Introduction
Deep-sea environments encompass a wide range of ecosystems, including continental margins, bathymetric highs, abyssal plains, and oceanic trenches. Despite the challenges posed by low temperatures, extreme hydrostatic pressure, absence of sunlight, and limited food availability, the deep sea supports a remarkably rich and diverse biota across these unique habitats. Nevertheless, these ecosystems remain among the least explored on Earth, collectively covering more than 90% of the ocean floor. The deep Indian Ocean has long represented a frontier in marine science, and its biodiversity gradually revealed through a succession of exploratory cruises, targeted programmes, and evolving technologies. Earlier key surveys include the HMS Challenger Expedition (1872–1876), the German Valdivia expedition (1898–1899), the Anglo-Egyptian John Murray Expedition (1930s), the Albatross Expedition (1947–1948), and the Danish Galathea expedition (1950–1952), and later Soviet investigations provided systematic collections of deep-sea fauna and sediments, laying the taxonomic and ecological foundations for the region. Although multinational oceanographic programmes such as the International Indian Ocean Expedition (1960–1965) greatly advanced our understanding of the basin-scale physical, chemical, and biological characteristics of the Indian Ocean, the information on benthos remained comparatively limited (Thomas et al., Reference Thomas, Bond, Kolbusz, Niyazi, Swanborn and Jamieson2024).
Specific deep-sea biodiversity research programmes of India began in the 1980s with surveys of the Central Indian Ocean Basin, initially linked to polymetallic nodule exploration (Ingole and Koslow, Reference Ingole and Koslow2005). These studies continued for environmental impact assessments for prospective deep-sea mining, and geological and geochemical studies of the mid-oceanic ridges concurrently facilitated the collection of deep-sea fauna. Collectively, these efforts have produced substantial datasets on the composition, distribution and diversity of benthic communities, leading to numerous species discoveries across the Indian Ocean (e.g. Wafar et al., Reference Wafar, Venkataraman, Ingole, Khan and LokaBharathi2011; Periasamy and Ingole, Reference Periasamy and Ingole2022; Periasamy et al., Reference Periasamy, Kurian and Ingole2023; Gaikwad et al., Reference Gaikwad, Sautya, Damare, Mascarenhas-Pereira, Gawas, Patil, Nanajkar and Bukhari2024). Parallel initiatives such as the Census of Marine Life have enabled international collaboration, standardised sampling protocols, and species inventories (Alexander et al., Reference Alexander, Miloslavich and Yarincik2011). Despite these advances, knowledge remains uneven. Early studies and later research motivated by commercial or managerial interests have left many bathymetric regions under sampled and, leading to the paucity of biodiversity data (O’Hara et al., Reference O’Hara, Ahyong, Alderslade, Bray, Criscione, Davey, Farrelly, Gomon, Hara, Horowitz, Hosie, Kirkendale, Kupriyanova, MacIntosh, Mackenzie, Mah, Miskelly, Pogonoski, Richards, Rowe, Schlacher and Williams2026).
Among deep-sea habitats, seamount ecosystems are characterised by complex topographic and oceanographic features, making them some of the most biodiverse environments in the deep ocean. As part of the Deep Ocean Mission initiated by the Government of India, exploratory research was conducted on seamounts in the southeastern Arabian Sea to document benthic biodiversity and improve the understanding of deep-sea ecosystems. During this survey, a fauveliopsid polychaete was collected from sediment samples obtained from the summit of a seamount. Detailed morphological examination revealed that the specimen represents a new species, which is described herein.
Fauveliopsidae Hartman, Reference Hartman1971 is a rarely abundant family of sedentary polychaetes, including about 33 species. Fauveliopsids are small benthic polychaetes characterised by blunt-ended bodies that may be cylindrical, club-shaped, or medially expanded. Their integument ranges from smooth to rugose, and interramal papillae are typically present between parapodial rami and are primarily detritivores (Salazar-Vallejo et al., Reference Salazar-Vallejo, Zhadan and Rizzo2019). Most of the fauveliopsids inhabit deep-sea sediments, though some species occur in shallower waters (<100 m) (Rouse et al., Reference Rouse, Pleijel, Tilic, Rouse, Pleijel and Tilic2022; Plathong et al., Reference Plathong, Plathong, Klangnurak and Salazar-Vallejo2026). The family includes three genera: Fauveliopsis McIntosh, Reference McIntosh1922 (17 species), Laubieriopsis Petersen, Reference Petersen2000 (11 species), and Riseriopsis Salazar-Vallejo, Zhadan & Rizzo, Reference Salazar-Vallejo, Zhadan and Rizzo2019 (5 species) (Petersen, Reference Petersen2000; Salazar-Vallejo et al., Reference Salazar-Vallejo, Zhadan and Rizzo2019; Plathong et al., Reference Plathong, Plathong, Klangnurak and Salazar-Vallejo2026). The members of this family have different life strategies. Genera Fauveliopsis usually live inside empty gastropod or scaphopod shells or foraminiferan tests. Laubieriopsis and Riseriopsis include species commonly regarded as free-living, although some species construct tubes composed of cemented sand grains. These genera are classified primarily by their body shape, body surface colour, and ecological traits. Fauveliopsis has a club-shaped body with a rugose body surface and inhabits molluscan shells or foraminiferan tests (López, Reference López2011); Laubieriopsis has a long, cylindrical body with a transparent, smooth body surface and free living (Magalhães et al., Reference Magalhães, Bailey-Brock and Rizzo2014); Riseriopsis has a body that is swollen at both ends with a transparent, smooth body surface and inhabits soft tubes made of sediment particles (Salazar-Vallejo et al., Reference Salazar-Vallejo, Zhadan and Rizzo2019).
From the Indian Ocean, only two species of Fauveliopsidae have previously been reported: Fauveliopsis challengeriae McIntosh, Reference McIntosh1922, from the southern Indian Ocean (midway between Australia and Antarctica), and Riseriopsis arabica (Hartman, Reference Hartman1976) from the Arabian Sea. Increased sampling efforts in this region may likely reveal additional species. The present specimen was collected from the summit of a seamount in the southeastern Arabian Sea at a depth of 760 m. This specimen is described as a new species, and a key to the species of the genus is provided. This study also represents the first record of this genus from the Indian Ocean.
Material and methods
The Laccadive Plateau (eastern Arabian Sea) constitutes the northernmost section of the Chagos-Laccadive Ridge in the northern Indian Ocean, and the Lakshadweep atolls (India) form the prominent above-water features of this ridge section. Bijesh et al. (Reference Bijesh, John Kurian, Yatheesh, Tyagi and Twinkle2018) described several submarine bathymetric highs (seamounts, guyots, hills, knolls, etc.) in the Laccadive Plateau and surrounding areas, one of these designated as seamount S9 (73.199°E, 9.732°N) (Figure 1). Specimens of Laubieriopsis were collected on 30th November 2024 during the research cruise of R/V Sindhu Sankalp, using a van Veen grab with a bite area of 0.8 m2 at a depth of 760 m from the summit of seamount S9 (9.729886°N, 73.20586°E). Sediments obtained from the grab operation were sieved on board through a 300-μm mesh sieve. Larger and easily visible organisms were handpicked and preserved in 95% ethanol for molecular analysis, then the residual sediments plus smaller organisms were fixed in 10% buffered formaldehyde solution. In the shore laboratory, detailed analysis of formalin-preserved residual samples revealed the occurrence of relatively smaller Laubieriopsis worms under Olympus SZX2-ILLD stereo microscope. Molecular analyses were not performed because the specimens were recovered from formalin-preserved samples. For scanning electron microscopic images, three preserved Laubieriopsis specimens were air-dried naturally at room temperature (∼ 28oC) before being coated with gold and observed using JEOL JSM-IT210. Material examined during this study was deposited in the reference collection of the Centre for Marine Living Resources and Ecology (CMLRE), Kochi, Kerala, India, which is the regional node of the Ocean Biodiversity Information System for the Indian Ocean. Complete information about these specimens is shown below in the Systematic Account section. The number of specimens in a sample is given in parentheses after the museum abbreviation and registration number.
Map of the eastern Arabian Sea showing the location of seamount S9 on the Laccadive Plateau, where the samples were collected.
Results
Systematics
Family Fauveliopsidae Hartman, Reference Hartman1971
Genus Laubieriopsis Petersen, Reference Petersen2000
Type species. Fauveliopsis cabiochi Amoureux, 1982, by original designation.
Fauveliopsids with cylindrical body, not swollen posteriorly. Anterior segments short, intersegmental grooves shallow or not defined; median and posterior segments long (as long as wide). Integument often transparent, smooth, without large papillae. Interramal papillae small, usually with a short stalk or sessile. Chaetae include uni or bidentate aciculars, straight or sigmoid spinulose, and smooth capillaries. Posterior end with pygidium retractile, often with small papillae and large, falcate aciculars. Genital papillae paired or unpaired in anterior chaetigers. Free-living, rarely in flexible, fibrous tubes, with or without foreign particles. (after Salazar-Vallejo et al., Reference Salazar-Vallejo, Zhadan and Rizzo2019). The species of Laubieriopsis can be separated with the key below.
Key to species of Laubieriopsis Petersen, Reference Petersen2000 (modif. Plathong et al., Reference Plathong, Plathong, Klangnurak and Salazar-Vallejo2026)
1a. Body with 15–18 chaetigers …………………………………………. 2
1b. Body with 20 or more chaetigers ………………………………………….6
2a. Body with 15–16 chaetigers ……………………………………………. 3
2b. Body with 18 chaetigers; anterior chaetigers with four slightly sigmoidal, bidentate acicular chaetae per ramus; genital papilla unpaired, on left side near posterior margin of chaetiger 8 …………………………………………………….. L. sankalpae sp. nov
3a. Body with 15 chaetigers ………………………………………………. 4
3b. Body with 16 chaetigers ………………………………………………. 5
4a. Genital papilla unpaired, on right side of chaetigers 6 and 7; anterior chaetigers with bidentate acicular chaetae ……………. L. petersenae Magalhães, Bailey-Brock & Rizzo, Reference Magalhães, Bailey-Brock and Rizzo2014
4b. Genital papillae paired, on chaetigers 6 and 7; anterior chaetigers with tapered acicular chaetae …………………………………. L. blakei Salazar-Vallejo, Zhadan & Rizzo, Reference Salazar-Vallejo, Zhadan and Rizzo2019
5a. Anterior chaetigers with two bidentate acicular chaetae and two capillaries per ramus; genital papillae paired, globose to verrucose, on chaetiger 6…………………………………………………………………………… L. brevis (Hartman, Reference Hartman1965)
5b. Anterior chaetigers with two tapered acicular chaetae and two capillaries per ramus; genital papillae short, globular, granulose, barely projected from body wall ……………………………………………..L. hartmanae (Levenstein, Reference Levenstein1970)
6a. Body with 20-21 chaetigers ……………………………………………. 7
6b. Body with ≥ 22 chaetigers………………………………………………8
7a. Body with 20–21 chaetigers; anterior chaetigers with 3–4 acicular chaetae per ramus, mostly bidentate, spinulose; genital papilla unpaired, on right side of chaetiger 8…………………………………………………………………… L. cabiochi (Amoureux, Reference Amoureux1982)
7b. Body with 21 chaetigers, Anterior chaetigers without bidentate aciculars……………… ………………… L. verrucosa Jimi, Hookabe, Fujimoto, Kise, Ogawa & Tsuchiya, Reference Jimi, Hookabe, Fujimoto, Kise, Ogawa and Tsuchiya2023
8a. Anterior chaetigers with acicula tapered, tips entire ……………………………. 9
8b. Anterior aciculars bidentate; Body with 22 chaetigers; genital papillae paired on chaetiger 8 ……………………………………. L. norvegica Zhadan & Atroshchenko, Reference Zhadan and Atroshchenko2012
9a. Anterior and posterior segments entire, not annulated ………………………… 10
9b. Anterior segments annulated …………………………………………… 11
10a. Anterior segments smooth, without annulations; body with 24 chaetigers; genital papilla unpaired on chaetiger 8……………………………. L. soyoae Jimi in Jimi et al., Reference Jimi, Fujimoto, Ogawa, Fujita, Shigenobu and Imura2020
10b. Body with 47 chaetigers……………………. L. fauchaldi Katzmann & Laubier, Reference Katzmann and Laubier1974
11a. Body with 23–43 chaetigers, anterior segments biannulate ………………. L. tantawan Plathong, Plathong, Klangnurak & Salazar-Vallejo, Reference Plathong, Plathong, Klangnurak and Salazar-Vallejo2026.
11b. Anterior segments multi-annulated, body with 25 chaetigers…….. L. arenicola Riser, Reference Riser1987
Laubieriopsis sankalpae sp. nov.
http://zoobank.org/urn:lsid:zoobank.org:act:D09F426E-E1D1-4108-BF93-2F439B7469BD
Diagnosis
Laubieriopsis with 18 chaetigers; chaetigers 1–3 with annulations, and 4 aciculars per rami; aciculars slightly sigmoidal, bidentate; 4th transitional chaetiger with one blunt acicular and one capillary, continuing posteriorly; genital papillae hemispherical, near posterior margin of chaetiger 8.
Type material. Holotype (CMLRE IO/OV/POL/00897): 5.34 mm long, 0.21 mm wide; 81 paratypes (CMLRE IO/OV/POL/00898): 3.10–4.79 mm long, 0.13–0.22 mm wide, 18 chaetigers, free living, collected from the summit of a seamount in the southeastern Arabian Sea by van Veen grab (Figure 1), at a depth of 760 m, three paratypes used for SEM observation.
Description
Holotype and paratypes complete, 3.10–5.34 mm long, 0.13–0.22 mm wide, 18 chaetigers. Body slender, cylindrical, body ends blunt (Figures 2a, 3a). First three chaetigers with annulations (Figure 2c), following segments smooth, non-annulated. Cuticle thick (about 10 μm), smooth, with triangular micropapillae visible under higher magnification (100×). Prostomium small, round, retractable. Peristomium forms a complete ring. Parapodia biramous, best developed along chaetigers 1–3. Interramal papillae short, globular, sessile, situated midway between noto- and neuropodia. Epidermal glands visible in each parapodium.
Laubieriopsis sankalpae sp. nov. (a, c, d) holotype (CMLRE IO/OV/POL/00897). B syntype. (a) Complete specimen, dorsal view. (b) Rose Bengal-stained specimen with male gonads, dorsal view. (c) Anterior end, ventral view. (d) Posterior end, dorsal view. Scale bars: a, b: 500 µm; c, d: 200 µm.
Laubieriopsis sankalpae sp. nov. syntype. (a) Dorsal view of male with gonads. (b) Dorsal view of anterior segments. (c) Cross-section of chaetiger 2. (d) Anterior bidentate acicular. (e) Posterior end dorsal view. (f) Cross-section of chaetiger 4. (g) Posterior acicular. (h) Posterior capillary chaetae. Scale bars: (a) 500 μm, (b) 100 μm, (c) 20 μm, (d) 5 μm, (e) 200 μm, (f) 40, (g, h) 10 μm.
Chaetigers 1–3 with thick, slightly sigmoidal, bidentate aciculars, with a rough surface along the convex side (Figures 3c, d, 5a, c, d). Each notopodium and neuropodium has 4 aciculars of similar width (2 long and 2 short) (Figures 3b, 4b).
Laubieriopsis sankalpae sp. nov. Paratype (CMLRE IO/OV/POL/00898). (a) Cross-section of chaetiger 2. (b) Anterior bidentate aciculars. (c) Cross-section of chaetiger 4. (d) Conical integumental micropapillae. (e) Interramal papilla. (f) Genital papillae. (g) Pygidium. Scale bars: (a, c, e–g) 20 µm, (b, d) 5 µm.
Chaetiger 4 transitional, with one blunt acicular and one capillary (Figure 4c).
Chaetigers 5–18 with one fine slender capillary and one blunt to knife-like acicular per ramus.
Genital papillae hemispherical, unpaired, situated on the left side at the posterior area of chaetiger 8 (Figure 5h, i).
Laubieriopsis sankalpae sp. nov. Paratype. (a) Anterior region, lateral view. (b) Another specimen, posterior end, frontal view. (c) Chaetiger 1. (d) Anterior slightly sigmoidal bidentate acicular. (e) Convex side of anterior acicular with rough texture. (f) Transitional chaetiger 4. (g) Posterior chaetiger. (h) Arrow mark indicates genital papilla anterior to chaetiger 9. (i) Close-up of genital papilla. Scale bars: (a) 100 µm, (b) 50 µm, (c, f, g) 10 µm, (d, i) 5 µm, (e) 1 µm, (h) 20 µm.
Pygidium with anus terminal, vertical slit separating it into two cushion-shaped lateral lobes, each with numerous digitate papillae. Anal cirri not seen (Figure 4g).
Testis visible in stained samples (Rose Bengal), extending through chaetigers 7–11, spermatids about 3 μm in diameter (Figure 2b).
Variation: 82 specimens examined. All individuals have 18 chaetigers, despite paratypes including larger specimens being two times as large as smaller ones; three anterior chaetigers each bearing four bidentate acicular chaetae per ramus. Prostomium variably everted, either fully extended or appearing as a blunt tip. Male gonads present in some specimens, extending from chaetigers 7–11; ovaries or oocytes not observed. Genital papillae observed only in a few individuals, and only on one side; paired condition uncertain. Posterior chaetigers in some specimens extend beyond the pygidium; in others not extending, due to retractile pygidium.
Remarks. Laubieriopsis sankalpae sp. nov. differs from all known congeners in having 18 chaetigers, anterior chaetigers (1–3) each with four slightly sigmoidal, bidentate acicular chaetae per ramus, a distinct transitional chaetiger 4 bearing one blunt acicular and one capillary chaeta, and a single unpaired, hemispherical genital papilla (GP) located near the posterior margin of chaetiger 8.
The new species is most similar to L. cabiochi (Amoureux, Reference Amoureux1982) from the Northeastern Atlantic by having only 20–21 chaetigers; GP unpaired on the right side of chaetiger 8; strongly or weakly bidentate spinulose aciculars on the thorax. However, L. sankalpae differs in having fewer chaetigers, anterior segments restricted to 1–3, lacking spinulose aciculars and GP situated on the left side near the posterior margin of chaetiger 8.
Among congeners, anterior bidentate acicular chaetae are also present in L. brevis (Hartman, Reference Hartman1965), L. norvegica Zhadan & Atroshchenko, Reference Zhadan and Atroshchenko2012, and L. petersenae Magalhães, Bailey-Brock & Rizzo, Reference Magalhães, Bailey-Brock and Rizzo2014. However, in these species, the anterior acicular chaetae occur in the first four chaetigers, whereas in L. sankalpae n. sp., they are restricted to the first three chaetigers. The structure and arrangement of the acicular chaetae also differ. In L. norvegica, the chaetae are thick, sigmoidal and hirsute, with two chaetae in notopodia and four in neuropodia (two long and two short). In L. brevis, the first 4 anterior parapodia have two bidentate aciculars and two capillaries per ramus. In L. petersenae, each noto- and neuropodium has two thick and two thin spinulose acicular chaetae, the thinner ones slightly longer and bidentate. In contrast, L. sankalpae n. sp. possesses four bidentate acicular chaetae per ramus in the first three chaetigers; these chaetae have a rough texture on the convex side.
The geographical distribution of members of Laubieriopsis was compiled based on type localities reported in the original taxonomic descriptions, additional material records summarised by Salazar-Vallejo et al. (Reference Salazar-Vallejo, Zhadan and Rizzo2019), and subsequent distribution records provided by Laguionie-Marchais et al. (Reference Laguionie-Marchais, Paterson, Bett, Smith and Ruhl2016), Alalykina (Reference Alalykina2018), and Zamora et al. (Reference Zamora, Parapar, Helgason and Moreira2019) (Figure 6). The genus exhibits a broad geographic and bathymetric distribution, with most species reported from relatively deep waters, although a few species occur in shallower environments. Laubieriopsis arenicola has been recorded from New Zealand at 22 m depth, L. tantawan from the Gulf of Thailand at 50–70 m, and L. fauchaldi from the Mediterranean Sea at 135 m. In contrast, the remaining species have been reported from depths greater than 200 m, ranging from approximately 200 m (L. cabiochi) to 6835 m (L. hartmanae). The present species was collected at 760 m depth, which falls within the known bathymetric range of the genus. Among its congeners, L. cabiochi and L. brevis exhibit comparatively broad bathymetric distributions that encompass depths comparable to those of to the present species (Zamora et al., Reference Zamora, Parapar, Helgason and Moreira2019). However, these species are morphologically and biogeographically distinct.
Global distribution of the genus Laubieriopsis based on published records and the present study. The type locality of L. sankalpae sp. nov. is highlighted with a red star.
Figure 6 Long description
The global distribution of Laubieriopsis species, marked by colored dots representing different species. Each color corresponds to a specific species, as indicated in the legend. The species and their markers are: L. brevis (green), L. cabiochi (orange), L. fauchaldi (purple), L. hartmanae (red), L. arenicola (blue), L. tantawan (yellow), L. petersenae (pink), Laubieriopsis sp. nov. (red star). The map shows these species distributed across various oceanic regions, with clusters in the Atlantic, Pacific and Indian Oceans. The red star highlights the type locality of Laubieriopsis sp. nov. Depths are indicated on the right, ranging from 0 to 7600 meters, showing the bathymetric distribution of these species.
The present record is also notable ecologically and biogeographically. To date, species of Laubieriopsis have been reported primarily from continental margins, continental slope sediments, and abyssal plains, whereas L. sankalpae sp. nov. represents the first record of the genus from a seamount ecosystem. Furthermore, this species is the first documented occurrence of Laubieriopsis in the Indian Ocean. Its occurrence on a southeastern Arabian Sea seamount broadens the known range and habitats of the genus, and its absence elsewhere in the Indian Ocean likely reflects limited deep-sea sampling rather than a true biogeographic gap. The broad geographic separation among known Laubieriopsis records suggests the potential for dispersal across distant deep-sea habitats, but current evidence is insufficient to identify specific pathways. The larval biology and dispersal capacity of Laubieriopsis remain poorly understood. Broader geographic sampling combined with molecular data in future studies may help to assess connectivity and clarify evolutionary relationships within the genus.
Etymology:. The species is named after the vessel R/V Sindhu Sankalp, on which it was collected. The specific name is a noun in the genitive case.
Distribution: Only known from the type locality. Summit of a seamount in the southeastern Arabian Sea at a depth of 760 m.
Acknowledgements
The authors thank the Director of the CSIR–National Institute of Oceanography (CSIR–NIO) in Goa, India, and the Scientist-in-Charge at the CSIR–NIO Regional Centre in Kochi, India, for their invaluable support. The authors gratefully acknowledge the Ministry of Earth Sciences, Government of India, for financial assistance through the project GAP3638. The authors also extend their appreciation to RV Sindhu Sankalp cruise team (Cruise No. SSK 178) for their dedicated efforts during the survey. This is NIO’s contribution No. 7723
Author contributions
V.M.L.: conceptualisation, investigation, visualisation, formal analysis, writing– original draft preparation. A.J.K.U.: conceptualisation, supervision, investigation, visualisation, and manuscript editing. S.I.S.-V.: supervision and manuscript editing. A.V.K.X.: formal analysis. A.R.M.A.: formal analysis. A.A.: investigation and project administration.
Declaration of interest
The authors declare none.
Data availability statement
The authors confirm that the data supporting the findings of this study are available within the article itself.
Funding statement
This work was carried out with financial support from the Ministry of Earth Sciences (MoES), Government of India (MoES/PAMC/DOM/87/2023(E-14693) dt.02/07/2024, GAP3638).
