Williamsella angusta Graeff, Kociolek & S.R. Rushforth, 2013

GRAEFF, C. L., KOCIOLEK, J. P. & RUSHFORTH, S. R., 2013, New and Interesting Diatoms (Bacillariophyta) from Blue Lake Warm Springs, Tooele County, Utah, Phytotaxa 153 (1), pp. 1-38 : 7-9

Williamsella angusta Graeff, Kociolek & S.R. Rushforth , sp. nov. ( Figs 19 –37; figure 22 = holotype)

Valve linear, narrow, with apices slightly protracted (except in the smallest cells), rounded ( Figs 19–27 ). Length 27.5– 106.0 µm, breadth 2.0– 2.5 µm. Central sternum very narrow, becoming slightly wider towards the center, but no central area is formed ( Figs 19–27 ). Striae opposite, irregularly punctate, more or less parallel ( Figs 26–27 ). Striae number 17–20/ 10 µm. A pore field is present at both apices. A single rimoportula is present at one end of the valve. In the SEM, the valve exterior is covered with rounded areolae that are nearly covered entirely with round occlusions ( Figs 29, 31 ). Beneath the occlusion is a second smaller, round opening that is recessed into the valve face ( Fig. 30 ). An oblong opening of the rimoportula is evident near the pole ( Fig. 29 ). On the margins at the apices there is an ocellimbus, the porelli of which are nearly the size of the areolae ( Figs 29–30 ). Areolae extend onto the margin ( Fig. 30 ) and are present in a single series on the cingulum ( Fig. 32 ). These also have rounded occlusions. Internally, the central sternum is narrow (Figs 34–37), and ribs oriented perpendicular to the sternum of a width similar to the sternum are present. Rimoportulae are sessile, with a narrow, slit-like opening about twice the length of areolae (Figs 34–35). The ocellimbi are indistinct at the poles (Figs 35, 36).

Type: — USA. Blue Lake , Tooele Co., Utah. ( COLO slide 439039, holotype (= Fig. 22 )!, designated here; COLO 8508 , BM 101676, isotypes)

Etymology:—The species is named for its straight and narrow form.

Distribution:— Williamsella angusta is abundant throughout the main basin of Blue Lake and the surrounding marsh.

Observations:— Kaczmarska & Rushforth (1983: 37) identify W. angusta as Synedra radians Kützing (1844: 64) using Patrick & Reimer (1966: 137, pl. 5, fig. 4). However, the description and illustration of S. radians indicate a species with a distinct central area. The description and illustration of S. tenera W. Smith (1856: 98), also featured in Patrick and Reimer (1966: 137, pl. 5, fig. 5), more closely resemble specimens of W. angusta . In addition, specimens of W. angusta possess a rimoportula at only one apex, excluding it from genus Synedra Ehrenberg (1830: 60) which has a rimoportula at each apex ( Round et al. 1990: 346, 370).

However, according to Round et al. (1990), commenting on work previously done by Williams & Round (1987), Fragilaria Lyngbye (1819: 182) should contain only taxa that occur in colonies, and Round et al. (1990) add that Fragilaria should remain a genus of freshwater species. Williams & Round (1987) note that some species grow singly but do not indicate whether there are any species lacking spines. We did not find any evidence of colony formation in W. angusta in the saline conditions in Blue Lake, and the cells lack spines and do not show any remnants of spines. The external appearance of the areolae, with the externally-placed coverings, sets this new genus apart from other known araphid genera such as Synedropsis Hasle, Medlin & Syvertsen (1994: 248) (in Hasle et al. 1994; Melo et al. 2003); Reimerothrix Prasad (in Prasad et al. 2001: 38; Catacombas Williams & Round 1986: 314 ; and Hyalosynedra Williams & Round 1986: 316 ). Prasad et al. (2001, table 2) provide a helpful comparison of seven ‘synedroid’ genera. Based on this table, and our review of the literature, we cannot find a genus with the unique suite of features described here, and therefore we present this as a new genus and species.

Figures

FIGURES 19–27. LM. Williamsella angusta. Valve views showing the size diminution series. Figure 22 is the holotype. Note the variable shape and breadth of the valve apices. Scale bar: 10 µm.

FIGURES 28–32. SEM. Williamsella angusta. External views showing valves from four specimens and the cingulum. Fig. 28, Full valve view of a large specimen with narrow apices. Fig. 29, View showing the external opening of the rimoportula and occluded areolae. Fig. 30, An apex with no rimoportula and the structure of the areolae beneath eroded occlusions. Fig. 31, Striae at the valve center sitting between very slight transapical ribs. Fig. 32, Components of the cingulum each with a row of occluded areolae of variable size. Scale bars: Fig. 28: 10 µm; Figs 29–32: 0.5 µm.