Hemicorallium indicodensum Hu, Zhang & Xu, 2025

Hu, Xuying, Zhang, Qian, Ge, Meiling, Li, Xinlong, Wang, Zongling, Zhang, Xuelei & Xu, Qinzeng, 2025, Two new species of deep-sea Red Corals (Coralliidae, Genus Hemicorallium Gray, 1867) from the western Indian Ocean, Zoosystematics and Evolution 101 (1), pp. 317-339 : 317-339

Hemicorallium indicodensum Hu, Zhang & Xu sp. nov.

Material examined.

Holotype • part of a colony branch with a broken edge on the stem; collected from the Guangde Ridge in the western India Ocean on 13 June 2022; Located at 3°26'39.35"N, 64°6'3.06"E, depth 1697 m; currently preserved at - 20 ° C in the Key Laboratory of Marine Eco-Environmental Science and Technology, First Institute of Oceanography, China with voucher number FIO-IND 72 - JLBEN 22003 . Paratype • A colony from the base of the main stem to the branches, but missing the top branches. Collected from the Guangde Ridge in the western Indian Ocean on 13 June 2022; Located at 3°26'30.31"N, 64°6'6.40"E, depth 1697 m; currently preserved in alcohol at the China National Deep Sea Center with NDSC-IND 72 - JLBEN 22001 .

Description.

Colony. The specimen FIO-IND 72 - JLBEN 22003 is part of a colony, nearly 22 cm in height and the thickest stem is less than 1 cm in diameter (Fig. 4 a ). Two main branches exhibit dichotomous branching in one plane with an acute angle. The branchlets are abundant and mostly arranged in an irregular pinnate pattern on one plane. Some of them grow at an acute angle, resulting in closely-spaced and overlapping branchlets. A brittle star is entwined amongst the branches. Paratype NDSC-IND 72 - JLBEN 22001 retained only the white basal part and the thicker lower branches (Fig. 4 i ). The branches are light red, with the colour becoming paler towards the base. Small branches with autozooids are distributed across half of the colony’s plane.

Polys. The autozooids are cylindrical and non-retracted, distributed on one side of the colony and are mostly alternately arranged on the branchlets (Fig. 4 b ). At the tips of the branchlets, two symmetrical autozooids form a Y-shape. Autozooids are 2.483 ± 0.348 mm in height and 1.515 ± 0.174 mm in width. They are abundant with most branchlets hosting fifteen or more. Each autozooid features eight longitudinal grooves that extend from the tentacle tips about two-thirds down to the base (Fig. 4 d ). Siphonozooids form subtle protrusions on the cortex.

Axis. The axis is solid and rigid, with no presence of canals inside (Fig. 4 c and Suppl. materials 7 – 9). It is covered by yellowish mesoglea tissue between the axis and the cortex (coenenchyme). The surface of the axis is uneven (Fig. 4 f ), featuring irregular, sharp, thorn-like projections (Fig. 4 d ).

Colour. In underwater in situ videos, it appears light pink. The base of the colony is white. The fresh coenenchyme and polyps are pale red, turning pink when dried. Specimens preserved in alcohol retain a pale red colour. The axis is red and sclerites observed under a microscope appear pale red.

Sclerites. The predominant sclerite type in this species is the distinctive double clubs (65.720 ± 5.630 μm in length, Fig. 5 d, e ), constituting about 70 % of all radiate sclerites (21 / 30 from tentacles, 24 / 33 from coenenchyme). This double club shapes are quite special, typically derived from 8 - radiates. According to the model of radiates (Fig. 3 k ), the R 1, R 4, R 6, R 7 and R 8 of the 8 - radiates in this specimen are enlarged to form smoother and more globular rays, while the R 2, R 3 and R 5 remain smaller with multilobular ornaments. Due to the fusion of rays, some double clubs cannot identify their terminal rays (R 1, R 8), making it uncertain whether they are derived from 8 - radiates (maybe 6 -, 7 - radiates). A few radiate sclerites may be in the process of developing where the rays become more globular (Fig. 5 f ).

A few 6 -, 7 - or 8 - radiates (not double clubs) are found in the coenenchyme with 74.520 ± 5.213 μm (Fig. 5 g ) and they are less numerous compared to the double clubs. Rods are found only in the tentacles, with an average length of 113.211 ± 9.298 μm (Fig. 5 a, b ). Some thick rods are elongated multi-radiates with 108.878 ± 13.299 μm (Fig. 5 c ). The surface and both ends of the rods exhibit distinct irregular protrusions. Additionally, crosses (Fig. 5 h ) and asymmetric multi-radiates (Fig. 5 i ) are presented in the coenenchyme.

Variation.

Paratype NDSC-IND 72 - JLBEN 22001 showed more details about the lower half of the colony, especially the basal part. The base of main stem is thicker, with the thickest diameter reaching 2.229 cm (Fig. 4 i ). There are slightly fewer autozooids and branches than the terminal branches shown in holotype FIO-IND 72 - JLBEN 22003 .

Etymology.

The specific name indicodensum is proposed for this new species. The name combines Indico, derived from “ Indicus, ” the Latin term for the Indian Ocean, with densi- meaning “ dense ” in Latin. This nomenclature reflects both the origin of the species in the expansive Indian Ocean and its distinctive, densely branched appearance.

Comparison.

The specimens FIO-IND 72 - JLBEN 22003 and NDSC-IND 72 - JLBEN 22001 have all its polyps distributed on one side of the colony, featuring non-retracted autozooids and rod-shaped sclerites in the tentacles. Therefore, these specimens conform to the description of the genus Hemicorallium and is classified under this genus.

H. indicodensum sp. nov. FIO-IND 72 - JLBEN 22003 resembles Hemicorallium bayeri Simpson & Watling, 2011 ( Simpson and Watling 2011) and Hemicorallium niobe Bayer, 1964 ( Bayer 1964; Sampaio et al. 2009) in that they feature abundant branchlets and dense autozooids, with double clubs as the primary sclerite type. However, they can be distinguished from each other based on geographic location, colour and the morphology of their double clubs. H. bayeri and H. niobe are only recorded in the Pacific and the colour of both colony and axis is white to yellow, which is different from the new species. The double clubs of H. bayeri and H. niobe have enlarged rays with obvious blunt projections, which distinctly differ from the smoother and more globular rays of the double clubs in H. indicodensum sp. nov. According to Tu et al. (2016) and Nonaka and Hayashibara (2021), the smooth double clubs’ morphologies are also found in Hemicorallium ducale Bayer, 1955 ( Bayer 1955; Tu et al. 2016), Hemicorallium abyssale Bayer, 1965 ( Bayer 1956; Tu et al. 2012; Nonaka and Hayashibara 2021) and Hemicorallium maderense Johnson, 1899 ( Johnson 1899). However, H. indicodensum sp. nov. can be readily distinguished from these three species based on the predominant proportion of double clubs. Besides, H. abyssale has double clubs with surfaces that are not completely smooth and H. maderense ’ s double clubs have the most thoroughly enlarged rays. For more information on morphology comparisons, see Table 2 .

In summary, the remarkable morphological features that distinguish this new species from other members of the genus Hemicorallium include:

Abundant branchlets bearing numerous and dense autozooids (> 2 mm).

An uneven surface on the axis, characterised by dense, fine and slightly sharp projections.

A predominance of globular double clubs sclerites (about 70 %), which are mostly derived from 8 - radiates and some of their enlarged rays possess a smooth, globular surface.

Remarks.

H. indicodensum sp. nov. represents a new species of Coralliidae first recorded in the Indian Ocean. In addition to holotype FIO-IND 72 - JLBEN 22003 and paratype NDSC-IND 72 - JLBEN 22001 , another highly similar Hemicorallium coral was found during a dive in the same area (Fig. 2 c and Suppl. material 3). Due to the rarity of deep-sea samples, the coral was not collected for conservation purposes, but in situ videos suggest it belongs to the same species because its branchlets are extremely dense and numerous. This indicates that H. indicodensum sp. nov. is commonly distributed on the Guangde Ridge.

Figures

Figure 2. In-situ images of deep-sea corals samples in this study. These images are from in-situ video screenshots and the camera on the Jiaolong. a. Sample in situ image of FIO-IND 72 - JLBEN 22003; b. Sample in situ image of NDSC-IND 72 - JLBEN 22001; c. Coral in the same area that was highly similar to FIO-IND 72 - JLBEN 22003; d. Sample in situ image of FIO-IND 72 - JLBEN 22005; e, f. Coral in the same area that was highly similar to FIO-IND 72 - JLBEN 22005.

Figure 3. Morphology of sclerites involved in this study and the diagram of radiates. a. Rods with sharp protrusions; b. Rods with blunt protrusions; c. Crosses; d. 6 - radiates; e. 7 - radiates; f. 8 - radiates with the rays closely spaced; g. 8 - radiates with the rays loosely spaced; h. The multilobular ornaments face of the double clubs; i. The enlarged rays face of the double clubs; j. The side view of the double clubs (both showing multilobular ornaments and enlarged rays); k. A simple diagram of 8 - radiates to show the direction of the eight rays, generated using the visualisation modelling software Sharp 3 D (version 5.661). Scale bar: 100 μm (a – j).

Figure 4. Morphological observation of Hemicorallium indicodensum sp. nov. FIO-IND 72 - JLBEN 22003 collected in the western India Ocean. Photographs by camera (a); Microscopic observations (b – d); Scanning Electron Microscope (SEM) observations (e – h). a. The holotype of H. indicodensum sp. nov. FIO-IND 72 - JLBEN 22003; b. The front and back of a branch; c. The cross-section of the axis; d. The autozooid of H. indicodensum sp. nov.; e. The details of the tentacles at the end of an autozooid; f. The axis of H. indicodensum sp. nov.; g. The cortex of the autozooid and axis; h. The surface details of the axis; i. The paratype of H. indicodensum sp. nov. NDSC-IND 72 - JLBEN 22001. Scale bars: 10 cm (a, i); 2 mm (b); 1 mm (c, d); 200 µm (e); 250 µm (f); 300 µm (g); 20 µm (h).

Figure 5. Morphological scanning electron microscopy observations of sclerites in Hemicorallium indicodensum sp. nov. a, b. Rods sclerites from the tentacles; c. Elongated radiates sclerites from the tentacles; d. Double clubs sclerites, showing the side with small multilobular ornaments and distributed in all parts; e. Double clubs sclerites, showing the side with smooth and globular rays and distributed in all parts; f. Radiates sclerites with rays becoming more globular at the ends, from autozooids; g. 6 -, 7 - or 8 - radiates from the coenenchyme; h. Crosses sclerites from the coenenchyme; i. Irregular multi-radiates sclerites from the coenenchyme. Scale bar: 100 μm.