Bicudoa C.E. Wetzel, Lange-Bertalot & Ector, 2012

Wetzel, Carlos E., Lange-Bertalot, Horst, Morales, Eduardo A., Bicudo, Denise De C., Hoffmann, Lucien & Ector, Luc, 2012, Bicudoa amazonica gen. nov. et sp. nov. (Bacillariophyta) - a new freshwater diatom from the Amazon basin with a complete raphe loss in the Eunotioid lineage, Phytotaxa 75 (1), pp. 1-18 : 5-7

Bicudoa C.E. Wetzel, Lange-Bertalot & Ector gen. nov. ( Figs 7–14 , 21–32 , 43–45 )

Frustula aspectu cinguli fere rectangulata. Valvae late ellipticae, lineari-ellipticae ad lineares apicibus cuneatis denique curte rostratis. Valvae dorsiventrales quia ad axem apicalem leviter ad levissime asymmetricae margine una semper convexa altera modice concava in mediis partibus quoad specimina media et maiora. Area axialis (id est sternum) angustissima plerumque excentrica sita. Areae angustae marginales prope limbos adsunt. Alterae areae absunt. Systema raphidea nulla. Striae transapicales, sicut in Eunotia , subparallelae saepe plusminusve irregulariter sitae inter se, sub apices transientes distincte radiantes. Interstriae comparate latiores. Structura chromatophorum incognita. Aspectus externus: Frontes valvarum fere planae cum foraminibus areolarum uniseriatis omnino comparate minimis circularibus. Series foraminum curte interruptae sterno angustissimo et latius interruptae ad iuncturam inter frontem et limbum valvae ubi series apicalis spinarum verrucosa num est tum series continuantes in limbis. Campi porellorum absunt. Aspectus internus: Superficies interna perforata areolis circularibus crateriformibus apertis. Depressiones transapicales id est alveoli vacant. Systema raphidea ut in familia Eunotiaceae sive ut in familia Bacillariaceae nulla. Etiam rimoportulae et porelli apicales ut in familia Fragilariaceae et alteris familiis diatomearum araphidearum vacant. Cingulum frustulorum: cingulum utraeque valvae 3–10 copulis apertis vel clausis constans. Omnes copulae preditae multis areolis uniseriatis transapicaliter (vel quodammodo pervalvariter ita ut in Eunotia nec in familia Fragilariaceae vel alteris familiis araphideis).

Frustules rectangular in girdle view. Cells forming band-like colonies. Valves isopolar, moderately dorsiventral, broadly linear with slightly constricted centre in large specimens; cuneate to sub-rostrate ends. Dorsal margins consistently convex. Ventral margins variable, either less concave in small valves or nearly straight to concave in medium-sized and large specimens. Axial area or sternum extremely narrow, sometimes displaced toward ventral side of valve or centrally positioned. Narrow lateral area located close to valve margins, visible in both valve and girdle view. Transapical striae sub-parallel often irregularly spaced, becoming radiate toward the ends. Costae comparatively much wider. Plastids unknown. In external view, valve face slightly curved with a shallow longitudinal depression near valve centre. Striae uniseriate situated both on valve face and mantle, interrupted by lateral area developing on junction between valve face and mantle. Areolae small and circular. Sternum narrow, sometimes displaced toward ventral side of valve. Apical pore fields absent. Cingulum composed of 3–10 open copulae, perforated by pervalvar multiporoid striae arranged in discernible rows. Raphe system, helictoglossae and rimoportulae lacking.

Type:— Bicudoa amazonica C.E. Wetzel, Lange-Bertalot & Ector

Figures

FIGURES 7–20. LM micrographs (bright field optics) of Bicudoa amazonica (Figs 7–14). Holotype population from the Rio Negro hydrographical basin, Brazil (SP–400.477). Fig. 7 Large cell showing slight dorsiventrality and irregular pattern of striae; Fig. 8 Holotype specimen; Figs 7–13 Micrographs illustrating size reduction series in the type population; Fig. 14 Two cells in connective view; Figs 15–17 Three Eunotia species similar in size and shape: Fig. 15 Eunotia pileus var. guianense (Ehrenberg) E. Reichardt; Fig. 16 Eunotia coringii Metzeltin & Lange-Bertalot; Fig. 17 Eunotia ventriosa R.M. Patrick. Note the helictoglossae present and visible on all Eunotia species (black arrow); Figs 18–20 Size reduction series of Fragilariforma hamiltonii Metzeltin & Lange-Bertalot. All specimens from the Rio Negro basin. Scale bar: 10 µm.

FIGURES 21–27. SEM micrographs of Bicudoa amazonica in external view. Fig. 21 Whole frustule in valvar view showing irregular silica deposition along the central part of the valve; central valve face with somewhat concentrated silica deposition at the apical axis; Fig. 22 Valve face apex detail showing the complete absence of raphe aperture system; small foramina forming the striae; Figs 23–24 Detail of both extremities showing reduced slightly radiate striae; note the absence of apical pore fields and external raphe openings on the mantle; axial area narrow, more or less eccentric; Fig. 25 Apex detail with small wart-like silicate on the valve edges (arrow); Fig. 26 Detail of apical valve outline in oblique view; Fig. 27 Detail of one valve and the cingulum in girdle view showing perforated Eunotia-like open copulae (white arrows) with multiporoid striae. Note the numerous open connective bands. Scale bars: Fig. 21: 10 µm; Figs 22–25: 2 µm; Figs 26–27: 5 µm.

FIGURES 28–32. SEM micrographs of Bicudoa amazonica in internal view. Apices details showing the absence of structures typical of Eunotiophycideae (raphe, helictoglossae and rimoportulae). Fig. 28 Valve with vestigial raphe at both extremities (arrows); Fig. 29 Detail of apex with no vestigial raphe; Figs 30–32 Vestigial slit (Figs 30–31) until the complete raphe loss (Fig. 32) in different individuals of the type population. Raphe slits rarely found. Striae interrupted and misaligned near the ventral margin. Scale bars: Figs 28–29: 10 µm; Figs 30–32: 5 µm.

FIGURES 40–54. SEM of several eunotioid elements: Figs 40–41 Perforated and opened valvocopula and detail of short internal raphe slit of Eunotia ventriosa var. brevis. Rimoportulae not visible; note the alternating short striae at the dorsal valve mantle and the thick apical cell wall; Fig. 42 Detail of internally uncovered areolae of E. ventriosa var. brevis; Figs 43–45 Bicudoa amazonica: Fig. 43 Detail of external uncovered areolae; Fig. 44 Transversal view of the valve face showing areolae aperture not covered and thick silicified cell wall; Fig. 45 Apex detail of B. amazonica with no external raphe nor apical pore field; Fig. 46 Detail of E. ventriosa var. brevis in oblique external view showing well developed raphe branch and perforated connective bands; Figs 47–54 SEM of different Eunotia species from the Brazilian Amazon showing distinct external and internal evolutionary patterns of raphe system: Fig. 47 Eunotia siolii Hustedt—Holotype (AM 1310). Raphe located mainly on the ventral valve mantle; Fig. 48 Eunotia synedraeformis Hustedt. Prominent “J” shaped raphe with dilated distal and proximal ends; rimoportulae aperture distinct; Fig. 49 Eunotia tapacumopsis Metzeltin & Lange-Bertalot. Large raphe slit located on the valve mantle and valve face; Fig. 50 Eunotia sp. from the Rio Negro with prominent spines and areolae covered externally by velum; Figs 51–52 Eunotia waimiriorum C.E. Wetzel. Reduced raphe system (white arrows) and poorly developed helictoglossae; Fig. 53 Eunotia bidens Ehrenberg. Note internal arrangement of the striae areolae aligned in a narrow “ditch” as in Fragilariforma hamiltonii; Fig. 54 Eunotia sp. showing a well developed helictoglossa. Scale bars: Figs 40–41, 45–47 and 53–54: 5 µm; Figs 42–44: 0.5 µm; Fig. 48: 2 µm; Fig. 49: 3 µm; Figs 50–52: 1 µm..