Rhoicosigma compactum (Greville) Grunow, 1877

Lobban, Christopher S., 2015, Benthic marine diatom flora of Guam: new records, redescription of Psammodictyon pustulatum n. comb., n. stat., and three new species (Colliculoamphora gabgabensis, Lauderia excentrica, and Rhoiconeis pagoensis), Micronesica 2015 (2), pp. 1-49 : 14-15

Rhoicosigma compactum (Greville) Grunow Figs 139–143

Ref. illus.: Van Heurck 1896, pl. 28, fig. 802; Peragallo & Peragallo 1897 –1908, pl. 35, fig. 8 Sample GU52N-7, GU52O-3; GU52P-1

Dimensions: Length 97–118 µm, width 16–23 µm, striae 20–22 in 10 µm.

Diagnostics: Frustule bent along the valvar plane (achnanthoid) ( Fig. 139 ); raphe on concave/ventral valve highly sigmoid, the other (convex valve) nearly straight (as for the genus) ( Figs 140, 141, 143 ). Plastids are several large plates ( Fig. 139 ). Central raphe endings overlap on both valves ( Fig. 142 ).

Comments: Hein & Lobban (2015) included two SEMs of this species for comparison with their new species R. parvum . Here I present full documentation of the record. Peragallo & Peragallo (1897 –1908) gave length = 77–180 µm, whereas Van Heurck (1896) gave rather larger dimensions of 140–180 x 35 µm, both gave stria density of 20–24 in 10 µm for R. compactum . R. oceanicum H. Peragallo ( Peragallo & Peragallo 1897 –1908, pl. 35, figs 3–7) is still larger (160–260 x 36– 45µm), with still coarser striae (17–18 in 10 µm). Hendey (1964, p. 250) gives dimensions of R. compactum as 90–150 µm, transapical striae 17–20 in 10 µm, longitudinal striae 22–24 in 10 µm, but he cites Peragallo & Peragallo (1897 –1908) figs 3, 4, 6, 7 and 8, whereas their figs 3–7 are identified as R. oceanicum .

Figures

Figures 139–149. Figs 139–143. Rhoicosigma compactum (GU52N-7). Fig. 139. Three views of a live cell in different orientations (DIC). Figs 140, 141. Acid-cleaned valves showing the two contrasting raphe paths: convex valve nearly straight raphe, concave valve highly sigmoid (DIC). Fig. 142. External detail, convex valve, showing overlapping central raphe endings and pattern of striae (SEM). Fig. 143. Internal surface of concave valve. Figs 144, 145. Seminavis robusta, DIC (GU52Q-1a) and SEM (GU52Q-10a). Figs 146–149. Surirella cf. fastuosa. Fig. 146. Live cell show lobed plastid (GU66C-7). Fig. 147. Valve in DIC (GU44AQ-3) (two stacked images). Figs 148, 149. External and internal views of valve (GU66A-1, GU66A-2) (SEM). Scale bars: Figs 139–141, 143–149 = 10 µm, Figs 142 = 5 µm.