Nitzschia incurva Grunow, 1878

Lobban, Christopher S. & Witkowski, Andrzej, 2023, Marine benthic diatoms of Guam: new records, Dictyoneis apapae sp. nov., and updates to the checklist, Micronesica 2023 (2), pp. 1-75 : 12

Nitzschia incurva Grunow Figs 75–78

Syn.: Nitzschia lorenziana Grunow in Cleve & Grunow (nom. illeg.)

Ref. illus.: Navarro 1982, pl. 35, fig. 4; Poulin et al. 1990, p. 87, figs 67, 68, 71, 72 ( SEM); Witkowski et al. 2000, p. 392, pl. 210, figs 24, 25, pl. 211, fig. 3, pl. 212, figs 1–3 (all as N. lorenziana )

Samples: GU58G-4A, GU44I-1, GU44Z-15

Dimensions: Length 18.5 µm, width 7.4 µm; striae 20 in 10 µm.

Diagnostics: Weakly sigmoid in girdle view, keel eccentric except in long-rostrate apices, striation bold in LM because of external costae between the striae; central striae biseriate towards to keel, triseriate towards the margin ( Fig. 78), becoming rings of pores with central row of pores and diminishing towards apex ( Fig. 77). Eccentric keels of the two valves are on opposite sides of the frustule (nitzschioid, not hantzschioid) ( Figs 77, 78). Keel canal is closed except for circular fenestrae

( Fig. 77).

Comments: Guiry (2017) explains why lorenziana is illegitimate and provides the earlier synonym. The ultrastructure of our specimens clearly matches SEM images in Poulin et al. (1990). It is apparently a brackish/estuarine species, with wide temperature tolerance (from Gulf of St. Lawrence to Florida + Guam). Abundant in Vietnam and Indonesia (Witkowski, unpubl.). Lopez-Fuerte et al. (2010: p. 63, pl. 37, fig. 21) show var. subtilis Grunow from Mexico and list the nominate variety. Nitzschia incurva somewhat resembles N. gaoi Bing Liu, S. Blanco & B.Q. Huang (2019) , but that has small conopeal canals and uniseriate striae. Lobban & Ashworth (2022b) did not transfer N. gaoi or their own N. maiae Lobban, Ashworth, Calaor & E.C.Theriot to Homoeocladia because of maintaining a narrow definition of the morphology of Homoeocladia and noted some other species in the region with doubtful formation of a conopeal canal that also have T-shaped external costae (one shown in Lobban 2023a, figs 4F, G).