Achnanthidium enigmaticum 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 : 6-8

Achnanthidium enigmaticum E.Morales & Manoylov sp. nov. Figs 43 –59 (LM), 60–65 (SEM)

Description:— Frustules rectangular in girdle view (not shown here). Valves linear to elliptic with rostrate apices ( Figs 43 –59). Length: 17.0–54.4 µm, width: 4.1–5.2 µm, stria density 23–26 (up to 29 toward apices) in 10 µm in RV, and 23–26 (up to 34 toward apices) in 10 µm in SV. Hymenes, girdle bands and plastids not determined.

RV with lanceolate, raised axial area (more in internal Fig. 61 than in external view Fig. 62 ), broad at valve ends ( Figs 60–63 ), becoming enlarged in the central area where it assumes a circular or rhomboid shape ( Figs 43 –53). Central area with slightly raised central nodule in external view ( Figs 60, 61 ), clearly raised in internal view ( Figs 62 and 63 ), and with shortened striae on both sides ( Figs 60–63 ). Virgae wider than striae becoming slender toward the valve apices, lower with respect to axial area and at same level as striae in external and internal views ( Fig. 61 ). Raphe filiform and straight, externally with proximal ends expanded and opposite to each other ( Fig. 60 , white arrows), and distal fissures expanded, close-hooked in shape and strongly deflected toward same direction, surfacing gradually at valve face/mantle junction, within the striation region ( Fig. 61 , white arrows). Internally, proximal raphe ends curved and deflected in opposite directions ( Fig. 63 , white arrow), distal raphe ends terminate in low helictoglossae located at a distance from valve apices ( Fig. 63 , black arrow). Striae varying from parallel at valve central area, only sometimes becoming slightly convex toward the apices, short striae on both sides of central area present ( Figs 60–63 ). Areolae visible under LM, round, elliptic or slit-like ( Figs 43 –53). A single row of elliptic areolae present on valve mantle, more slit-like in shape and interrupted by clear area at valve apex ( Figs 60, 63 ). A clear area runs around the edge of the face/mantle junction ( Figs 60–61 ).

SV with sunken lanceolate axial area (only observed in external view), broad at valve ends (seen as an expansion as the distal end of striation is reached) ( Figs 64, 65 ). Virgae wider than striae, becoming slender toward valve apices, but in general wider than those in RV at the same region (compare Figs 63 and 65 ). Striae at same level as virgae. Central area with evenly spaced striae, rarely shortened ( Figs 64, 65 ).

Type locality:— 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. 48 )

Etymology:— The species epithet refers to the fact that this diatom has remained undiscovered despite the rather long history of studies of algae, particularly diatoms, in Texas.

Ecology for both taxa:— While A. lucectorii sp. nov. was found in 3.1% relative abundance in a count of 600 valves, A. enigmaticum sp. nov. was only found in a relative abundance of <1%. The dominant accompanying taxa in the sample were Encyonema neogracile Krammer (1997a: 177 , with a relative abundance of 26.8%), Delicata delicatula (Kützing) Krammer (2003: 113 , 20.5%), Encyonopsis microcephala (Grunow) Krammer (1997b: 91 , 11.8%), Ulnaria ulna (Nitzsch) Compère (2001: 100 , 7.8%), Achnanthidium caledonicum (Lange-Bertalot) Lange-Bertalot (1999: 277 , 5.3%), Navicula cryptotenella Lange-Bertalot (1985: 62, 3.0%) and other species in relative abundances <2%.

Though the new taxa were in low abundances as to consider the Sabinal River their optimal habitat, the available physico-chemical for July 31, 1996 were: river discharge 3.7 L̔ s-1, water temperature 31 °C, electric conductance 476 µS̔ cm-1, dissolved oxygen 6.9 mg ̔L- 1, pH 7.8, total nitrogen 0.31 mg ̔L- 1, and orthophosphates <0.01 mg ̔L- 1.

Based on the autecology of the assemblage found in type material, the habitat of the new species can be characterized as an oligotrophic site with water of good quality, with low nutrients, low electric conductance and circumneutral pH, which corresponds to the actual parameters measured on site. Based on the presence of A. caledonicum and N. cryptotenella , the type locality could have a calcareous, alkaline influence ( Van Dam et al. 1994, Hofman et al. 2011).

K. Manoylov recently found, in periphyton collections from May 2022, A. lucectorii sp. nov. in Roy Creek, Austin, TX (relative abundance <1%) and A. enigmaticum sp. nov. in Hamilton Creek and Deadmans Creek, close to Austin, TX (relative abundance <1%). Specific sampling locations were not available to us, but based on public reports the three creeks close to Austin, TX are either under private protection or within rangeland.

Figures

FIGURES 43–52. Achnanthidium enigmaticum sp. nov. (LM) from type material. 43–54. RV arranged in size diminution series, Figures 53 and 54 correspond to the same frustule, 54 being a SV; Figure 48 corresponds to the holotype. 55–59. SV arranged in increasing size order. Notice axial area expanded in central area and clearly observable areolae along striae. Scale bar = 10 µm.

FIGURES 60–65. Achnanthidium enigmaticum sp. nov. (SEM) from type material. 60, 61. RV in external view. Notice expanded proximal raphe ends (white arrows in 60), and distal raphe ends bent and deflected in the same direction (white arrows in 61). 62, 63. Internal views of RV showing the proximal raphe ends that are slightly bent and deflected in opposite directions (white arrow in 63) and the low helictoglossum separated from the valve apex (black arrow in 63). 64, 65. External view of SV. Scale bars = 5 µm (60, 61, 63, 65); 10 µm (62, 64).