Systematics and biogeography of Chrysogorgiidae from the Indian Ocean: discovery of new species on deep-sea ridges

Deep beneath the surface of the Indian Ocean, along ridges that few research vessels have ever visited, scientists have discovered a collection of luminous deep-sea corals that rewrite what we know about life in one of the planet’s least explored marine realms. A new study published in Zoosystematics and Evolution describes three brand-new species of Chrysogorgiidae, a family of golden deep-sea octocorals, and reports two known species from Indian Ocean ridges for the very first time. The findings, drawn from a 2022 survey of the Carlsberg Ridge and Ninetyeast Ridge, suggest that these underwater mountain chains may harbor an overlooked diversity hotspot for deep-sea corals.

The expedition used the human-occupied submersible JiaoLong, launched from the research vessel ShenHai YiHao during cruise DY72, to collect 13 chrysogorgiid specimens from the two ridge systems. Two specimens came from the Carlsberg Ridge in the northern Indian Ocean, while the remaining eleven were gathered from the Ninetyeast Ridge, a vast north-south undersea mountain chain. Every colony was photographed in its natural position on the seafloor before being preserved in 75 percent ethanol, allowing researchers to document both the living appearance and the microscopic anatomy of each animal.

Chrysogorgiidae is a family of octocorals found worldwide in deep-sea environments, currently comprising more than 100 species in seven valid genera. Nearly all of its members are restricted to deep-sea habitats, and the family includes the genera Chrysogorgia, Iridogorgia, Metallogorgia, Radicipes, Parachrysogorgia, Pseudochrysogorgia, and Ramuligorgia. Yet despite this global distribution, most recorded species have come from the Atlantic and Pacific Oceans, with only a handful of reports from the Indian Ocean, particularly from its ridge regions. This gap in taxonomic and genetic data has long hindered understanding of how Indian Ocean populations connect to those in other oceans.

The first new species, Chrysogorgia pugnioides Ge, Hu and Xu, sp. nov., was collected from the Guangde Ridge at a depth of 2,674 meters. Its name derives from the Latin adjective pugnioides, meaning fist-shaped, a reference to the remarkable appearance of its polyps, which resemble clenched fists when their tentacles contract. The holotype colony is bottlebrush-shaped, about 58 centimeters long and 20 centimeters wide, with a brown stem bearing an aeruginous metallic luster. Under the microscope, the species reveals warty spindles in the tentacles, flat and irregular spindles and elongate scales in the polyp body wall, and slender, smooth scales with toothed to irregular edges in the coenenchyme, the tissue connecting the branches.

The second and third new species belong to the genus Iridogorgia, famous for its upward-spiraling axes with brilliant iridescence and unbranched branches arising from one side. Iridogorgia acutisclerita Ge, Zhang and Xu, sp. nov., collected from the Ninetyeast Ridge at depths between 1,897 and 1,977 meters, builds closely coiled helices up to 73 centimeters tall, with 12 helical turns each roughly 6 centimeters in height. Its name combines the Latin acutus, meaning sharp, with the Greek skleros, meaning hard, referring to the characteristic sclerites, the tiny calcified structural elements that all feature two sharp ends. The species resembles I. splendens in colony form but differs by possessing needles and pointed elongate scales in the polyp body wall.

Iridogorgia levisquama Ge, Zhang and Xu, sp. nov., also from the Ninetyeast Ridge at a depth of 2,073 meters, is an even more dramatic animal. Its holotype colony stands approximately 120 centimeters tall, with six loosely coiled helical turns each 18 centimeters in height, growing counterclockwise on a calcareous holdfast about 6.5 centimeters in diameter. The name derives from the Latin levis, meaning smooth, and squama, meaning scale, referring to the nearly smooth scales in the basal part of the polyp body wall. It is differentiated from its congener I. squarrosa by slender, smooth scales with sharp ends in the polyp body wall and needles with fine wartiness in the tentacles.

Beyond the three new species, the team reported two known species as new records for the Indian Ocean ridges. Parachrysogorgia chryseis, previously known from the Pacific Ocean including the Indonesian Ceram Sea and seamounts on the Caroline Ridge, was collected from the Carlsberg Ridge at 1,700 meters, where it grew as a bi-flabellate yellow colony on the white branch of a dead coral. Metallogorgia melanotrichos, first described from Ascension Island in the South Atlantic and recorded across the central Indo-Pacific, western and central Pacific, and Atlantic, was found at six sites on the Ninetyeast Ridge at depths between 1,976 and 2,073 meters. Notably, nearly all M. melanotrichos specimens hosted the same ophiuroid species, Ophiocreas oedipus, consistent with North Atlantic records and supporting a highly specific epibiotic association between the two species.

To confirm the new species, the researchers integrated morphological examination with molecular phylogenetic analysis using two genetic markers: the partial mitochondrial mutS homolog, known as mtMutS, and the nuclear 28S ribosomal DNA region. Tissues were digested to isolate the microscopic sclerites, which were then examined under a Regulus 8100 scanning electron microscope after gold-coating. DNA was extracted from polyps and amplified by polymerase chain reaction, with sequences aligned in MAFFT and trimmed to conserved blocks using Gblocks. Phylogenetic trees were constructed using both maximum likelihood and Bayesian inference algorithms, with the mtMutS dataset comprising 85 chrysogorgiid sequences and the 28S rDNA dataset comprising 58, each with keratoisidid outgroups.

The genetic results strongly support the validity of all three new species. In the 28S rDNA phylogeny, Chrysogorgia pugnioides formed a distinct, well-supported lineage as the sister group to a clade containing C. pinniformis, C. cylindrata, C. pendula, and C. acanthella. Iridogorgia acutisclerita formed a well-supported clade, while I. levisquama was recovered as a strongly supported independent lineage sister to I. squarrosa. Genetic distance analyses showed that the new species diverge from their congeners by measurable margins, with C. pugnioides differing from other Chrysogorgia species by 1.53 to 8.22 percent in 28S rDNA. Interestingly, the two new Iridogorgia species were genetically identical to some congeners in mtMutS, highlighting the value of combining both markers, since rate heterogeneity between mitochondrial and nuclear gene evolution in octocorals can produce different topologies.

Perhaps most significantly, the two new Iridogorgia species represent the first confirmed taxonomic records of this genus in the Indian Ocean. Iridogorgia previously comprised 14 valid species concentrated in three high-diversity regions: the tropical western Pacific with 10 species, the waters around Hawaii with five, and the Northwest Atlantic with five. Although recent remotely operated vehicle surveys documented Iridogorgia in the Perth Canyon of the southeastern Indian Ocean, those observations had never been formally described. The establishment of these two new species validates the occurrence of Iridogorgia in the northern Indian Ocean and significantly expands its known biogeographic range beyond the traditional diversity centers of the Pacific and Atlantic. Together with the new records of Parachrysogorgia chryseis and Metallogorgia melanotrichos, the study suggests the Indian Ocean Ridge system may represent an overlooked diversity hotspot for Chrysogorgiidae, providing a foundation for future biogeographic and evolutionary studies of deep-sea octocorals in this region.

Subject of Research: Systematics and biogeography of Chrysogorgiidae from the Indian Ocean: discovery of new species on deep-sea ridges

Article Title: Systematics and biogeography of Chrysogorgiidae from the Indian Ocean: discovery of new species on deep-sea ridges

Article References: Ge, M., Xu, N., Hu, X., Tian, X., Li, X., Xu, Y., Wang, Z., Zhang, X., & Xu, Q. (2026). Systematics and biogeography of Chrysogorgiidae from the Indian Ocean: discovery of new species on deep-sea ridges. Zoosystematics and Evolution, 102(2), 621-647. https://doi.org/10.3897/zse.102.182492

Image Credits: AI Generated

DOI: 10.3897/zse.102.182492

Keywords: Systematics, biogeography, Chrysogorgiidae, Indian, Ocean, discovery, species, deep-sea, ridges, scientific research