Placoneis pseudabundans Levkov, 2011

Levkov, Zlatko & Williams, David M., 2011, Fifteen new diatom (Bacillariophyta) species from Lake Ohrid, Macedonia, Phytotaxa 30, pp. 1-41 : 14-16

Placoneis pseudabundans Levkov , sp. nov. ( Figs 92–95 )

Valvae late lanceolatae apicibus curte protractis rostratis, 21–28 µm longae, 9–10.5 µm latae. Raphe filiformis, linearis poris centralibus paulo incrassatis. Area axialis angusta, linearis, area centralis angusta, elliptica striis circum aream centralem regulariter abbreviatis utrimque. Duae stigmata adsunt dense sita ad striam mediam. Striae transapicales paulo radiantes in media parte, tum valde radiantes sub apices 13–15 in 10 µm. Puncta striarum non aspectabilia microscopio photonico.

Type:— MACEDONIA. Lake Ohrid, St. Naum , shore, mud 5 m depth, collection date: 17 March 2007 (accession No. MKNDC 001128). Slide BM 101472 (holotype). Slide MKNDC 001128 (isotype) .

Valves broadly lanceolate with short, protracted to rostrate ends, 21–28 µm long, 9.0– 10.5 µm wide. Raphe branches filiform and straight, central pores weakly broadened. Axial area narrow, linear; central area narrow, elliptical, formed by regular shortening of central striae. Two stigmata present in central area, located close to central striae. Striae weakly radiate becoming strongly radiate towards valve ends, 13–15 in 10 µm. Areolae not resolved in LM (LM).

Observations:—One of the main features of P. pseudabundans is the presence of two stigmata in the central area. This character is present in several Placoneis species such as Placoneis abundans Metzeltin et al. (2005 : figs 73: 1–4), P. clementis (Grunow) Cox (1988 : figs 5: 1, 2) and P. clementoides (Hustedt) Cox ( Hustedt 1944: figs 8: 19, 20; Simonsen 1987: figs 476: 4–6). Placoneis abundans is characterized by ellipticlanceolate valves with abruptly protracted and subcapitate ends, while the valves of P. pseudabundans are lanceolate with short, protracted to rostrate ends. Additionally, there are differences in the shape of the central area. The central area in P. pseudabundans is narrow and elliptical, formed by regular shortening of the central striae, while in P. abundans the central area is wider and bordered by alternating short and long striae. Placoneis clementis (Grunow) Cox is a fossil species ( Grunow 1882) characterized by broadly lanceolate valves with short, protracted and obtusely rounded ends, and coarsely punctated striae with 18–20 areolae in 10 µm ( Figs 90, 91 ). There are probably two different entities described as Navicula clementoides Hustedt , with valves of both depicted in Hustedt (1944: figs 8: 19, 20) and Simonsen (1987: figs 476: 6–10) with two stigmata in the central area. The first type of valve has elliptic-lanceolate outline with abruptly protracted and capitated ends ( Hustedt 1944, fig. 20), while the second has strictly lanceolate valves but neither protracted nor cuneate ends ( Hustedt 1944, fig. 19). The differences between P. pseudabundans and P. clementoides are found in the shape of the valve ends and size of the central area. Placoneis gracilis Metzeltin et al. (2005 : figs 15–17) has a similar valve outline to P. pseudabundans , but lacks stigmata in the central area.

In Lake Ohrid there are several Placoneis species groups. In species belonging to the P. placentula group, stigmata are absent, while species similar to P. tumida and P. significans Lange-Bertalot (in Metzeltin et al. 2005) (= N. exigua var. signata Hustedt ) have a single, isolated stigma. So far, only two species present in Lake Ohrid have two stigmata in the central area, Placoneis juriljii Miho & Lange-Bertalot (2006 : figs 1–11, 27–35) and P. clementis (Grunow) Cox (1987: 155 ; figs 28–33). Valves of Placoneis juriljii ( Fig. 96 ) are broadly lanceolate with short, protracted, broadly rounded to truncate ends, length 40–73 µm and width 16–20 µm; P. clementis ( Levkov et al. 2007: figs 89: 8–12, Miho & Lange-Bertalot 2006: fig. 63), while being very similar to P. pseudabundans , can be differentiated by the shape of the valve ends.

According to current understanding, species in Placoneis have highly variable valve morphology and associated ultrastructure: large differences in almost every ultrastructural feature can be noticed in different species. For example, the stigmata vary from none to one, two or four; the striae can be uniseriate, biseriate, or partly biseriate; the external distal raphe endings can be symmetrical or asymmetrical; the internal proximal raphe endings can be deflected, recurved, or hook-like; the external areolae openings can be surrounded by rims, or round, or elongate, or lineola-like; the internal areolae openings can be occluded, either entirely with vola-like occlusions or not occluded at all. These variations suggest that there may be several lineages that might best be named as subgenera. For example, Kociolek & Thomas (2010) recently described the new genus Rexlowea Kociolek & Thomas based on their observations of North American populations of Navicula navicularis Ehrenberg [= Placoneis navicularis (Ehrenberg) Cox (2003: 71) ]. The genus Rexlowea is characterized by the presence of septa and pseudosepta, several isolated stigma–like structures around the central area and condensed pores at the poles. However, no information about chloroplast features (number, shape, position) was given and comparison with species of Placoneis , and other naviculoid and cymbelloid genera, is not yet possible.

Figures

FIGURES 90–107: LM micrographs. Figs 90, 91. Placoneis clementis. Figs 92–95. Placoneis pseudabundans. Fig. 96. Placoneis juriljii. Figs 97–107. Placoneis subelegans. Scale bar = 10 µm. Figs 90, 91, fossil deposit Köpecz, Romania, Figs 92–107. Lake Ohrid, recent.