LM observations: Valves elliptic-lanceolate, very slightly constricted in the median region, with slightly cuneate to rounded apices and longitudinal fold slightly noticeable, 16.30–27.56 μm long and 7.17–10.00 μm wide. Raphe eccentric and fibulae weakly distinguishable (7–10 in 10 μm). Striae inconspicuous and axial area absent. The valvar face presents two different patterns, one side presents transapical ribs, parallel to somewhat radiate, unequally spaced (13–16 in 10 μm). The other side lacks these or other kind of structures.

SEM observations: The transapical ribs are asymmetric, becoming larger closer to the valve center, assuming a spatulate shape (Figs 14, 16, 17). Striae parallel (40–42 in 10 μm), formed by a single line of rounded poroids (80 in 10 μm). The striae are more easily recognized among the transapical ribs, in the other side of the valve face they are less conspicuous and closer to valvar margin they are formed by a transapically elongate perforations (Figs 14, 17). The valve margin is thickened in the raphe side and a flattened silica projection is deflected towards the valve face, as well as distal raphe endings (Figs 18, 19). The cingulum is composed by several non-perforated bands (Fig. 86). The mantle is striated (40–42 in 10 μm) (Figs 14–16). The longitudinal fold is more remarkable internally (Fig. 15). It was not possible to confirm the presence of helictoglossae (Figs 20, 21).

Holotype: — BRAZIL. Rio Grande do Sul State, Rio Grande city, Ilha da Pólvora (32º01’14.424’’S – 52º05’59.095’’W), epiphyton attached to Bolboschoenus maritimus L., sample collected by D. Talgatti and L. Donadel, February 18, 2011. Circled specimen on slide 6611 (HAS — Museu de Ciências Naturais - Fundação Zoobotânica do Rio Grande do Sul, Porto Alegre, Brazil). Figures 2 and 8 correspond to the holotype specimen.

Isotype: — BRAZIL. Rio Grande do Sul State,Rio Grande city,Ilha da Pólvora(32º01’14.424’’S – 52º05’59.095’’W), epiphyton attached to Bolboschoenus maritimus L., sample collected by D. Talgatti and L. Donadel, February 18, 2011. Circled specimen on slide 78330 (UPCB - Herbarium of the Universidade Federal do Paraná, Curitiba, Brazil).

Etymology: the epithet “ confusa ”, which means confusing or unclear, was attributed due to the difficulty to define the species identity and the former misidentification as Tryblionella debilis.

Occurrence in the sites: Tryblionella confusa was observed in epiphyton and micrphytobenthos both in winter and summer.