Crucicostulifera bebourensis Coste, 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 : 119-120

Crucicostulifera bebourensis Coste, Le Cohu & Gassiole sp. nov.

Figures 1–27 , Figs 1–8 are of the holotype.

Description: Valves lanceolate with obtusely rounded apices. Frustule more or less rectangular in girdle view. Length: 25.4–69.0 µm; breadth: 7.9–12.5 µm. Raphe lateral near the poles; terminal fissures hooked to the secondary side of the valve; proximal raphe endings almost straight and enlarged; central pores inconspicuous. Axial area lanceolate without distinct central area. Striae slightly radiate and distinctly punctate, 12–14 in 10 µm in the middle part of the valve, up to 18 in 10 µm towards the poles. SEM. External view: raphe slit curved to the secondary side of the valve becoming axial with proximal endings almost straight and enlarged; terminal fissures strongly hooked to the secondary side of the valve and terminating at the valve face / mantle junction. Vaulted valves with junction between valve face and mantle unclear, and a system of longitudinal ribs discernible. Striae with x-shaped areolar openings and extending without interruption onto the mantle. Internal view: raphe fissure straight with proximal endings hooked towards the same side of the valve. Striae in depressions with areolar openings oval and occluded. Cingulum composed of at least three open bands.

Holotype: PC0143222 . Laboratoire de Cryptogamie. Muséum d'Histoire Naturelle. Paris.

Isotype: BR–4277 . Jardin Botanique National de Belgique. Département de Bryophyta et Thallophyta. Domaine de Bouchout. Meise. Belgique .

Type locality: Bébour, rivière des Marsouins, Île de la Réunion (Longitude: 55°33'58.0"E –Latitude: 21°06'45.3"S)

Etymology: The specific epithet refers to the sample site.

LM Morphology ( Figs 1–12 )

The cells ( Figs 9–12 ) are more or less rectangular in girdle view (6 – 11 µm broad). The valves (number of valves observed: 196) are lanceolate with obtusely rounded apices ( Figs 1–8 ). The raphe is lateral near the poles and the terminal fissures are strongly hooked to the secondary side of the valve ( Figs 1, 7 and 8 ). The proximal raphe endings are almost straight and enlarged; the central pores are inconspicuous. The distance between the proximal raphe endings decreases from the largest to the smallest individuals. The axial area is lanceolate, relatively wide (up to 4 µm), slightly asymmetric in the middle part of the valve and without a distinct central area. The striae, slightly radiate throughout, appear distinctly punctate, 12–14 in 10 µm in the middle part of the valve, up to 18 in l0 µm towards the poles.

Ecology and distribution

C. bebourensis was most abundant (51% relative abundance) in a sample with a pH of 5.5, very low conductivity (6–8 µS.cm -1) and very low nutrient concentration ( Table 2 ). It was associated with Kobayasiella bebourensis Le Cohu, Gassiole & M. Coste (2012: 402) and Stenopterobia schweickerdtii (Cholnoky) Brassac, Ludwig & Torgan (2003: 186) which contributed respectively 11% and 4% of the total number of diatoms counted. Other taxa not yet identified were represented by Achnanthidium sp .. C. bebourensis was recorded from an other sampling site of the “rivière des Marsouins” (Takamaka) but with a very low relative abundance (0.2%).

Figures

FIGURES 1–12. Crucicostuliferabebourensis LM. Scale bar: 10 µm. FIGURES 1–8. Views of the valve face: size variability of the holotype population from Bébour, rivière des Marsouins. FIGURES 9–12. Girdle views.

FIGURE 13. External view with longitudinal ribs hardly discernible. FIGURE 14. External view showing the raphe course and laterally the longitudinal ribs (arrows). FIGURE 15. (tilted 20°). External view of pole showing the terminal fissure terminating onto the valve face and the closed endings of both copulae (C1, C2). FIGURE 16. (tilted 20°). External view of valve centre showing the longitudinal ribs system. FIGURE 17. External view of valve face showing the longitudinal ribs disappearing on the distal part of the valve (arrows). FIGURE 18. Girdle view showing the striae extending uninterruptedly on the mantle and the cingulum with valvocopula (VC), first copula (Cl), and second copula C2).

FIGURE 19. Internal view of raphe fissure. FIGURE 20. Internal view of a broken valve showing oblique raphe slit, hollow helictoglossa (arrow) and hooked proximal raphe endings. FIGURE 21. Internal views of occluded areolar openings. FIGURE 22. Girdle view showing the closed ending of valvocopula (VC), and the first copula (C1) disappearing underneath the valvocopula. FIGURE 23. Advalvar view of valvocopula or first copula. FIGURE 24. Detail of closed ending of VC or C1 with crenulate pars interior and a row of pores separating pars interior and exterior.

FIGURE 25. Advalvar view of the second copula. FIGURE 26. Advalvar view of the second copula with a small septum and a row of pores at the junction pars interior and exterior. FIGURE 27. Detail of the apex showing the valvocopula (VC) and the closed endings of the first (C1) and the second copula (C2).