Rumarcanella yanbaruensis

Description. Colony tubular, branching, transparent; brownish transparent only in basal part (Figs 6 A, B). Floatoblast (Fig. 6 D) oblong-elliptical; large, 350 – 416 (387 ± 21) μm long (n = 16) by 205 – 262 (231 ± 21) μm wide; length / width ratio about 1.8; sides almost parallel (Figs 6 C, D); ventral fenestra oval, 203 – 249 (242 ± 11) μm long by 168 – 196 (192 ± 7) μm wide; dorsal fenestra 171 – 202 (192 ± 9) μm long by 137 – 161 (153 ± 6) μm wide. Surface of floatoblast fenestrae entirely covered by low, rounded tubercles topped with hypertubercles (Figs 7 A – D). Floatoblast valves nearly symmetrical in lateral and transverse views; both valves nearly flat; fenestra of ventral valve convex; floatoblast suture line finely denticulate (Figs 7 B, D). Annulus wider at both ends; annulus of ventral valve 15 – 20 μm wide on sides, 70 – 80 μm wide at ends; annulus of dorsal valve about 20 – 25 μm wide on sides and 90 – 120 μm wide at ends; annulus sculpturing comprising a coarse pavement of circular elements. Sessoblast (Fig. 7 E) rounded-elliptical, 258 – 310 (288 ± 21) μm long by 205 – 237 (219 ± 13) μm wide (n = 5), length / width ratio about 1.3; fenestra covered with small cylindrical tubercles; annulus weakly reticulate; margin of annulus coarsely serrate (Fig. 7 F).

Plumatella sp. 1. Hirose et al. 2008: 65.

Remarks. Although colonies of Rumarcanella yanbaruensis resemble Hyalinella colonies, the tuberculate sessoblast and thin colony wall are very similar to those in R. vorstmani. The floatoblast of R. yanbaruensis resembles that of Hyalinella lendenfeldi in the wide annulus and flat form, but is smaller with larger fenestrae than in H. lendenfeldi. The fenestrae of R. yanbaruensis are similar in size to fenestrae in old illustrations of Plumatella princeps (e. g. Kraepelin 1887), but R. yanbaruensis clearly differs from P. princeps, P. emarginata, and related species in having a large dorsal fenestra, wide flat annulus and denticulate suture line. A molecular phylogeny (Hirose et al. 2008) showed R. yanbaruensis to be basal in a clade containing the four species of Rumarcanella.

Distribution. Known only from the Yambaru area, northern Okinawa.

Etymology. The species name is from the Ryukyuan language yanbaru, in reference to the Yanbaru area, where the type locality is located.

Material examined. Holotype. Colony with floatoblasts, collected 2 November 2007 from Benoki Dam, village of Kunigami; specimen deposited in National Science Museum, Tokyo (NSMT-Te 680). Paratype. Colony with floatoblasts, same collection data as for holotype; specimen deposited in National Science Museum, Tokyo (NSMT-Te 681). Other material examined. Floatoblasts, Fukuji Dam, village of Kunigami.