Achnanthidium paulistanum L.F.Costa & D.Bicudo, 2022

Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, Phytotaxa 575 (3), pp. 187-223 : 203-204

Achnanthidium paulistanum L.F.Costa & D.Bicudo sp. nov. ( Figs 117–143 )

Description:— LM: Valves narrowly linear-lanceolate, with subrostrate to rostrate apices ( Figs 117–134 ); 8.4–15.0 µm long, 2.1–2.6 µm wide. Raphe valve: axial area narrow and linear; central area small rounded, almost absent, with 1–2 shortened striae on both sides of the valve, and never forming a fascia ( Figs 117–126 ). Raphe filiform straight. Transapical striae slightly radiate throughout the entire valve, becoming denser and more strongly radiate towards the apices, 30–31 in 10 µm ( Figs 117–126 ). Rapheless valve: axial area narrow and linear, slightly widening towards the central area; central area very small rounded or absent, with 1–2 shortened striae on both sides of the valve ( Figs 127–134 ). Transapical striae slightly radiate throughout the entire valve, becoming denser towards the apices, 30–32 in 10 µm ( Figs 127–134 ). Girdle view rectangularly arched, with apices recurved to the rapheless valve ( Figs 135–137 ).

SEM: Raphe terminating right after the last stria ( Figs 138–140 ). Central raphe fissures close, straight and slightly expanded ( Figs 138–140 ). Terminal raphe fissures short, deflected ( Figs 138–140 ). Striae mainly composed of 3–5 rounded to elongated areolae, sometimes slit-like near the valve margin ( Figs 138–143 ). Mantle with one row of slitlike areolae ( Figs 139, 140 ).

Type:— BRAZIL. São Paulo state: São Pedro, SP –304, km 127, periphyton from a small lake, A.A.J.Castro & C.E.M.Bicudo, 20th March 1990 (holotype SP-255724! = Fig. 118 , isotype: BR-4617) .

Etymology: —The specific epithet refers to people born in the state of São Paulo (in Portuguese, the “ paulistas ”).

Taxonomical remarks:— Achnanthidium paulistanum sp. nov. is characterized by a linear or narrowly lanceolate valve shape, with subrostrate apices. In contrast, A. brasiliense sp. nov. has linear-lanceolate valves and rostrate apices. Notably, the slightly deflected terminal raphe fissures are always present in A. paulistanum sp. nov. valves, a feature that was not observed in A. brasiliense sp. nov. specimens. The central area never forms a fascia and is almost absent in the new species. In this way, the main characters differing in both species are the terminal raphe fissures and the central area shape of the raphid valve.

The deflected terminal raphe fissures differentiate the species from others, including A. indistinctum specimens with valve outline very similar to A. paulistanum sp. nov. It also can be distinguished by the girdle view: the Brazilian species has apices recurved to the rapheless valve and mantle striae almost indiscernible in LM, whereas the Antarctic one presents girdle view with apices straight and mantle striae thickened in LM. In valve view, A. indistinctum has slightly higher stria density and slightly narrower valves (1.8–2.2 µm wide, 30–35 striae in 10 µm, Van de Vijver & Kopalová 2014). Achnanthidium maritimo-antarcticum , another Antarctic species, differs by having parallel to slightly convex margins, appearing sometimes undulated, and clearly protracted apices, whereas A. paulistanum sp. nov. has convex to slightly convex margins and apices not clearly differentiate from the main body. In ultrastructure, A. maritimo-antarcticum possesses terminal raphe fissures straight to weakly curved ( Van de Vijver & Kopalová 2014).

Due to its linear shape, Achnanthidium acutum sp. nov. is morphologically close to this taxon, but A. paulistanum sp. nov. can be narrowly lanceolate. These species also differ with respect to the apices, which are acutely rounded in the first and subrostrate in the latter. Nevertheless, the slightly deflected terminal raphe fissures and the higher stria density in A. paulistanum sp. nov. can be used to separate both taxa.

Distribution and ecological information:— The taxon was rare and occurred only in the epiphytic type sample.

Figures

FIGURES 117–143. LM and SEM images of Achnanthidium paulistanum sp. nov. from the type (SP-255724) from a small lake, São Pedro city, São Paulo state. Figure 118 corresponds to the holotype. 117–126. LM views of raphe valves. 127–134. LM views of rapheless valves. 135–137. Girdle views of the new taxon. 138–140. SEM external view of raphe valves showing striation pattern, axial and central areas, the terminal raphe fissures deflected and the row of slit-like areolae on mantle. 141. SEM internal view of the raphe and rapheless valves. 142, 143. SEM external view of the rapheless valves showing striation pattern, axial and central areas. LM scale bar = 10 μm. SEM scale bars indicated in each figure.