The genus Chrysogorgia contains 72 species distributed in the world oceans, with water depths ranging from 10 m to 4492 m. Three branching forms have been recognized in the colonies of the genus: a single ascending spiral (clockwise or counterclockwise) producing a bottlebrush shape, a single fan (planar colony) and two fans emerging from a short main stem (biflabellate colony). Based on the shapes of rods or scales in the body wall and tentacles, a rough grouping has been built for the separation of Chrysogorgia species.
In a paper published in the journal PeerJ on 25 March 2020, Yu Xu of the Laboratory of Marine Organism Taxonomy and Phylogeny at the Institute of Oceanology of the Chinese Academy of Sciences, the Laboratory for Marine Biology and Biotechnology at the Pilot National Laboratory for Marine Science and Technology, the Center for Ocean Mega-Science, and the University of Chinese Academy of Sciences, Zifeng Zhan, also of the Laboratory of Marine Organism Taxonomy and Phylogeny at the Institute of
Oceanology of the Chinese Academy of Sciences, the Laboratory for
Marine Biology and Biotechnology at the Pilot National Laboratory for
Marine Science and Technology, and the Center for Ocean Mega-Science, and Kuidong Xu, again of the Laboratory of Marine Organism Taxonomy and Phylogeny at the Institute of
Oceanology of the Chinese Academy of Sciences, the Laboratory for
Marine Biology and Biotechnology at the Pilot National Laboratory for
Marine Science and Technology, the Center for Ocean Mega-Science, and
the University of Chinese Academy of Sciences, describe three new species of Chrysogorgia collected while studying the benthic diversity in the tropical Western Pacific Ocean.
Specimens were obtained by the Remotely Operated Vehicle FaXian (Discovery) from an unnamed seamount (temporarily named as M2) adjacent to the Mariana Trench and the Kocebu Guyot in the Magellan Seamounts in the tropical Western Pacific Ocean during the cruises of the Research Vessel KeXue (Science) in 2016 and 2018. These specimens were photographed in situ before sampled, photographed on board and then stored in 75% ethanol after collection. Some branches were detached and stored at -80° C for molecular analysis.
Sampling sites on a seamount (M2) adjacent to the Mariana Trench and the Kocebu Guyot in the Western Pacific Ocean. Yu Xu in Xu et al. (2020).
The general morphology and anatomy were examined by using a stereo dissecting microscope. The sclerites of the polyps and branches were isolated by digestion of the tissues in sodium hypochlorite, and then were washed with deionized water repeatedly. Polyps and sclerites were air-dried and mounted on carbon double adhesive tape and coated for the Scanning Electron Microscope (SEM) to investigate their structure.
The first new species described is named Chrysogorgia dendritica, from the Latin adjective 'dendriticus' (dendritic) in reference to the dendritic shape of the colony. The species is known from a single specimen, collected from the Kocebu Guyot at a depth of 1821 m below sealevel, where the colony was attached to a dead Sponge. The water temperature at the collection site was about 2.31° C and the salinity about 35.8 psu.
The external morphology and polyps of Chrysogorgia dendritica. (A) The holotype in situ; (B) The holotype immediately after collection; (C) A single polyp under light microscope; (D) Single polyp under scanning electron microscope. (E) Tentacles with rods under scanning electron microscope. Yu Xu & Shaoqing Wang in Xu et al. (2020).
The specimen is about 57 cm long with the holdfast not recovered. Colony treeshaped, composed of a 36 cm long, straight and unbranched stem and a 21 cm long branched part with branching sequence 1/3L. The whole stem monopodial from bottom to top with lateral branches producing on the top. Stem surface almost smooth with a strong golden metallic luster, about two mm in diameter at base. Branches subdivided dichotomously, up to seventh order, most broken after collection. Distance between adjacent branches 16- 22 mm, and orthostiche interval 50 55 mm. First branch internodes 15 -20 mm long, with the terminal branchlets up to 50 mm. Polyps with a long neck and an expanded base, about three mm long and two mm wide at bases, composed of one or two on the first internodes, one to five in middle internodes, and up to six in terminal branchlets. No polyps on main axis internodes. Golden eggs often occurred at the expanded bases.
Rods longitudinally arranged in the back of tentacles, occasionally branched, with many small warts on surface, measuring 77 -330 x 15 -34 μm. Sclerites rarely extend into the pinnules, which are otherwise sclerite-free. Spindles and rods longitudinally arranged in the long polyp neck, slender with many small warts on surface, usually slightly curved, measuring 193- 800 x 25- 56 μm. Scales and rare plates transversely and crosswise arranged at body base, flat and amoeba-shaped with irregular edges, measuring 69 -248 x 11- 79 μm. Scales of coenenchyme sparse, flat and lobed with irregular edges, measuring 68 -268 x 10- 70 μm.
Sclerites of Chrysogorgia dendritica. (A)-( G) Sclerites of the polyp neck. (H )-(M) Sclerites in the back of tentacles; (N)-( X) Sclerites at the body base; (Y )-(GG) Sclerites in coenenchyme. Scale bars: (A)-( G) and (H)-( M), (N)-( X) and (Y)-( GG) at the same scale, respectively. Yu Xu in Xu et al. (2020).
The second new species described is named Chrysogorgia fragilis, from the Latin adjective 'fragilis' (fragile), and refers to the fragile stem and branches of the species. This species has been found only on the Kocebu Guyot in the Magellan Seamount chain. Colonies attached to rocky substrate. The holotype was attached with an egg-shaped structure and the paratype with an individual of the Squat Lobster genus Galathea. The water temperature at the collection site was about 3.2°C and the salinity about 35.8 psu.
The external morphology and polyps of Chrysogorgia fragilis. (A) The holotype in situ. Laser dots spaced at 33 cm used for measuring dimensions; (B) The paratype in situ; (C) The holotype immediately after collection; (D) The paratype after fixation; (E), (F) A single polyp under light microscope; (G)-( I) Three polyps under scanning lectron microscope. Yu Xu & Shaoqing Wang in Xu et al. (2020).
The holotype specimen is about 55 cm in height excluding the holdfast. Colony tree-shaped, composed of a sympodial branching part on the top and a fragile, slender and unbranched stem about 35.5 cm long and 1.5 mm in diameter at base. Stem surface almost smooth with a few scars and aeruginous metallic luster, and sometimes covered with a layer of pink mucous membrane. Branching part produced a slightly zigzag pattern at the top portion with branching sequence 1/3L. Branches subdivided dichotomously, nearly perpendicular to the axis, up to fifth order, most broken after collection. Distance between adjacent branches and the first branch internodes both 15 -22 mm long, orthostiche interval 50 -65 mm, and the terminal branches up to 75 mm. Polyps with a long neck and an expanded body base, 2 4 mm long, 1 2 mm wide at base, with the neck up to two mm long and less than one mm wide. Up to two polyps on the first internodes, two to four in middle internodes, up to ten in terminal branchlets. No polyp on main axis internodes. Golden eggs present in expanded body bases. Polyps pink immediately after collection, colour gradually faded in alcohol.
Rods longitudinally arranged in the back of the tentacles, rarely branched, with many small warts on surface, measuring 105- 442 x 14- 50 μm. Rare sclerites extend into the pinnules, and pinnules free of sclerites. Spindles and rods longitudinally arranged in the polyp neck, slender with many small warts on surface, sometimes with one or two sharp ends, measuring 170- 600 x 17- 62 μm. Scales longitudinally and transversally arranged at base of expanded polyp body, elongated with a few warts and irregular edges, sometimes branched, thicker and wider than those in coenenchyme, measuring 144- 551 x 34- 106 μm. Scales of coenenchyme flat and elongate, rarely with distinctly irregular edges, measuring 122- 435 x 28- 83 μm.
Sclerites of Chrysogorgia fragilis. (A)-( G) Sclerites of the polyp neck; (H )-(L) Sclerites in the back of tentacles; (M)-( V) Sclerites at the expanded polyp body base; (W)-( EE) Sclerites in coenenchyme. Scale bars are all at the same scale. Yu Xu in Xu et al. (2020).
The paratype specimen is 65 cm in height with unbranched stem about 35 cm long and one mm across at base. Branching part relatively longer and more zigzagging.
The third new species described is named Chrysogorgia gracilis, from the Latin adjective gracilis (gracile), and refers to the gracile stem and branches of this species. The species is known from a single specimen from an unnamed seamount (temporarily named as M2) adjacent to the Mariana Trench with water depths of 298 m. Colony attached to a rocky substrate with a small holdfast.
The external morphology and polyps of the holotype of Chrysogorgia gracilis. (A) -(C) The holotype in situ (A) and after collection (B) and fixation (C); Laser dots spaced at 33 cm used for measuring dimensions; (D) A single polyp under light microscope; (E) A single polyp under scanning electron microscope; (F) Tentacles under scanning electron microscope; (G) Mesozooids at the base of branch under light microscope; (H) Four mesozooids under scanning electron microscope; (I) A single mesozooid under scanning electron microscope. Yu Xu & Shaoqing Wang in Xu et al. (2020).
The specimen was orange to reddish after collection, became yellow in alcohol, about 51.8 cm long. Stem and branches golden with slightly glaucous metallic luster. Colony tree-shaped. Unbranched stem curved, up to 40.5 cm in arc length and 1.0 -2.9 mm in diameter, emanating in regular 1/4L spiral on the top of a tall. Holdfast small and rounded, about 9.8 -12.5 mm in diameter. Distance between adjacent branches in stem 2.0 -4.5 mm long and orthostiche interval 11- 16 mm. The first branch internodes 3 7 mm. Branches subdivided 2 7 times and the angle between bifurcating branches particularly obtuse: 18 °-62° . Terminal branchlets slender, usually whip-like, up to 90 mm long.
Polyps translucent, 0.9 -1.5 mm long, 0.2 -0.4 mm wide, uniserial spaced 2 5 mm on the branches by one side, with angle random to the branches. Polyp body base golden, without sclerites. Tentacles up to 1.0 mm in length, became white in alcohol. Three to 20 polyps on terminal branchlets and up to ten polyps in branch internodes. Axial internodal polyps not observed in the stem, where dense mesozooids occurred along the internodes of the stem and branch bases. Mesozooids bud-like shaped, orange in situ and yellowish in alcohol, without sclerites, about 0.3 -0.5 mm wide and up to 0.4 mm high.
Rods and rod-like scales slender, sometimes one end flat and the other end cylindric, mostly aggregated in the joints between the tentacles and bodies, or longitudinally along the back of the tentacles, with dentate projections at one or both ends and coarse, granular warts on surface, measuring 90 -450 x 15- 20 μm. Coenenchyme in branches with a thin pellucid and calcareous layer in outside of the central axis, sometimes with regular scales oriented along branches or without scales on branches. Scales elongated with smooth surface and edges, occasionally with finely serrated edges, usually becoming narrow in middle, rare to absent in coenenchyme, measuring 50 -250 x 12- 38 μm. All sclerites colourless.
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