Encyonopsis cilaosensis Le Cohu & Gassiole, 2014

Cohu, René Le, Gassiole, Gilles & Coste, Michel, 2014, Three new species of Cymbellales (Bacillariophyceae) from Réunion Island, Phytotaxa 156 (3), pp. 117-132 : 124-126

Encyonopsis cilaosensis Le Cohu & Gassiole sp. nov.

Figures 28–53 , Figs 28–46 are of the holotype.

Valves dorsiventral with a convex dorsal margin and a straight to weakly convex ventral margin in large individuals; valves nearly symmetrical in the smallest individuals. Apices rostrate to subcapitate. Length: 7.3–12 µm; breadth: 3.3–4.1 µm; maximum length/breadth ratio: 3.3. Axial area narrow. Central area absent. Raphe filiform, slighty curved to the ventral side with dorsally deflected proximal endings; central pores inconspicuous. Striae parallel becoming weakly radiate near the poles, 21–24 in l0 µm; areolae not discernible in LM. SEM: raphe with terminal fissures strongly hooked towards the ventral side and internally presence of a very short intermissio. Areolae: around 50 in 10 µm.

Holotype: PC0143223 . Laboratoire de Cryptogamie. Muséum d'Histoire Naturelle. Paris.

Isotype: BR–4278 . Jardin Botanique National de Belgique. Département de Bryophyta et Thallophyta. Domaine de Bouchout. Meise. Belgique.

Type locality: Le Bras des Etangs aux anciens thermes . (Longitude: 55°28'09.7"E –Latitude: 21°07'51.5"S)

Etymology: The specific epithet refers to the “Cirque de Cilaos” where the type locality is located

LM Morphology ( Figs 28–46 )

The valves (number of valves observed: 103) are generally dorsiventral with a clearly convex margin and a straight to weakly convex ventral margin ( Figs 28–39 ). In the smallest specimens the valves are nearly symmetrical ( Fig. 42–46 ). The apices, broadly rounded, are rostrate to subcapitate. The length /breadth ratio varies from 2.1 to 3.3. The axial area is very narrow and linear; the central area is absent. The raphe is filiform, slighty curved to the ventral side but with dorsally deflected proximal endings terminating in inconspicuous central pores. The striae are parallel becoming barely radiate near the poles.

SEM Morphology ( Figs 47–53 )

Externally, the striae are composed of 2–7 more or less rounded areolae, around 50 in l0 µm ( Figs 47–48 ). The central area is only marked by a slightly shorter stria on either side of the valve ( Fig. 47 , arrow). The raphe fissure is curved towards the ventral side with almost straight and enlarged proximal endings hardly pointed to the dorsal side ( Figs 47–48 ). The terminal fissures are first dorsally bent, then are strongly hooked to the ventral side and end onto the mantle ( Figs 48–49 ). Internally, the raphe slit, interrupted by a very short intermissio ( Fig. 50 , arrow) is straight and terminates in well developed helictoglossae ( Fig. 51 ). The striae appear as beaded strips with rounded areolae ( Fig. 50 ). The cingulum is distinctive. Each level is composed of two opposite bands which are closed at the poles of the valves ( Figs 52–53 ); they are open near the valve centre and terminate in more or less cuneate endings ( Figs 52–53 , arrows). A row of pores is visible, probably at the junction pars interior-pars exterior (fig. 53).

Ecology and distribution

E. cilaosensis was found in highest relative abundance (3%) in the type locality “Le Bras des Etangs aux Anciens Thermes” in waters with a pH of 8.4 and high conductivity ( Table 3 ). It was associated with Cymbella excisa Kützing (1844: 80) (28.6%), Amphora pediculus (Kützing) Grunow in Schmidt et al. (1875: 26) (19.8%), Cocconeis euglypta Ehrenberg (1854: 34) (7%) and Achnanthidium sp. (14.6%). E. cilaosensis was recorded from six other sampling sites with a relative abundance reaching 0.6% at the maximum.

Figures

FIGURES 28–46. Encyonopsis cilaosensis LM. Scale bar: 10 µm. Valves views of the holotype population from Le Bras des Etangs aux anciens thermes showing variation in size and outline.

FIGURES 47–51. Encyonopsis cilaosensis SEM.Scalebars:1 µm (Figs 47,48 and51), 100nm(Figs 49 and 50). FIGURE 47. External valve view. Open ending of copulae (arrow). FIGURE 48. External view showing the raphe course. FIGURE 49. External view of pole with terminal fissure. FIGURE 50. Internal view of valve centre:intermissio (arrow), beaded strip striae. FIGURE 51. Internal view of the whole valve.

FIGURES 52–53. Encyonopsis cilaosensis SEM.Scalebars:1 µm (Figs 52and53) FIGURE 52. Internal view showing open bands (arrows). FIGURE 53. Internal view showing two copulae with cuneate endings and a row of pores (arrows).