Chrysogorgia dendritica Xu, Zhan & Xu, 2020

Xu, Yu, Zhan, Zifeng & Xu, Kuidong, 2020, Morphology and phylogenetic analysis of five deep-sea golden gorgonians (Cnidaria, Octocorallia, Chrysogorgiidae) in the Western Pacific Ocean, with the description of a new species, ZooKeys 989, pp. 1-37 : 1

Chrysogorgia dendritica Xu, Zhan & Xu, 2020 Figures 1 , 2 , 3 , 4 ; Table 1

Chrysogorgia dendritica Xu, Zhan & Xu, 2020: 6-8, figs 2, 3.

Type locality.

Kocebu Guyot in the Magellan Seamount chain, 1821 m depth.

Voucher specimens. MBM286353, station FX-Dive 71 (11°20.83'N, 139°15.87'E), a seamount (tentatively named as M2) near the Mariana Trench, depth 1375 m, 28 March 2016. MBM286442, station FX-Dive 211 (10°02.97'N, 140°10.48'E), a seamount (tentatively named as M5) located on the Caroline Ridge, depth 1475 m, 29 May 2019. MBM286443, station FX-Dive 211 (10°03.27'N, 140°10.70'E), a seamount (M5) located on the Caroline Ridge, depth 1387 m, 29 May 2019. MBM286444, station FX-Dive 227 (10°37.92'N, 140°05.62'E), a seamount (tentatively named as M8) located on the Caroline Ridge, depth 1702 m, 15 June 2019. GenBank accession number: MT269888.

Extended diagnosis.

Chrysogorgia "group A, Spiculosae" with 1/3L Branching sequence and a monopodial or a little zigzagging stem. Juvenile with a bottlebrush-like colony, while adult usually having a tree-shaped colony. Branches nearly perpendicular to stem, subdivided dichotomously. Polyps with a long neck and an expanded base. Rods and rare scales in tentacles, longitudinally arranged. Rods/spindles and elongate scales in polyp neck longitudinally arranged, coarse with many warts on surface. Scales and rare plates at the basal polyp body irregularly and alternately arranged, irregular and often amoeba-shaped. Scales in coenenchyme sparse and elongate, usually lobed with irregular edges.

Description.

For morphological measurements, see Table 1 .

Distribution.

Kocebu Guyot, 1821 m ( Xu et al. 2020); a seamount adjacent to the Mariana Trench and seamounts on the Caroline Ridge, 1375-1702 m depth.

Remarks.

The four specimens match the holotype of Chrysogorgia dendritica Xu, Zhan & Xu, 2020 in having a monopodial stem and the same sclerite form, for example, rods and rare scales in tentacles, rods/spindles and elongate scales in polyp neck, irregular scales at the basal polyp body, and elongate scales in coenenchyme. Moreover, their mtMutS gene sequences are identical (see the genetic analysis below). Thus, we identified the four specimens as C. dendritica . The sclerites in the four voucher specimens and the holotype showed some differences: (1) rod-like scales with an obvious medial contraction often present in MBM286353 and MBM286442, while rare in the holotype, MBM286443 and MBM286444 (Figures 1A , 2H vs. Figures 3D , 4D ); (2) large cured spindles are rarely present in the polyp neck of the holotype, MBM286443 and MBM286444, while absent in the other two specimens (Figures 3F , 4F vs. Figures 1F , 2I ); (3) scales at the basal polyp body of the holotype and MBM286444 are more irregular and amoeba-shaped than the other specimens (Figure 4E vs. Figures 1H , 2K , 3E ); and (4) scales in coenenchyme are more elongate in MBM286442 than the other specimens (Figure 2J vs. Figures 1G , 3G , 4G ). However, these differences are minor and not constant, and we thus treate as the conspecific variation.

The four specimens of C. dendritica showed a series of growth stages, from bottlebrush-like colony (juvenile) to tree-shaped colony (adult). Considering the diameter size of the stem base and scars on the stem, the specimen MBM286442 is likely a juvenile with a narrow stem and without scars, while the other specimens have wider stems and some old scars of the past branches (Table 1 ). The juvenile has a bottlebrush-like colony (Figure 2B ), while the adult has a long monopodial stem with branches occurring on the top, forming a tree-shaped colony (Figures 1B , 3B ). Compared to the changing colony shapes, the sclerite forms showed small variation in the growth stages and can be used as a main character to identify the species.

Figures

Figure 1. External morphology, polyps and sclerites of Chrysogorgia dendritica MBM 286353 A, B the tree-shaped colony in situ and after collection (likely an adult), with a broken stem and a branching part C a single polyp under a light microscope D long polyp neck under SEM E sclerites in tentacles F sclerites of the polyp neck G sclerites in coenenchyme H sclerites at the basal polyp body. Scale bars: 10 cm (B); 1 mm (C); 200 μm (D); 50 μm (E), 100 μm (F, G, H).

Figure 2. External morphology, polyps and sclerites of Chrysogorgia dendritica MBM 286442 A, B the bottlebrush-like colony in situ and after collection (likely a juvenile) C a single polyp under a light microscope D a single polyp under SEM E tentacular part under SEM F coenenchyme under SEM G sclerites in tentacles H sclerites extending to pinnules I sclerites of the polyp neck J sclerites in coenenchyme K sclerites at the basal polyp body. Scale bars: 10 cm (B); 1 mm (C, D); 300 μm (E, F); 200 μm (G, I-K at the same scale), 100 μm (H).

Figure 3. External morphology, polyps and sclerites of Chrysogorgia dendritica MBM 286443 A, B the tree-shaped colony in situ and after collection (likely an adult) C a single polyp under SEM D sclerites in tentacles E sclerites at the basal polyp body F sclerites of the polyp neck G sclerites in coenenchyme. Scale bars: 10 cm (B); 1 mm (C); 300 μm (D-G at the same scale).

Figure 4. External morphology, polyps and sclerites of Chrysogorgia dendritica MBM 286444 A, B the colony in situ and after collection (likely an intermediate state) C a single polyp under SEM D sclerites in tentacles E sclerites of the polyp neck F sclerites at the basal polyp body G sclerites in coenenchyme. Scale bars: 20 cm (B); 2 mm (C); 300 μm (D-G at the same scale).