Stenopterobia amazonica 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 : 69-72

Stenopterobia amazonica Sala, Guerrero, Núñez Avellaneda & Kociolek sp. nov. (Figs 38–56)

(= Stenopterobia delicatissima (Lewis) Brébisson ex Van Heurck sensu Sala et al. 2002a )

Description: —Valves are linear to lanceolate with long, protracted apices (Figs 38–53). Valve margins are convex to almost parallel. Externally the valve face has irregularly shaped transapical costae (Figs 54–55). The median area is wide and conspicuous, sunken or elevated, not extending up to the poles (Fig. 52). The transapical costae are broad and flat, interconnected at the median area by short, irregular thickenings, which leave round or transapically elongated depressions among them; towards the valve margin costae become more cylindrical (Figs 54–55). Near the raphe canal, 2–3 costae fuse to the keel while 2–4 thinner costae extend beneath the keel to form the fenestral bars. This pattern is disorganized at the valve apices. The striae are hardly visible in external valve view due to the valve face thickenings and internally they are bi-triseriate and short, occupying 1/4 of the valve width and positioned at the same level of the interstriae (Fig. 56). Internally, the areola openings are not rimmed and portulae are ovoid or round, regularly distributed (Figs 53, 56).

Dimensions (n=26): length: 68–92 µm; width: 5.5–7.0 µm; portulae: 56–75 in 100 µm; striae: 24–34 in 10 µm; areolae: 80–100 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 5610-2 ! finder: S/31–2–4, here illustrated as Fig. 39) .

Etymology: —The epithet refers to the geographic area where this new species was collected.

Comments: —The new species has also been found in Lake Bufeo (Peruvian Amazon).

Its main morphometric features and those of the most closely related taxa are summarized in Table 3. Sala et al. (2002a, figs 61–62) described specimens of S. delicatissima from Lake Tarapoto, that differ from those found in this study in their shorter valves and less protracted ends. Nevertheless, they are coincident in other morphometric data and ultrastructural features, so we consider that those materials also belong to S. amazonica .

Our materials are similar in dimensions to S. delicatissima but differ from materials collected at the type locality and described in Siver & Camfield (2007) in their valve outline with long protracted ends, clearly differentiated from the main body of the valve, and also in their valve morphology. Stenopterobia delicatissima has a narrower axial area (occupying no more than 20% of the total width), not traversed by the interstriae, only two costae connected to the raphe canal, striae mostly triseriate and areolae externally capped with domed coverings. Additionally, areola density is slightly higher in S. delicatissima than in the Amazonian specimens.

The studied materials also resemble in valve outline and dimensions the specimens of a taxon from Minnesota ( USA) described and illustrated as S. cf. delicatissima in Ruck & Kociolek (2004); however, SEM images reveal certain differences in their ultrastructure. In North American specimens the axial area is quite smooth, more distinctly delimited and narrower—this is especially evident in internal view—and in the valve interior generally three striae proceed into the alar canals instead of one as it occurs in the specimens from Colombia and Peru. Although similarly sized, the individuals of S. delicatissima encountered in Goiás (central-western Brazil) ( Oliveira et al. 2012) differ from our specimens in their apices gradually attenuated and thus less differentiated from the main body of the valve.

Figs 38–56. Stenopterobia amazonica sp. nov. LM (Figs 38–51) and SEM (Figs 52–56) images from type material LPC 5610-2. 38–51. Specimens in valve view showing size variation. 52. Valve in external view. 53. Valve in internal view. 54. Tilted external view of a valve showing mantle and fenestrae. 55. Detail of the same specimen in external valve view showing irregularly shaped transapical costae and axial area. 56. Internal valve view showing biseriate striae, portulae and wide axial area. Scale bars: 1 µm (Figs 54–56), 10 µm (Figs 38–51), 20 µm (Figs 52–53).

Another similar taxon is S. hulli , collected in Lake Jurucuí, Brazilian Amazon ( Metzeltin & Lange-Bertalot 2007). The protologue does not include SEM photographs; however, the differences in valve width (5.5–7.0 vs 7.0–7.5 µm) and in the stria density (24–34 vs 15–17 in 10 µm) are sufficiently clear to differentiate these two taxa with LM.

The valve shape of the new species also shows some resemblance to S. fusiformis , however, the latter species has highly protracted, needle-like apices, a higher density of alar canals (56–75 vs 100–105 in 100 µm) and very short striae, restricted to the margins. Finally, the fine valve morphology of the studied specimens is closest to S. gracilis , a species described for North Carolina, USA. Nonetheless, the specimens from the Amazon region have some differences with those collected in North America in their valve dimensions since they do not produce such long valves (68–92 vs 65–125 µm), and also in details of the valve morphology such as the occurrence of 2–3 ribs fused to the raphe canal, areolae externally not capped nor rimmed and lack of wings on the mantle costae.