Lunella excentrica, 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 : 26-28

Lunella excentrica, Graeff, Kociolek & S.R. Rushforth , sp. nov. ( Figs 125–137 ; figure 129 =

holotype)

Valves distinctly asymmetrical to the apical axis, with the dorsal margin convex and the ventral margin straight to slightly concave in the center, apices not produced, rounded ( Figs 125–131 ). Length 9.0–20.0 µm, breadth 2.5–3.0 µm. Axial area very narrow, arched, with central area linear and indistinct ( Figs 125, 129 ). Axial area and raphe positioned much closer to the ventral margin than the center of the valve. Raphe filiform, arched ( Fig. 126 ). External distal raphe ends deflected towards the ventral margin ( Figs 132–133 ). Striae indistinctly punctate and radiate along the dorsal margin. Along the ventral margin, striae longer at the apices and quite short at the middle of the valve ( Figs 129 , 132 ). Striae number 22–29/ 10 µm. Stigmoids and apical pore fields wanting ( Figs 133, 134–137 ).

In the SEM, the valve exterior has areolae that are oval to nearly lineolate ( Figs 132–134 ). The axial area is straight ( Fig. 132 ), as are the proximal raphe ends ( Fig. 134 ). External distal raphe ends are curved towards the ventral margin ( Figs 132–133 ). Internally, the raphe has an elongated flange of silica near the dorsal margin that fuses the helictoglossae ( Figs 135–136 ). A small, slightly thickened central nodule is present ( Figs 135–137 ). Areolae occur in shallow troughs ( Fig. 137 ). Especially at the valve center, striae on the ventral margin are coarser than on the dorsal margin ( Figs 135–137 ).

Type:— USA. Small pool near Blue Lake , Tooele Co., Utah. (COLO 439048, holotype! (= Fig. 129 ), designated here; COLO 8512, BM 101682, isotypes)

Etymology:—This species is named for its shape.

Distribution:— Lunella excentrica occurs only rarely in a sample from a small pool at the north side of the Blue Lake basin (COLO 8512), and this is the only location in which we found the taxon. Of the waterbodies in the Blue Lake area that we studied, the small pool has the highest salinity, conductivity, and pH and the lowest temperature ( Table 1).

Observations:—This is the first report of this genus in the USA. Snoejis (1996: 144) first described Lunella Snoejis from the Baltic Sea. Though species of Lunella are generally asymmetrical to the apical axis like many amphoroid or cymbelloid genera, members of Lunella have a catenuloid valve structure, where the ventral and dorsal valve margins and the ventral and dorsal girdle are of equal depths (Snoejis 1996). Snoejis (1996) compares Lunella with the naviculoid genus Parlibellus and Catenula Mereschkowsky (1903: 97) , finding that Parlibellus differs largely in its cingulum structure and that the fine structure of Catenula is very different from that of Lunella . Encyonopsis Krammer (1997a: 156) is a genus that looks similar to Lunella , and while Krammer (1997a) does not note in the genus description whether it has a catenuloid valve structure, numerous scanning electron micrographs of Encyonopsis species suggest that its valve margins are equal depths ( Graeff & Kociolek 2013, Krammer 1997b). Encyonopsis and Lunella are distinguished by the differing structure of their raphes ( Krammer 1997b: pls 145–147, 188): in Lunella the external distal raphe ends shortly beyond the helictoglossa, the internal and external raphe ends are straight and visible, and the raphe is arched but not displaced. Snoejis (1996) hypothesizes that because of its many similarities with Parlibellus , Lunella might belong in family Berkeleyaceae Mann in Round et al. (1990: 659). Images of Parlibellus provided by John (1991) and Cox (1988) show taxa that have very simple raphe structures like that in Lunella . While Parlibellus possesses internal occlusions on its areolae, whether occlusions are present in Lunella or have been destroyed in the process of cleaning is unclear (Snoejis 1996). We did not detect internal occlusions on L. excentrica .

Three other species of Lunella have been described from coastal areas in waters with a wide range of salinities. L. ghalebii Witkowski et al. (2000: 229) is a marine taxon ( Ulanova & Snoeijs 2006) that was originally described from Qurum Beach in Oman and also reported from the western Baltic Sea and Narvik, Norway ( Witkowski et al 2000: 229, pl. 163, figs 5–10). L. bisecta Snoeijs (1996: 145) is a low salinity, cold water species originally described from Forsmark, Sweden ( Snoeijs 1996). Finally, L. garyae Ulanova & Snoeijs (2006: 416) is from a supralittoral habitat in Dorset, England with an extremely high salinity of 240 psu ( Ulanova & Snoeijs 2006). L. excentrica represents an inland taxon living in salinities below those of seawater at the time of collection. Because the pool from which we collected L. excentrica is small and shallow, the pool likely experiences great fluctuations in conductivity, salinity, and temperature throughout the year.

L. excentrica can be quickly distinguished from other Lunella taxa by its strongly dorsi-ventral valves, noticeably arched raphe branches, and nearly lineolate areolae.

Figures

FIGURES 125–131. LM. Lunella excentrica. Valve views showing the size diminution series. Figure 129 is the holotype. Scale bar: 10 µm.

FIGURES 132–134. SEM. Lunella excentrica. External views of three specimens. Fig.132, Full valve view showing radiate striae along the dorsal side of the valve. Fig. 133, Apex with ventrally deflected distal raphe ends. Fig. 134, Central area with straight proximal raphe ends. Scale bars: Fig. 132: 2 µm; Figs 133–134: 1 µm.

FIGURES 135–137. SEM. Lunella excentrica. Internal views of three specimens. Fig. 135. Full valve view showing radiate striae. Fig. 136, Full valve view showing arched raphe branches, prominent helictoglossae, and relatively short striae on ventral side of the valve at the center. Fig. 137, Nearly lineolate areolae and straight proximal raphe ends. Scale bars: Figs 135–136: 2 µm; Fig. 137: 0.5 µm.