New records of three vent scale worms fill Central Indian Ridge gaps

Three species of vent-endemic scale worms have been documented at six hydrothermal vent fields across the northern Central Indian Ridge, filling geographic gaps in their previously known Indian Ocean distributions. The taxonomic study records 31 newly collected specimens of Cladopolynoe jiaolongae , Photinopolynoe kaireiensis and Themis longqiensis from Cheoeum, Maru, Onbada, Onnare, Onnuri and Saero. Morphological comparisons with type material and cytochrome c oxidase subunit I, or COI, DNA barcodes confirmed the identifications, while also revealing that the new northern populations have low genetic divergence from populations recorded thousands of kilometres away.

The three animals belong to Lepidonotopodini, a tribe within the scale-worm family Polynoidae whose members inhabit deep-sea chemosynthetic ecosystems globally. The family is species-rich and ecologically important, but its taxonomy is difficult because polynoids frequently lose fragile diagnostic structures, including elytra, dorsal cirri and body segments, during collection and recovery. The northern Central Indian Ridge contains multiple hydrothermal vent fields, yet its fauna has been sampled less extensively than the well-studied Onnuri field.

The new records therefore add distributional data as well as anatomical observations that can help distinguish these species in future surveys of undersampled deep-sea populations. All specimens were collected during cruises aboard the research vessel R/V Isabu in 2023 and 2024. A suction sampler and scoop mounted on the remotely operated vehicle ROPOS were used at depths ranging from 2,009 to 3,273 metres.

For each animal, a piece of an elytron or parapodium was removed and preserved in 99% ethanol for molecular analysis, while the complete body was retained for morphological examination in 10% neutral buffered formalin or 70% ethanol. The specimens were examined with stereomicroscopes and, in some cases, optical microscopes. The investigators documented head anatomy, elytra, branchiae, jaws, pharyngeal structures, body bristles and the sex-specific structures on the ventral side.

All material was deposited at the Korea Research Institute of Bioscience and Biotechnology in Daejeon, South Korea, with unique specimen vouchers. DNA analysis produced 10 new COI barcodes for C. jiaolongae , 18 for P. kaireiensis and three for T. longqiensis . The sequences were aligned with previously deposited specimens, and p-distances were calculated to measure variation within species, between populations and between species.

Intraspecific COI variation across the newly sampled and previously available populations remained between 0.00% and 1.55%, substantially below the 7.77–24.47% divergence reported among different species in the three genera. The barcode gap supported discrimination of the species, while the low values between regional populations indicated no significant genetic barrier in the sampled material. For C. jiaolongae , divergence among northern Central Indian Ridge, Southwest Indian Ridge and Carlsberg Ridge populations was 0.20–0.31%.

Corresponding interpopulation divergence was 0.37–1.09% for P. kaireiensis and 0.61% for T. longqiensis . Each species extended its documented range in a different way. C. jiaolongae was previously known from the Southwest Indian Ridge and the Carlsberg Ridge, regions separated by approximately 6,500 kilometres along the ridge system.

Ten new specimens came from Onnuri at 2,009 metres and Saero at 3,273 metres, providing the first northern Central Indian Ridge record and an intervening occurrence. P. kaireiensis had been reported from the southern Central Indian Ridge and the Carlsberg Ridge, about 4,000 kilometres apart. The new material came from Maru, Cheoeum, Onnuri, Saero and Onbada, filling the gap between those regions.

T. longqiensis had been known only from the Southwest Indian Ridge; three specimens collected at Onnare and Onbada extended its range northward. Because only three new individuals of this species were analyzed, the authors cautioned that its genetic diversity and population structure remain insufficiently characterized. The expanded material allowed the researchers to revise all three species diagnoses.

For C. jiaolongae , the northern specimens showed that arborescent branchiae may terminate on segment 16 rather than always continuing to segment 21 or the posterior end. The study also presented the first photographic sequence of all ten pairs of elytra. These dorsal scales are oval to subreniform, have smooth surfaces and are crossed by branching veins.

A complete series is useful because scales can detach during sampling, leaving incomplete specimens that may be difficult to identify using published descriptions. For P. kaireiensis , the observations revised the reported number of pharyngeal papillae. The original description recorded three dorsal and two ventral papillae from a dissected southern specimen, whereas the northern animals consistently showed five dorsal and four ventral papillae in naturally everted pharynges.

The researchers attributed the discrepancy to the difficulty of observing an inverted, dissected structure and amended the diagnosis to reflect the more clearly visible arrangement. This count was supported by their assessment that pharyngeal papillae are stable diagnostic characters within Polynoidae and rarely vary within a species. One northern specimen also carried three copepods between its parapodia beneath the elytra.

Similar associations have been reported for other polynoids, but the ecological relationship between the copepods and P. kaireiensis remains unknown. The T. longqiensis specimens were more complete than the truncated material used in its original description. The revised diagnosis records a body of 25 segments, 11 pairs of elytrophores, a round pygidium and long, slender anal cirri.

The species lacks eyes, lateral antennae and branchiae, while its jaws are smooth rather than toothed. Its stout notochaetae have serrated distal portions and pointed tips coated with polymetallic sulfide; some long neurochaetae also carried a thin layer of the material. The study documented pronounced sexual dimorphism for the first time with photographs: males have two pairs of elongated ventral papillae on segments 11 and 12, with spiraled tips on the second pair, whereas females have no ventral papillae.

The elytra remain unknown because the animals are small and their scales readily detach during sampling. These records broaden the known scale-worm fauna of the northern Central Indian Ridge and support the hypothesis that the region may act as a stepping stone for dispersal along Indian Ocean ridges. The three species nevertheless show unusually low COI variation across ridge systems, including separations of roughly 6,500 kilometres.

The pattern is consistent with recent range expansion or sustained gene flow, although the genetic data alone do not distinguish between those possibilities. Planktonic larval dispersal, intermediate vent fields along the continuous ridge and deep-sea currents were identified as possible mechanisms. More specimens from the Southwest Indian Ridge, the northern and southern Central Indian Ridge, the Carlsberg Ridge and intervening regions will be needed to test the extent of connectivity and to characterize the population structure of these sparsely sampled species.

Subject of Research: Morphology, DNA barcoding and distribution of three vent-endemic scale worms in the northern Central Indian Ridge

Article Title: New records of three vent-endemic scale worms ( Annelida , Polynoidae ) from the northern Central Indian Ridge fill regional distributional gaps

Article References: Lee, W.-K., & Kim, S.-J. (2026). New records of three vent-endemic scale worms (Annelida, Polynoidae) from the northern Central Indian Ridge fill regional distributional gaps. ZooKeys, 1287, 273-289. https://doi.org/10.3897/zookeys.1287.198002

Image Credits: AI Generated

DOI: 10.3897/zookeys.1287.198002

Keywords: deep-sea biology, hydrothermal vents, scale worms, Polynoidae, Central Indian Ridge, DNA barcoding, taxonomy, species distribution, Indian Ocean, population connectivity, records, three