Craticula obaesa Van de Vijver, Kopalová & Zidarova, 2015

Vijver, Bart Van De, Kopalová, Kateřina & Zidarova, Ralitsa, 2015, Three new Craticula species (Bacillariophyta) from the Maritime Antarctic Region, Phytotaxa 213 (1), pp. 35-45 : 38-39

Craticula obaesa Van de Vijver, Kopalová & Zidarova , sp. nov. ( Figs 10–20 )

LM ( Figs 10–15 ): valves lanceolate to elliptic-lanceolate with distinctly convex margins, tapering gradually towards the small, weakly protracted, subrostrate apices. Valve dimensions (n=20): length 59–90 μm, width 14–18 μm. Axial area linear, very narrow, formed by a raised sternum, slightly widening towards the small, narrowly elliptic central area. Raphe branches almost straight to weakly curved, slightly lateral for most of its length, almost filiform near the central area. Proximal raphe endings weakly deflected terminating into enlarged pores. Distal raphe endings elongated, distinctly hooked opposite to the proximal raphe endings ( Figs 13, 14 ). Striae more distantly spaced near the central area, almost parallel to weakly radiate throughout the entire valve, 13–15 in 10 μm, becoming parallel and finally weakly convergent and more densely spaced near the apices, 16–18 in 10 μm. Areolae visible in LM, ca. 26–31 in 10 μm. Heribaudii-stages often observed ( Fig. 15 ). SEM ( Figs 16–20 ): external proximal raphe endings short, expanded, clearly hooked ( Fig. 16 ). Distal fissures elongated continuing onto the mantle ( Fig. 17 ). Striae composed of narrow, apically elongated slit-like areolae ( Figs 16, 17 ). Rarely, areolae in the valve middle almost rounded or only weakly transapically elongated ( Fig. 16 ). Frets separating the areolae weakly raised forming longitudinal grooves in which the areolae are positioned ( Fig. 17 ). Near the central area and the apices, grooves absent. Internally, central nodule raised ( Fig. 18 ). Proximal raphe endings short, unilaterally weakly deflected. Areolae occluded by perforated hymenes ( Fig. 19 ). Heribaudii stage composed of a longitudinal, axial sternum with irregularly and distantly-spaced, thickened transverse bars ( Fig. 20 ).

Type:— JAMES ROSS ISLAND. Clearwater Mesa: sample CLW60, L. Nedbalová, 09 February 2009 (holotype BR! slide no. 4393, isotype PLP! Slide no. 277, University of Antwerp, Belgium).

Ecology and Distribution: — Craticula obaesa has been found in several lakes on the Clearwater Mesa, a volcanic tableland in the northern part of James Ross Island. Their littoral zone is covered by microbial mats dominated by filamentous cyanobacteria (L. Nedbalová, pers. comm.). The largest populations of the new species were found in lakes with an alkaline pH (8.1–8.7) and a moderate to rather high conductivity (500–1000 μS/cm).

Etymology: — The specific epithet describes the general outlook of the species, showing a rather obese aspect.

Figures

FIGURES 10–15. Craticula obaesa. LM pictures taken from the holotype population (sample CLW60) from Clearwater Mesa, James Ross Island. 15. LM view of a heribaudi stage. Scale bar represents 10 μm.

FIGURES 16–20. Craticula obaesa. SEM pictures taken from the holotype population (sample CLW60) from Clearwater Mesa, James Ross Island. 16. Detail view of the central area. Note the difference in shape of the central areolae and the expanded, hooked proximal raphe endings. 17. External detail of a valve apex with the typical longitudinal lines formed by the frets separating the areolae. 18. Internal detail of the central area with focus on the central nodule and the differences in areola structure near the central area. 19. Internal detail of the areolae occlusions. 20. Heribaudii stage. Scale bar represents 10 μm, except for Fig. 19 where scale bar = 1 μm.