Hemicorallium ryukyuense, Nonaka & Takata & Yasuda, 2025

Hemicorallium ryukyuense sp. nov.

[New Japanese name: Ryukyu-sango] ( Figs 2–18 View Fig View Fig View Fig View Fig ; Tables 1, 2)

Material examined. Holotype: NSMT-Co 1914, around Okinawa Island , 860 m depth, 10 September 2013, collected by Takashi Ganaha.

Diagnosis. Colony branched in one plane with some anastomoses; irregularly dichotomous branching, at acute or right angles near base and at acute angles in twigs. Contracted autozooids distributed three-dimensionally throughout colony, at 4 mm intervals on stem and 2.5 mm intervals on twigs on average. They tend to be arranged in two pairs at branch tips, without clusters. Autozooid diameter 1.1–2.0 mm (avg. 1.66 mm) and height 1.2–2.0 mm (avg. 1.53 mm). Coenenchyme 0.07–0.15 mm (avg. 0.11 mm) thick, pale pink, with inconspicuous warts. Stout axis, no pits beneath autozooids, and smooth and red surface. Tentacles contain blunt warty rods, mainly elongate multi-radiates; coenenchyme contains 8-radiates and multi-radiates. Double clubs absent.

Description of the holotype. Colony form. The specimen is an almost complete colony ( Fig. 2 View Fig ), measuring approximately 125 mm in height and 100 mm in width. The branching is fan-shaped and irregularly dichotomous in one plane. The angles of branching are acute to right angle in the stem and acute in the twigs. The diameter of the thickest part of the stem is approximately 5 mm, whereas the thinnest branch tips measure 1–2mm. The branches tend to gradually thin from the base to the tip. There are three instances of anastomosis on the branchlets ( Fig. 3 View Fig ), although some pairs of branches may appear to be joined in photographs. The branch cross-sections are almost rounded ( Fig. 4 View Fig ).

Polyps. Autozooids are not retracted into the coenenchyme but form cylindrical mounds with eight longitudinal striations ( Figs 5 View Fig , 8–10 View Fig View Fig View Fig ), distributed three-dimensionally across the colony (on both sides and laterally). There are almost no autozooids on the opposite side of the colony ( Figs 2 View Fig , 7). They are sparsely distributed along the stem, at intervals of 4.08 ± 1.00 mm ( N = 13) ( Fig. 6), but are relatively crowded and regularly spaced on the branchlets, at intervals of 2.47 ± 0.39 mm ( N = 15) ( Figs 8 View Fig , 9 View Fig ). The autozooids tend to be distributed in pairs on the branchlets, but without forming clusters ( Figs 8 View Fig , 9 View Fig ). The diameter of the autozooid mounds is 1.66 ± 0.18 mm ( N = 60), and their height is 1.53 ± 0.18 mm ( N = 19). Inconspicuous siphonozooids are distributed mainly on the opposite side and around the autozooids, with a diameter of 0.035 ± 0.008 mm ( N = 13) ( Figs 8 View Fig , 9 View Fig , 11).

Axis. The axis is smooth without commensal burrows on the surface ( Figs 10 View Fig , 12–14). No rounded pits were observed at the autozooid positions.

Coenenchyme. The coenenchyme is rather thin, measuring 0.11 ± 0.025 mm ( N = 17) ( Figs 4 View Fig , 14 View Fig ). Small and inconspicuous warts, approximately 0.24 ± 0.03 mm in diameter ( N = 26), are distributed on both sides, along with weak longitudinal grooves ( Figs 4 View Fig , 6, 11). At high magnification, the gastrovascular system was visible in the cross-section of the branch ( Fig. 4 View Fig ) and through the thin coenenchyme ( Fig. 13 View Fig ).

Color. The preserved specimens were pale pink under wet conditions and cream yellow under dry conditions ( Figs 2 View Fig , 6–9, 11). The axis is red ( Figs 4 View Fig , 10 View Fig , 12–14).

Sclerites. The tentacles predominantly contain rods (49%; 0.099 ± 0.011 mm long, 0.028 ± 0.006 mm wide; N = 74), elongated multi-radiates (42%; 0.085 ± 0.010 mm long, 0.039 ± 0.005 mm wide; N = 63), with a few 8-radiates and 6-radiates ( Figs 15 View Fig , 17 View Fig ; Table 1).

The autozooid mounds mainly consist of elongated multi-radiates (83%; 0.089 ± 0.005 mm long, 0.046 ± 0.003 mm wide; N = 97). Some 6-, 7-, and 8-radiates as well as other types, are present ( Figs 15 View Fig , 17 View Fig ; Table 1).

The branch tips primarily contain asymmetrical 8-radiates (46%; 0.062 ± 0.005 mm long, 0.037 ± 0.003 mm wide; N = 42), elongated multi-radiates (41%; 0.076 ± 0.010 mm long, 0.041 ± 0.004 mm wide; N = 37), with a few 6-, 7-radiates, symmetrical 8-radiates, and other types ( Figs 16 View Fig , 17 View Fig ; Table 1).

The coenenchyme on the colony base mainly contains symmetric 8-radiates (41%; 0.054 ± 0.007 mm long, 0.033 ± 0.004 mm wide; N = 33), asymmetric 8-radiates (26%; 0.056 ± 0.004 mm long, 0.034 ± 0.003 mm wide; N = 21), and multi-radiates (25%; 0.059 ± 0.009 mm long, 0.035 ± 0.003 mm wide; N = 20). A few 6- and 7-radiates are also present ( Figs 16 View Fig , 17 View Fig ; Table 1).

Etymology. The species is named after the region from which it was collected off Okinawa Island. The Ryukyu Archipelago, which includes Okinawa Island, was once the center of the prosperous Ryukyu Kingdom. Today, both “ Okinawa ” and “Ryukyu” are used to describe the area, but “Ryukyu” carries a more traditional resonance. In naming this new species, we have chosen to honor that tradition and selected “Ryukyu” for the specific name.

Remarks. Tu et al. (2015) described the morphological characteristics of the genus Hemicorallium as follows: contracted autozooids that are not retracted into the coenenchyme, cylindrical in shape, and typically distributed on one side of the colony. The tentacles contain rod-shaped sclerites. All of these characteristics were found in the holotype of H. ryukyuense .

Among the species of Hemicorallium described from Hawaii and the western Pacific, the holotype of this species has larger autozooid mounds (1.66 ± 0.18 mm in diameter, 1.53 ± 0.18 mm in height). The autozooid mounds of H. abyssale ( Bayer, 1956) , H. boshuense (Kishinouye, 1903) , H. guttatum Tu, Dai, and Jeng, 2016 , H. halmaheirense (Hickson, 1907) , H. meraboshi Nonaka, Hanahara, and Kakui, 2023 , H. regale ( Bayer, 1956) , H. reginae (Hickson, 1905) , H. sulcatum , and H. variabile (Thomson and Henderson, 1906) are also large, measuring more than 1.5 mm in both diameter and height. Among these species, only three ( H. abyssale , H. regale , and H. sulcatum ) exhibit a pinkish or reddish cortex and axis.

Hemicorallium abyssale View in CoL has relatively large autozooids (2.0 mm in diameter and height), which are arranged biserially on the branch. Moreover, H. abyssale View in CoL predominantly contains double-club sclerites in the cortex. Hemicorallium regale View in CoL exhibits a pinnate branching pattern and is characterized by spherical sclerites in the cortex. Hemicorallium sulcatum View in CoL shares the most similar characteristics with the new species. Although H. sulcatum View in CoL features some double clubs, crosses, and irregularly shaped sclerites in the cortex, the holotype of this new species has almost no or extremely few of these sclerites. Recently, H. indicodensum Hu, Zhang, and Xu, 2025 and H. jiaolongensis Hu, Zhang, and Xu, 2025 were described from the west coast of the Indian Ocean ( Hu et al. 2025). These two species have particularly tall autozooids ( H. indicodensum : 2.5 mm; H. jiaolongensis : 3.3 mm), which distinguish them from other species of Hemicorallium View in CoL , and from the holotype of H. ryukyuense .

Many species belonging to the genus Hemicorallium View in CoL are known to host symbiotic organisms, such as polychaetes, within their coenenchyme or axis (e.g., Nonaka and Hayashibara 2021). The presence of symbionts can cause grooves in the axes of these species. The specific name of H. sulcatum View in CoL means “coral with grooves” in Latin, and it has also been given the Japanese name “Mizosango” (meaning “grooved coral”), because of the grooves on the surface of their axis. A unique characteristic of the holotype of H. ryukyuense is the absence of commensal burrows in the axis ( Figs 2 View Fig , 10 View Fig , 13 View Fig ). Interestingly, no grooves are observed in the axis of the specimen in this study, and no symbiotic organisms were observed. While the relationship between symbionts and the host may be characteristic of the species, it is likely also influenced by the habitat environment, so this information is included here for reference.

Nonaka and Muzik (2016) examined a specimen stored at the Smithsonian Institution, collected from 800 m deep near Tsuken Island, Okinawa Islands , and identified it as H . cf. sulcatum. This specimen ( USNM 76105 About USNM ) was collected from a similar area and depth to the holotype of H . ryukyuense and appears morphologically similar. However, the autozooid mounds of this specimen are smaller ( 1.16 mm in diameter and 1.04 mm in height), and the composition and size of the sclerites differ, making it clear that it is not the same species.

Molecular analysis. All MIG-seq data obtained in this study were deposited in the International Nucleotide Sequence Databases through the DNA Data Bank of Japan (DDBJ) [Sequence Read Archive (SRA) accession numbers DRX639955–DRX639976]. A total of 95995598 raw reads [mean ± standard error (SE) = 2181718 ± 54249] were generated. After filtering out adapter sequences and low-quality reads, we retained 81452308 reads per individual (mean ± SE = 2181718 ± 54249). In total, 5252 SNPs ( R = 0.3) and 1209 SNPs ( R = 0.5) were detected from the 22 individuals.

The ML phylogenetic tree recovered well-supported topologies that were consistent with the clades ( Corallium , Hemicorallium , and Pleurocorallium ) hypothesized by Tu et al. (2015), confirming that H. ryukyuense belongs to the Hemicorallium clade ( Fig. 18B View Fig ). However, within the genus Hemicorallium , the interspecific tree topology was inconsistent across datasets with varying rates of missing data, and clear genetic differences between morphologically distinct species could not be identified ( Fig. 18 View Fig ).

Consistent with the phylogenetic tree results, the genetic distance analysis revealed that the genus Pleurocorallium exhibited larger distances compared to the other two genera, while smaller values were observed between Corallium and Pleurocorallium ( Fig. 19 View Fig ). When calculating intra-generic and intraspecific genetic distances, the intra-generic distances within Hemicorallium were significantly smaller than those within Pleurocorallium [Tukey’s honestly significant difference (HSD) test, p <0.05]. Furthermore, the genetic distance within the genus Hemicorallium was greater than the intraspecific variation within C. japonicum (Tukey’s HSD test, p > 0.05), but significantly smaller than the intraspecific genetic distances of the two Pleurocorallium species (Tukey’s HSD test, p <0.05).