Rhoicosigma parvum n. sp., a benthic marine diatom from The Bahamas and Western Pacific

Abstract

A new diatom species, Rhoicosigma parvum Hein & Lobban, is described from benthic marine macroalgae and seagrasses northwest of Andros Island, The Bahamas, and from macro- and filamentous seaweeds of Guam and Palau. Specimens observed ranged in length from 30 to 70 µm, and in width from 6 to 9 µm, with 29–32 transverse striae in 10 µm. The frustule is slightly sigmoid in valve view and arched in girdle view. The two valves are dissimilar. The convex valve has an almost-straight raphe and the concave valve has a strongly sigmoid raphe. The chloroplasts are deeply lobed plates, one along each side of the girdle. Scanning electron microscopy revealed that the valves have loculate areolae, with the areolae opening to the valve exterior as apically elongated slits. By contrast, the internal areolar openings are rectangular, elongated apically. Externally, the central raphe endings are deflected in the same direction and terminate in pore-like expansions. The terminal fissures are strongly bent in opposite directions and in the same direction as the sigmoid curvature of the valve end. Internally, the central raphe branch endings are simple and straight, whereas the apical raphe endings terminate in well-developed helictoglossae. There are siliceous thickenings (central bars) on both sides of the central nodule. Specimens are similar to R. compactum (Greville) Grunow but are smaller and have finer striae.