Chrysogorgia tenuis sp. nov.

Figs. 42, 43; Table 9

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Material examined. Holotype: MBM286858, station FX-Dive 214 (10°5′17″N, 140°10′09″E), a seamount (tentatively named as M5) on the Caroline Ridge, 1357 m, 1 June 2019.

Diagnosis. Chrysogorgia with a broad bottlebrush-shaped colony and discoidal holdfast. Branching sequence 1/ 3L. Distance between adjacent branch 12–14 mm. Scales in tentacle rachis elongate and nearly smooth with irregular shape, often one end forked and broad, and the other end narrow. Scales in pinnules slender and nearly smooth, usually one end forked or broad and the other end narrow or sharp. Scales in polyp body wall elongate and smooth, usually lobed with irregular shape. Scales in coenenchyme slender and smooth, some of them with lobed edges and irregular shape.

Description. Specimen of holotype broad bottlebrush-shaped with a part of unbranched stem, about 31 cm long and 16 cm wide in maximum with the holdfast not recovered (Fig. 42B). Stem about 1.5 mm in diameter at base with golden metallic luster. Branching sequence 1/ 3L. Branches tenuous and thin, subdivided dichotomously, up to 10 orders with branching angle 60°–90°. Distance between adjacent branch 12–14 mm, orthostiche intervals 36–40 mm, the first internode of branch 14–22 mm long and terminal branchlets up to 20 mm long. Polyps usually cylindrical with eight long tentacles, 1–2 mm tall and 1.0– 1.5 mm wide, some of them up to 3 mm tall in terminal (Fig. 42C–F). Polyps arranged one or two and often small on first internode, one to four in medial internodes and up to four in terminal branchlets. Polyps small, about 0.5 mm tall, arranged up to two on each internode of stem. Verrucae absent.

Scales in aboral face of tentacle rachis transversely and alternately arranged in a single row, elongate, thick and nearly smooth with irregular shape, often one end became broad or forked, and the other end narrow, some of them with finely toothed edge surface and sparse fine warts, measuring 104–430 × 22–170 μm (Fig. 43A). Scales in pinnules longitudinally arranged, slender and a little curved, usually one end forked or broad and the other end narrow or sharp, nearly smooth and occasionally with sparse fine warts and finely toothed edge surface, measuring 110–333 × 7–72 μm (Fig. 43C). Scales in polyp body wall transversely or obliquely arranged, elongate and smooth, often lobed with irregular shape, some of them with finely toothed edge surface, measuring 156–496 × 32–228 μm (Fig. 43B). Scales in coenenchyme longitudinally arranged, slender and smooth, some of them with broad lobed edges forming irregular shape, measuring 143–457 × 26–146 μm (Figs. 42G, 43E). Scales in polyp mouth area near the tentacles usually thick and coarse with many warts and obvious toothed edges (Fig. 43D).

Type locality. A seamount (tentatively named as M5) on the Caroline Ridge with water depth of 1357 m.

Etymology. The Latin adjective tenuis (tenuous) refers to the tenuous and thin branches of the species.

Distribution and habitat. Found only from a seamount on the Caroline Ridge in the Western Pacific with water depth of 1357 m. Colony attached to a rocky substrate (Fig. 42A). The water temperature was about 3.32°C and the salinity about 36.46.

Remarks. Chrysogorgia tenuis sp. nov. is most similar to C. sibogae Versluys, 1902 and C. axillaris (Wright & Studer, 1889) in colony shape and branching sequence, but differs from the former by having more irregular scales with broad lobes in polyp body wall (vs. relatively regular with finely serrated edges), more slender scales in coenenchyme (vs. round or oval), and from the latter by different scales in coenenchyme (slender scales and some of them lobed and irregular vs. small oval and biscuit-like scales and plates) (Table 9). It is also similar to C. delicata Nutting, 1908, but differs by relatively regular sclerites with less lobes in polyp and coenenchyme (vs. lobed and branched with more irregular shape) (Table 9).