle, stopping short of the very apex (unclearly seen in Fig. 40 ). A clear area runs around edge of the face/mantle junction ( Figs 37–39 ).

SV ( Figs 41, 42 ) with wider, flat (only observed in internal view) lanceolate axial area, more or less broad at valve ends. Central nodule is only slightly raised internally. Virgae of same characteristics as in RV, but at same level as striae and axial area (only internal views available). Striae of same general characteristics as those of RV, except they are evenly spaced (some are short) at valve central area, they become less convex toward the valve apices, and single row of elongated striae on the valve mantle are continuous around valve apex. The clear area around edge of the face/mantle junction is less noticeable.

Type:— U.S.A. Texas, Uvalde County, Sabinal River , 29.491 N, 99.493 W, NAWQA, August 7 th, 1996 (holotype slide ANSP GC 102382 a = Fig. 10 )

Etymology:— The species epithet honors our late colleague Luc Ector. Luc promoted a major advancement in the taxonomy of the genus Achnanthidium through a series of courses and workshops that were the foundation of the taxonomy used in the European Water Framework Directive. The special issue in Algological Studies (2011: 136/137), edited by Luc and the result of one of those workshops, presented critical analyses of type material and new species in Achnanthidium from around the world, a foundation for current studies in this genus.

Figures

FIGURES 1–36. Achnanthidium lucectorii sp. nov. (LM) from type material. 1–21. RV views arranged by size. Notice bow-tie fascia and stauros in central area, Fig. 10 corresponds to the holotype. 22, 23. Girdle views of two complete frustules. 24–36. SV arranged in size diminution series. Notice lanceolate axial area and clearly seen striae. Scale bar = 10 μm.

FIGURES 37–42. Achnanthidium lucectorii sp. nov. (SEM) from type material. 37–39. RV in external view. Striae are more densely packed toward valve apices (open arrows in 38), Figure 39 shows straight distal ends of raphe extending beyond the striation region (white arrows). 40. Internal view of RV showing small capitate termini of proximal raphe ends (white arrows) and valve apex low thickening attached to low helictoglossum (black arrow). 41, 42. External view of RV and internal view of SV. Figure 42 shows internal view of SV. Scale bars = 5 μm.