Knowledge of deep-water benthic fauna in the eastern Indian Ocean remains limited. Prior to 2020, no systematic deep-water benthic sampling had been conducted in waters around Christmas Island and the Cocos (Keeling) Islands, Western Australia (Murray et al. 2025; Kupriyanova et al. 2026). Two biodiversity surveys were recently carried out in 2021 and 2022 in the waters off Australia’s Indian Ocean Territories (IOT) (O’Hara 2024). Kupriyanova et al. (2024) provided a catalogue of the annelid morphospecies collected during these surveys. Using three collection methods (beam trawl, Gandalf net, and Sherman sled), approximately 867 specimens representing 34 families were obtained. However, several annelid families that are typically abundant and diverse in benthic samples were absent or only poorly represented, including cirratulids, dorvilleids, and spionids, likely because of their small body size (Kupriyanova et al. 2024). A similar pattern was reported by Chuar et al. (2021) in a study of deep-water annelids, collected from depths of up to 1796 m off southwest Java in the eastern Indian Ocean using beam trawls and dredges.

Polynoid scale worms were among the better represented taxa in the IOT collections. Twelve morphospecies were recognised, including a Bathyeliasona morphospecies likely allied with Bathyeliasona nigra (Hartman, 1967) from depths of 3780–4990 m (Kupriyanova et al. 2024). In the most recent study treating annelid material from the IOT, three new species of Polynoidae, a potential new polynoid genus, and one known species of Anotochaetonoe Britayev & Martin, 2006 were reported and described (Kupriyanova et al. 2026).

The genus Bathyeliasona Pettibone, 1976 currently comprises four valid species (Read and Fauchald 2026), mostly with wide geographical distributions and broad bathymetric ranges. The four species were originally described from widely separated deep-sea regions: Bathyeliasona abyssicola (Fauvel, 1913) from the Gulf of Gascony (Bay of Biscay) in the northeastern Atlantic at 4380 m; B. kirkegaardi (Uschakov, 1971) from the Aleutian Trench in the North Pacific at 7180 m; B. mariaae Bonifácio & Menot, 2018 from the Clarion-Clipperton Zone in the eastern Pacific at 4328–4823 m; and B. nigra (Hartman, 1967) from the South Sandwich Islands in the South Atlantic at 2553–2575 m. Subsequent records have expanded the known distributions of three species: B. abyssicola was later reported from the Aleutian Trench (Levenstein 1961), B. kirkegaardi from the Kermadec Trench, Sunda Trench, and the northeastern Atlantic near Portugal (Kirkegaard 1956; Hartmann-Schröder 1975; Pettibone 1976), and B. nigra from southwest of Sri Lanka in the Indian Ocean (Kirkegaard 1995) as well as from southern and eastern Australia (Murray et al. 2025). In contrast, B. mariaae is currently known only from the eastern Pacific nodule fields (Bonifácio and Menot 2018). Gonzalez et al. (2025) summarised the recorded depth ranges of all valid Bathyeliasona species and observed that B. abyssicola has the deepest record within the genus, at 7974 m, and is also the most frequently recorded polynoid species from depths of 3000 m or greater.

Following the ratification of the Agreement on the Conservation and Sustainable Use of Marine Biological Diversity of Areas beyond National Jurisdiction (BBNJ, or High Seas Agreement) in September 2025 (United Nations 2025), the National University of Singapore and OceanX jointly conducted a deep-sea biodiversity expedition in October 2025 to international waters between Christmas Island and the Cocos (Keeling) Islands (Fig. 1). A range of sampling equipment was deployed, including a remotely operated vehicle (ROV), free-fall benthic lander, multinet plankton sampler, large volume water transfer system, and box corer, enabling the collection of biological material across a broad size range from pelagic and benthic environments. Among the specimens collected across seven ROV dives was a single large polynoid scale worm from the abyssal plain, estimated to be ~ 10 cm long based on ROV camera observations (Fig. 2A). The animal was conspicuous on an otherwise sparsely populated seafloor and was collected with the ROV suction sampler (Suppl. material 1).

In this paper, we describe a new species of Bathyeliasona, increasing the number of valid species in the genus from four to five. We provide a morphological description together with molecular data (cytochrome c oxidase subunit I [COI], 16S rRNA, 18S rRNA, 28S rRNA), using phylogenetic analyses to support species delineation rather than to resolve broader relationships among genera. This is the first species of Bathyeliasona for which the elytra are described and figured. In previously described species, elytral morphology could not be documented because the elytra are readily shed and are often lost during collection (Pettibone 1976; Bonifácio and Menot 2018; Murray et al. 2025). This also highlights the value of ROV- or submersible-based sampling which often yield more intact specimens than using bottom trawling or dredging. The absence of key diagnostic features, particularly the elytra and chaetae in damaged specimens is problematic for polynoid taxonomy (Kupriyanova et al. 2026).

Bathyeliasona oceanxplorer sp. nov. is the first species of the genus described from the eastern Indian Ocean, adding to the sparse but growing knowledge of abyssal annelids in this region (Kupriyanova et al. 2026). As some Bathyeliasona species have wide recorded geographical distributions, as noted in the Introduction, the new species is unlikely to be restricted to this region. Increased sampling effort will therefore be important for establishing its true distribution, which is likely to be underestimated owing to the limited sampling effort at abyssal depths.

All described species of Bathyeliasona show a high degree of overall morphological resemblance, particularly in the shape of the parapodia and the general pattern of serrations on the noto- and neurochaetae. This close resemblance among species, combined with the limited and often damaged material available, means that few characters are currently reliable for distinguishing them. It is therefore useful to distinguish characters that appear more promising for species discrimination from those that remain uncertain because of specimen condition.

Several potentially informative characters cannot yet be evaluated across the genus because the relevant features are often damaged, lost, or not described in the available literature. Nuchal organs, reported in the present study, are not yet reliable as diagnostic characters. Even in the new species, the nuchal organ depressions were clearly visible only in fresh material (Fig. 2E) and were indiscernible after preservation (Fig. 3C). Elytral morphology is similarly limited; although the shape of the elytra and the presence of marginal papillae are described here for B. oceanxplorer sp. nov., the elytral morphology of the currently valid species remains unknown, because the elytra detach easily and were not retained with previous material (Pettibone 1976; Bonifácio and Menot 2018; Murray et al. 2025). The central rib on the subacicular neurochaetae, which gives them a bilimbate appearance (Fig. 5J–L), is a comparable case. Although Bonifácio and Menot (2018) reported this feature in some specimens of B. mariaae, neurochaetae are frequently lost or damaged in preserved collections of Bathyeliasona (Pettibone 1976). While we include the presence of central rib on neurochaetae in the diagnosis of the new species, it cannot yet be assessed with high confidence across the genus, as it remains unclear whether this feature is genuinely absent in other species or has simply gone unobserved because of specimen condition. We regard it as a potentially informative feature that requires examination across additional, well-preserved specimens of Bathyeliasona species before its diagnostic value can be established.

Recent studies of Bathyeliasona have used the relative stoutness of the notochaetae and neurochaetae as a character in the generic diagnosis (Bonifácio and Menot 2018; Murray et al. 2025). This character traces back to Pettibone (1976), who stated: “Notosetae stout, spinous, more slender than neurosetae”. Bonifácio and Menot (2018) appear to have interpreted ‘slender’ as meaning less stout, noting that B. mariaae differs from the other three species in having notochaetae that are stouter, although always narrower, than the neurochaetae. Murray et al. (2025), by contrast, appear to have interpreted the character in terms of the relative thickness of the notochaetae. Based on the scaled drawings and species descriptions, we interpret Pettibone’s use of ‘slender’ to refer to width rather than robustness, that is, the notochaetae are narrower than the distally broadened and flattened neurochaetae. Taking the present study into account, the notochaetae appear to be consistently stouter across species, whereas the neurochaetae are broader distally but more fragile. This shows that even a long-used character can be applied inconsistently.

A few characters appear more promising for distinguishing species. The buccal organ of polynoids is generally described as an axial, muscular, eversible pharynx with terminal papillae and two pairs of dorsoventral jaws (Hutchings 2000; Hourdez 2022; Rouse et al. 2022). In the dissected pharynx of the new species, we observed a sac-like appendage at the distal end (Fig. 3I, J) which, when everted, superficially resembles the buccal bulb of Aphrodita aculeata Linnaeus, 1758, as illustrated in the figure redrawn by Hourdez et al. (2022). The same figure of buccal organ was labelled as the mouth in Rouse (2000). As far as we are aware, a comparable structure has not previously been reported at the distal end of the pharynx in any described species of Bathyeliasona. We include the buccal bulb in the diagnosis as a character observed in this species, while recognising that its broader taxonomic significance across the genus remains to be confirmed. Since the pharynx was not necessarily dissected in previous descriptions, and comparative material was not re-examined here, its apparent absence in congeners cannot be assumed. We therefore document this character with clear figures to flag it for future workers to examine and either validate or dismiss its diagnostic value. The shape of the pygidial lobes appears to be another useful character for distinguishing the new species from its congeners (Fig. 3H; Table 2), consistent with the taxonomic value of pygidial morphology at the species level noted previously by Bonifácio and Menot (2018).

Taken together, these observations suggest that the main challenge for species-level taxonomy in Bathyeliasona is not the absence of useful characters, but the difficulty of evaluating them consistently in the available material. Many promising characters, including nuchal organs, elytra, and delicate neurochaetae, are easily lost, damaged, or obscured during conventional trawling and dredging. Even so, B. oceanxplorer sp. nov. is clearly distinguished from its congeners by a combination of assessable morphological characters. The near-complete specimen also allowed the elytral morphology to be described for the first time in Bathyeliasona. The intact condition of this new species therefore highlights the value of targeted ROV sampling for observing and recovering large, delicate deep-sea annelids. As the present description is based on a single specimen, intraspecific variation could not be assessed, and additional material will be valuable for confirming the consistency of these characters. Improved ROV collections of fresh material will therefore help resolve which characters are consistently diagnostic within Bathyeliasona and clarify the diversity and distribution of the genus in poorly sampled regions such as the eastern Indian Ocean.