Platessa guianensis Le Cohu, Tudesque & C. E. Wetzel 2016, sp. nov.
Creators: name: Tudesque, Loïc
name: Cohu, René Le
name: Wetzel, Carlos E.
Publisher: name: Zenodo
Year: 2016
Subjects: Biodiversity
Taxonomy
Chromista
Bacillariophyta
Bacillariophyceae
Achnanthales
Achnanthaceae
Platessa
Platessa guianensis

description: Platessa guianensis Le Cohu, Tudesque & C.E.Wetzel, sp. nov. (Figs 2−71)   
LM (Figs 2−37, 46−64): Valves elliptic-rhomboidal with broadly rounded apices. Length: 9.0−15.3 μm, width: 6.7−8.9 μm, ratio length/ width: 1.5−1.8 (n= 67). Raphe-sternum valve (RSV) (Figs 2−13, 14a−25a, 56−64): axial area very narrow almost non-discernible; central area transapically expanded and bordered by several alternately longer and shorter striae (3 to 5—mostly 5). Striae more irregularly spaced from one other in the central area. Striae clearly radiate throughout the valve, 15−17 in 10 μm. Raphe filiform with expanded proximal endings. Sternum valve (SV) (Figs 14b−25b, 26−37, 46−55): wide rhomboidal to elliptical axial area with numerous dots. Short striae, 11−13 in 10 μm, parallel in the center and becoming radiate to the apices. On both valves the optical focus on the entire valve is not possible, suggesting the curvature of the frustule. SEM (Figs 38−45, 65−71): externally, RSV slightly concave. Raphe branches straight with proximal endings surrounded by a shallow groove extending on either side of the raphe fissure on a short distance between the central and the terminal pore (Figs 38, 39, 67 arrows). Distal raphe endings located in a cone-shaped (delta) depression bordered by a shallow groove (Figs 40, 67 arrows). Internally raphe branches with proximal endings deflected in opposite direction and distal endings slightly curved to opposite sides and terminating in poorly developed helictoglossae (Figs 41, 44). Central striae clearly sunken between raised virgae (Figs 41, 68). A very shallow depression is observed on the central nodule between the proximal raphe endings (Figs 41, 68). SV slightly convex (Figs 43−44). Externally, axial area covered with numerous scattered depressions (around 0.5 μm diameter) irregularly distributed on the surface (Figs 43, 44, 65, 69). Internally, trace of vestigial raphe (Fig. 45, arrow) generally present. On both valves, striae biseriate composed of offset areolae relative to each other and separated by a strongly silicified vimines (Figs 38−45, 65−71). Internally, areolae occluded by a cribrum perforated by delicate slits (Fig. 42). Externally, on both valves, narrow hyaline area all around the valve separating the biseriate striae from elongated “poroids” located in a continuous groove on the mantle (Figs 38, 43, 44), which can be inconspicuous or irregular in some specimens (Figs 65, 68 arrows). External “poroids” continuous on SV (Figs 43, 44), discontinuous in the RSV (missing poroids at the apices, Figs 38, 40). Internally, at the marginal end of each striae, presence of a very tiny “outgrowth” surrounded by a groove and structurally different from the areolae (Fig. 42, arrow). Notches present on both valves (Figs 41, 43, 44, arrows).  
 Type: — FRENCH GUIANA. River Nouvelle-France: municipality of Saül, 3.572° N, 53.201° W, benthic periphyton, Loïc Tudesque, November 2011 (holotype: PC! 00145186, Muséum National d’Histoire Naturelle de Paris, Paris, France; isotype: ANSP! GC 26825 and raw material GCM 10311, the algae collection of the Academy of Natural Sciences, Philadelphia, U.S.A.).   
 Habitat:—freshwater periphyton from tropical and subtropical moist broadleaf forest.   
 Etymology:—in reference to the location where this species was sampled.   
 Paratypes:— BRAZIL. Amazonas: Barcelos Municipality, Quimicuri river (‘igapó’), a tributary from the right bank of the Rio Negro hydrographical basin (0.6935° S, 63.20964° W), phytoplankton sample, C. E. Wetzel, 2 March 2005 (Figs 46−68). Sample (SP-400.274) deposited at the herbarium from the Instituto de Botânica de São Paulo (SP!). BRAZIL. Amazonas: Barcelos Municipality, small name-less river (‘igapó’), from the right bank of the Rio Negro hydrographical basin (0.488409° S, 64.612874° W), phytoplankton sample, C. E. Wetzel, 4 March 2005. (Figs 69−71). Sample (SP-400.315) deposited at the herbarium from the Instituto de Botânica de São Paulo (SP!).   
 Distribution and ecology:—The type locality is the same as that of Lacuneolimna novagallia Tudesque et al.   
 (2015: 24). At the place where it was sampled, the trophic status and the saprobic degree are very low, respectively oligotrophic and oligosaprobic. The same ecological status also applies to the localities where the species was found in the Rio Negro basin (Amazon basin), including low pH preferences (ca. 4.5).   
 In the type locality, the diatom community consists of a high diversity of genera with many undetermined species, mainly in the genera Eunotia, Nupela and Sellaphora.Although the exhaustive taxonomic inventory has not been finished yet, many accompanying species of P. guianensis have been inventoried such as: Chamaepinnularia brasiliana Metzeltin & Lange-Bertalot (1998: 31), Cocconeis feuerbornii Hustedt (1937: 188), Eunotia yanomami Metzeltin & Lange-Bertalot (1998: 86), Gomphonema brasiliense ssp. pacificum Moser, Lange-Bertalot & Metzeltin in Moser et al. (1998: 185), Gomphonema lepidum Fricke in Schmidt (1904: 248), Lacuneolimna novagallia Tudesque & Le Cohu in Tudesque et al. (2015: 24), L. zalokariae (Lange-Bertalot & Metzeltin 1998: 39) Tudesque, Le Cohu & Lange-Bertalot in Tudesque et al. (2015: 21), and Planothidium magnificum (Hustedt 1952: 385) Lange-Bertalot (1999: 283).
description: Published as part of Tudesque, Loïc, Cohu, René Le & Wetzel, Carlos E., 2016, Two new Platessa (Bacillariophyceae) from Amazonia: Platessa guianensis spec. nov., and P. itoupensis spec. nov., pp. 237-255 in Phytotaxa 267 (4) on pages 240-245, DOI: 10.11646/phytotaxa.267.4.1, http://zenodo.org/record/13667166