Iridogorgia curva Xu, Watling & Xu, 2025

Iridogorgia curva Xu, Watling & Xu , sp. nov.

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Figs. 3 View FIGURE 3 , 4 View FIGURE 4

Material examined: Holotype: MBM286456 View Materials , station FX-Dive 215 ( 10°05.05′N, 140°10.60′E), a seamount (temporarily named as M5) located on the Caroline Ridge in the tropical northwest Pacific, 1081 m, 2 June 2019. GoogleMaps

Diagnosis: Colony relatively short. Axis closely coiled with helical turn 5–7 cm high. Polyps small and curved with tentacular part nearly perpendicular to the body. Sclerites rods and needles without scales, all slender and nearly smooth, occasionally with sparse small warts. Polyps and branches with a few low verrucae.

Description: Holotype about 50 cm long with the holdfast not recovered after collection, with only a few branches alive on the top in situ ( Fig. 3A, B View FIGURE 3 ). The direction of growth clockwise. Axis closely coiled spiraling upward and with dark iridescent metallic luster, about 4 mm in diameter at base. Branches not present on lower part, upper section of about 30–40 cm with 6 helical turns and each turn 5–7 cm long and 1–3 cm in diameter. Branches arc-shaped, up to 15 cm long with as many as 29 polyps counted, producing from the middle to top and arranged along one side of the axis, 2–3 mm apart. Polyps small and curved, 1.0– 1.5 mm in height and up to 2.5 mm in width at base, 3–4 mm apart ( Fig. 3C–F View FIGURE 3 ). Tentacles incomplete contractile, up to 1 mm long and 1 mm wide. Polyps and branches with a few low verrucae.

Sclerites all slender and nearly smooth, occasionally with sparse small warts, and scales absent. Rods and needles in the back of tentacle rachis longitudinally arranged, some of them with sparse small warts or shallow cracks on surface, and measuring 148–607 × 12–49 μm (length × width, the same below, Fig. 4A View FIGURE 4 ). Rods and needles in the polyp body wall usually longitudinally arranged in the upper part and transversely in the basal part, occasionally crossed and with sparse small warts, and measuring 125–559 × 19–56 μm ( Fig. 4B View FIGURE 4 ). Needles in coenenchyme arranged along the branch, thick and usually with one or two sharp ends, some of them a little curved and with an irregular outline, and measuring 167–946 × 14–70 μm ( Fig. 4C View FIGURE 4 ). Sclerites usually abundant in the coenenchyme on the side bearing the polyps, and sparse to absent on the other side ( Fig. 3E, F View FIGURE 3 ).

Type locality: A seamount (temporarily named as M5) located on the Caroline Ridge with water depth of 1081 m.

Etymology: The species name curva is derived from the Latin adjective curvus, referring to the curved shape of the polyps.

Distribution and habitat: Found only from a seamount located on the Caroline Ridge. Colony growing on a rocky bottom with a white holdfast and with some fouling hydrozoans ( Fig. 3A View FIGURE 3 ). The water temperature was 4.2°C and the salinity 36.6.

Remarks: Iridogorgia curva sp. nov. is characterized by small and curved polyps, slender sclerites without scales. It is similar to I. splendens Watling, 2007 , I. densispiralis Xu, Zhan & Xu, 2021 and I. verrucosa Xu, Zhan & Xu, 2021 in short colony and close helical turns. Iridogorgia splendens , originally described by Watling (2007), was inadequately characterized; a revised description is provided in this study (see below). This species is distinguished by abundant rods and needles in the polyp body wall, featuring fine warts, sparse warty rods at the tentacle bases, and rare to absent needles in the inter-polyp coenenchyme. Iridogorgia curva sp. nov. differs from I. splendens in having relatively sparse and slender sclerites with fewer warts in both the polyp body wall and tentacles (vs. numerous, stout sclerites with abundant warts) ( Watling 2007).

Iridogorgia densispiralis and I. verrucosa , both described by Xu et al. (2021), differ in their morphological features. Iridogorgia densispiralis possesses rods, needles and a few flat rods (previously mischaracterized as thick elongated scales) in the polyp body wall, sparse warty rods at the tentacle bases, and rods and needles in the coenenchyme. In contrast, I. verrucosa is characterized by flat rods and needles with irregular edges in the polyp body wall and coenenchyme, sparse warty rods at the tentacle bases. Iridogorgia curva sp. nov. distinguishes itself from these two species through its distinct slender sclerites in the polyp body wall and coenenchyme, along with the absence of flat and relatively stout rods in the polyp body wall.