Stenopterobia caquetaensis Sala, Guerrero, Núñez Avellaneda & Kociolek, 2021

Sala, Silvia E., Guerrero, José M., Avellaneda, Marcela Núñez & Kociolek, John P., 2021, New species of Stenopterobia (Bacillariophyta) from Colombia and Peru, and new nomenclatural transfers in Iconella, Phytotaxa 514 (1), pp. 61-76 : 64-67

Stenopterobia caquetaensis Sala, Guerrero, Núñez Avellaneda & Kociolek sp. nov. ( Figs 2–19 )

Description: —Frustules are solitary and valves are slightly sigmoid with both sides parallel up to the narrow, protracted and curved ends ( Figs 2–15 ). Externally, the transapical costae are interrupted in the median line of the valve delimiting an irregular thickened and sunken axial area ( Figs 16–17 ). Pairs of these costae have the same breadth lengthwise and are connected to the upper part of the raphe canal, while others (1–3) are thinner near the margin and form the fenestral bars ( Fig. 17 ). Costae alternate with striae composed of 2 rows of small round areolae arranged in a quincunx pattern; towards the margin a third, shorter row of areolae is usually present ( Fig. 16 , arrowheads). Exceptionally, striae are tetraseriate ( Figs 16–17 , arrows). The striae are broader than costae. This pattern is the same at the valve mantle ( Fig. 17 ). The raphe canal is located on a slightly developed wing, connected to the interior of the frustule by short alar canals. On the valve interior, the interstriae are slightly elevated; at the margin the striae proceed into the alar canals or are separated from the mantle striae by a hyaline area ( Fig. 18 , arrows); the areolae openings are not rimmed ( Fig. 18 ). Portulae are ovoid, irregularly spaced ( Figs 18–19 ).

Dimensions (n=37): length: 95–145 µm; width: 4.0–5.5 µm; portulae: 60–77 in 100 µm; striae: 28–32 in 10 µm; areolae: 87–100 in 10 µm.

Type: — COLOMBIA. Caquetá River Basin: Orteguaza River , flooded forest creek, 1º 38’ 1.46” N, 75º 28’ 45.98” W, M. Núñez Avellaneda, 1 st November 2003 (holotype slide LPC 5610-1 ! finder: K/34–2–4, here illustrated as Fig. 4 ) .

Etymology: —The specific epithet caquetaensis refers to the Caquetá River Basin, where the new species was found.

Comments: — Table 2 summarizes the main morphometric parameters of the new species and those of the most closely related, sigmoidal-shaped taxa. Compared to the lectotype valves of S. curvula depicted in Krammer & Lange- Bertalot (1997, pl. 171, figs 6–7), S. caquetaensis is similar in the overall sigmoid outline of the valve but apices of the new species are thinner and more differentiated from the main body of the valve. Also valve morphometrics differ, though slightly, from S. curvula since valves of the new species are narrower (4.0–5.5 vs 6–9 µm) and both stria and portula densities are higher (28–32 vs 18–24 in 10 µm and 60–77 vs 30–60 in 100 µm, respectively) while valve length falls within the range given for S. curvula ( Krammer & Lange-Bertalot 1997) . SEM observations of the type material of S. curvula in Krammer (1989, figs 1–8) also show differences between both species in their fine valve morphology. The valve face in S. curvula is more transapically undulated, the axial area is narrower, portulae comprise two striae instead of one as in S. caquetaensis and striae are mainly triseriate all across their width. Siver & Camfield (2007, figs 13–18) provide SEM micrographs with details of the valve morphology of the type specimens of S. intermedia (Lewis) Van Heurck ex Hanna (1933: 90) , a later synonym of S. curvula . Although the specimens illustrated there show different stages of valve face coverings—not observed in the Amazonian materials—which make comparisons difficult, they clearly differ from our material in the narrower axial area; other features such as stria and areola densities match our specimens. Moreover, the original illustrations of Surirella intermedia Lewis (1864: 339 , figs 2a and 2b) depict valves with apices barely attenuated and thus clearly different from S. caquetaensis .

Stenopterobia densestriata (Hustedt) Krammer also resembles our specimens but according to the original illustration of S. intermedia var. densestriata Hustedt (1914: 115 , Simonsen 1987, pl. 19, fig. 3) and the specimen illustrated in Krammer & Lange-Bertalot (1997, pl. 172, fig. 6), the valve outline of this species is different, with its apices being much less protracted. Further differences are portula and areola densities, while the other morphometric parameters overlap.In relation to valve morphology, both species have a narrow axial area but thickenings of the interstriae as shown in Krammer & Lange-Bertalot (1997, pl. 170, figs 3–4) have not been observed in S. caquetaensis .

Comparison with S. planctonica Metzeltin & Lange-Bertalot (1998: 221) , described from Guyana, is difficult since this species is poorly documented. The studied materials differ from S. planctonica in valve outline, size and areola density; furthermore, the only SEM picture provided by Metzeltin & Lange-Bertalot (1998, pl. 210, fig. 4) shows that in S. planctonica striae are tri- to tetraseriate instead of bi- or tri- (tetra-) seriate as in the new species. Another similar taxon is S. pelagica ; although both taxa are similar in valve shape and main morphometric features, our specimens have lower stria densities. Moreover, SEM observations made by Siver & Camfield (2007, figs 130–132) on Hustedt’s materials revealed some differences in their ultrastructure that are (a) costae in S. pelagica are uniformly spaced and evenly thickened, without differentiation between those attaching to the alar canals and the rest of the costae, and (b) striae are composed of two rows of areolae instead of 2–3 (up to 4) as it occurs in the specimens from Colombia.

Stenopterobia rautenbachiae Cholnoky (1958: 138) may also be difficult to distinguish from the new species. Both taxa have similar dimensions and valve shape, though in the South African taxon valve apices are somewhat less attenuated and both portula and stria densities are lower (40–45(50) in 100 µm and 24–26 in 10 µm, respectively) than in the Amazonian species. Comparison with S. intermedia f. subacuta Fricke in Schmidt et al. (1912, pl. 284, fig. 6) is limited as there is only one specimen illustrated in the literature. Both taxa coincide in their valve outline and narrow axial area but the forma subacuta is larger (177 µm) and has lower alar canal and stria densities (52–54 in 100 µm and 22 in 10 µm, respectively).

Finally, specimens that more closely resemble the studied materials are those collected in Minnesota, USA, and described as S. cf. densestriata in Ruck & Kociolek (2004). Both species overlap in their morphometric features and are coincident in valve outline, type of stria, costa structure and smooth raphe canal, but S. cf. densestriata has a very narrow, almost linear axial area ( Ruck & Kociolek 2004: pl. 58, figs 1–5). Nevertheless, more details of the valve structure are needed to determine if this taxon is conspecific with the Colombian materials.

Figures

FIGURES 2–19. Stenopterobia caquetaensis sp. nov. LM (Figs 2–13) and SEM (Figs 14–19) images from type material LPC 5610-1. 2–13. Specimens in valve view showing size variation. 14. External view of a valve. 15. External view of a broken frustule, partially showing valve internal view. 16. Detail of the external valve view showing striae, transapical costae and raphe canal. Arrowheads point 2- seriate striae with a third row of marginal areolae.Arrow points a tetraseriate stria. 17. Tilted external view of the same specimen showing valve mantle and fenestrae. Arrow points a tetraseriate stria. 18. Internal valve view with detail of the striae and the ovoid, irregularly spaced portulae. Arrows point the hyaline area separating valve face striae from mantle striae. 19. Internal view of the valve apex. Scale bars: 1 µm (Figs 16–19), 10 µm (Figs 2–13), 20 µm (Figs 14–15).