Introduction
Trochochaeta Levinsen 1884 (a replacement name for Disoma Örsted, Reference Örsted1843, a junior homonym to protozoan Disoma Ehrenberg, Reference Ehrenberg, Hemprich and Ehrenberg1831) is a small genus of polychaetes comprising 12 species described to date, most of which occur in the northern hemisphere (Read and Fauchald, Reference Read and Fauchald2025; Radashevsky et al. Reference Radashevsky, Peixoto and Peixoto2018: fig. 1). The species are usually rare, mostly occurring offshore. Adults are long and fragile, living in firm tubes deeply embedded into sediment. Posterior chaetigers bear strong spines used to anchor individual inside the tube. Therefore, Trochochaeta worms fragment easily upon collection, thus usually anterior fragments are only present in samples.
Map showing type localities (stars) and worldwide records (circles) of Trochochaeta species discussed in the present study: T. chakara sp. nov. – red star – type locality: Kerala, Alappuzha mud bank, India. Trochochaeta cirrifera – turquoise star – type locality: Gulf of Oman, Iran; turquoise circles – records from Maharashtra, India. Trochochaeta diverapoda – pink star – type locality: Samar Sea, Philippines; pink circles – records from Honk Kong, China. Trochochaeta kirkegaardi – yellow star – type locality: Gulf of Guinea, Ghana; yellow circles – records from West Africa. Trochochaeta orissae – green star – type locality: Odisha, India. Trochochaeta watsoni – blue star – type locality: off Nova Scotia, Canada; blue circles – records from Northwest Atlantic.
The phylogenetic relationships and systematic placement of Trochochaeta have long remained controversial (see Radashevsky et al. Reference Radashevsky, Peixoto and Peixoto2018). Cladistic analysis of morphological characters and the few available molecular data confirmed its close relationship with spionid polychaetes but led to two phylogenetic hypotheses. One of them proposed the origin of Trochochaeta within the family Spionidae (Blake and Arnofsky, Reference Blake and Arnofsky1999; Rouse et al. Reference Rouse, Pleijel and Tilic2022; Wang et al. Reference Wang, Xu, Qiu, Ji, Yu and Ke2022), while the other proposed Trochochaeta to be a sister taxon to Spionidae (Bogantes et al. Reference Bogantes, Meiβner, Waits, Kocot, Kenneth and Halanych2024). The former hypothesis was supported by Radashevsky et al. (Reference Radashevsky, Peixoto and Peixoto2018) and is followed in the present paper, in contrast to the more conservative view of treating Trochochaeta within a monogeneric family Trochochaetidae Pettibone, Reference Pettibone1963 (e.g., Blake and Maciolek, Reference Blake, Maciolek, Purschke, Böggemann and Westheide2020).
From the Indian Ocean, Fauvel (Reference Fauvel1932) described Disoma orissae Fauvel, Reference Fauvel1932 (=Trochochaeta orissae) from Orissa, on the east coast of India (off Puri, Odisha, Bay of Bengal), and Hartman (Reference Hartman1974) described Disoma cirrifera Hartman, Reference Hartman1974 (=Trochochaeta cirrifera) from the Gulf of Oman (Iran) (Figure 1). Both Fauvel’s (Reference Fauvel1932) and Hartman’s (Reference Hartman1974) descriptions were based on single anterior fragments and, therefore, were brief and incomplete. Reviewing Trochochaeta described to that time, Pettibone (Reference Pettibone1976) re-examined type specimens of three species and provided an identification key to six species, but overlooked T. cirrifera and noted that specimens of T. orissae were not available to study.
Kirkegaard (Reference Kirkegaard1959) for the first time reported D. orissae from the East Atlantic Ocean (West Africa): off Ghana, Liberia, and Nigeria (Figure 1). Pettibone (Reference Pettibone1976) re-examined Kirkegaard’s specimens and assigned them to a new species, Trochochaeta kirkegaardi Pettibone, Reference Pettibone1976. Intes and Lœuff (Reference Intes and Lœuff1977, Reference Intes and Lœuff1986a, Reference Intes and Lœuff1986b) reported D. orissae from off Ivory Coast (Côte d’Ivoire, West Africa). Although these identifications have never been confirmed further, the specimens reported by these authors are tentatively referred to T. kirkegaardi and not considered in the present study.
Shin (Reference Shin, Morton and Tseng1982) for the first time reported T. cf. orissae from the West Pacific Ocean (Hong Kong, China) (Figure 1). Mackie (Reference Mackie and Morton1990) re-examined those specimens and referred them to T. diverapoda (Hoagland, Reference Hoagland1920), which was originally described from the Philippines (Figure 1).
From Indian waters, after Fauvel’s (Reference Fauvel1932) study, Achari (Reference Achari1968: 100) first described the morphology of two Trochochaeta specimens collected ‘off Azhikode (15 miles North of Ernakulam along southwest coast of India) at 15–30 fathoms depth’ (Kerala, southwestern coast of India). Achari (Reference Achari1968) referred these specimens to T. watsoni (Fauvel, Reference Fauvel1916), which was originally described from the Northwest Atlantic, off Nova Scotia (Canada) (Figure 1), from the depth of 1332 m, and until that never reported from outside of the Northwest Atlantic. Pettibone (Reference Pettibone1976: 15) noted that ‘Except for the absence of a median antenna on the prostomium, the description and figures [by Achari Reference Achari1968] appear to agree more closely with T. orissae, than with T. watsoni’. With the caveat that if specimens described by Achari (Reference Achari1968) appear closer to T. orissae, Mackie (Reference Mackie and Morton1990: 342) assumed that ‘perhaps T. orissae is indeed synonymous with T. diverapoda’. Despite this assumption, Paxton and Chou (Reference Paxton and Chou2000) included T. cf. orissae, and Sun et al. (Reference Sun, Lei, Zhou and Liu2008) included T. orissae in the lists of polychaetous annelids from the South China Sea. Hernández-Alcántara and Solís-Weiss (Reference Hernández-Alcántara and Solís-Weiss2011: 411) interpreted Mackie’s (Reference Mackie and Morton1990) assumption as he ‘agreed with Pettibone’s (Reference Pettibone1976) decision to synonymize it [T. orissae] with T. diverapoda’, despite the fact that Pettibone (Reference Pettibone1976) never did that.
On the world distribution map of the described Trochochaeta species, Hernández-Alcántara and Solís-Weiss (Reference Hernández-Alcántara and Solís-Weiss2011: fig. 5) marked the presence of T. diverapoda to the southwest of India, from where Achari (Reference Achari1968) identified worms as T. watsoni, and omitted the type locality of T. orissae in the east coast of India, Bay of Bengal, where this species was originally described by Fauvel (Reference Fauvel1932). Such mapping did not consider Pettibone’s (Reference Pettibone1976) concept of Trochochaeta, which distinguished T. orissae and T. diverapoda as different species in the identification key. Radashevsky et al. (Reference Radashevsky, Peixoto and Peixoto2018) suggested postponing synonymization of T. orissae with T. diverapoda until the morphology of the former species is fully understood.
Damodaran (Reference Damodaran1973) reported D. orissae as abundant in 4–21 m depth in almost the same locality off the Kerala coast, where Achari (Reference Achari1968) collected his specimens. Hartman (Reference Hartman1974) reported D. orissae from off Kakinada Bay (Andhra Pradesh, Bay of Bengal). Later, T. orissae was reported in environmental studies in the Arabian Sea (Goa, Maharashtra) (Sivadas et al. Reference Sivadas, Ingole and Fernandes2013, Reference Sivadas, Ingole and Nanajkar2010, Reference Sivadas, Nagesh, Gupta, Gaonkar, Mukherjee, Ramteke and Ingole2016; Sukumaran et al. Reference Sukumaran, Vijapure, Mulik and Ridha2021).
In addition to T. orissae, Srinivasa Rao and Rama Sarma (Reference Srinivasa Rao and Rama Sarma1981) reported Trochochaeta sp. from the Vasishta Godavari Estuary (Andhra Pradesh, Bay of Bengal), Sukumaran et al. (Reference Sukumaran, Vijapure, Kubal, Mulik, Rokade, Salvi, Thomas and Naidu2016, Reference Sukumaran, Vijapure, Mulik and Ridha2021), Vijapure and Sukumaran (Reference Vijapure and Sukumaran2019), and Vijapure et al. (Reference Vijapure, Sukumaran, Neetu and Chandel2019) reported Trochochaeta sp. from off Malvan and Malvan Bay (Maharashtra, Arabian Sea), and Rengaiyan et al. (Reference Rengaiyan, De and Ingole2017) reported Trochochaeta sp. from Trivandrum (Kerala). Mulik et al. (Reference Mulik, Sukumaran and Dias2020a, b) noted that Trochochaeta sp. was one of the dominant species (6.1%) in the macrobenthic abundance of Savitri estuary (Maharashtra). Vijapure and Sukumaran (Reference Vijapure and Sukumaran2019), Vijapure et al. (Reference Vijapure, Sukumaran, Neetu and Chandel2019), and Sukumaran et al. (Reference Sukumaran, Vijapure, Mulik and Ridha2021) for the first time reported T. cirrifera from off Mumbai (Maharashtra). These were the only reports of T. cirrifera outside its type locality in the Gulf of Oman.
To summarize, T. orissae was first described based on an incomplete specimen from off the east coast of India (Odisha), later widely reported in environmental studies from off the west coast (Maharashtra, Goa, Kerala) and occasionally reported from off the east coast (Andhra Pradesh) of India. The species has never been re-described since its original poor description. The taxonomic status of this species needs to be clarified since its identity with T. diverapoda has been suspected. Trochochaeta cirrifera and T. watsoni have been recorded for India, but these records require confirmation.
During environmental surveys in Alappuzha mud banks off the southwest coast of Kerala, we found Trochochaeta worms, which appeared similar to the specimens described by Achari (Reference Achari1968) and different from other species known to date. The purpose of this study was to describe the morphology of the collected worms and clarify the taxonomy of Trochochaeta occurring in Indian waters.
Materials and methods
Collections were conducted across the Alappuzha mud bank along the Kerala coast on the southwest coast of India from November 2021 to October 2022 (Figure 1). Sediments were collected using a van Veen grab with a sampling area of 0.04 m2. Collected sediments were sieved onboard on a 300-µm mesh sieve, and the organisms retained in the residue were fixed in 5% formalin solution. In the laboratory, these organisms were rinsed in fresh water and transferred to 70% ethanol. Trochochaeta worms were removed and examined using a stereo microscope Olympus SZX2-ILLD. Some Trochochaeta specimens were air-dried, coated with gold and viewed with a JEOL JSM 7610F Plus field emission scanning electron microscope equipped with a digital camera.
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 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.
Sampling localities of Trochochaeta spp. reported in this study are plotted on a map using QGIS 3.28 software and the geodata provided by the OpenStreetMap Project (https://osmdata.openstreetmap.de). When no coordinates were provided for sampling sites in old studies, they were estimated using Google Earth Pro 7.3.6.9345 according to the original descriptions of the locations. Final map and figures were prepared using CorelDRAW®2022 software.
Characterization of the study area
The Alappuzha mud bank is a unique ecosystem along the southwest coast of India, ranging from 3 to 15 m in depth. The mud banks represent a distinct coastal oceanographic feature marked by calm, turbid waters with high concentrations of suspended sediments and high biological productivity. These phenomena are frequently observed along the southwest coast of India, particularly off the coast of Kerala, locally known as Chakara. The Alappuzha mud banks experience seasonal variability in environmental conditions. They become active during the southwest monsoon season (June to September), when unconsolidated fluid mud and high concentrations of suspended sediment (turbidity: 45 ± 17 NTU) are abundant in the area. At this season, the mud banks experience hypoxia (dissolved oxygen 77 ± 28 µM) caused by the upwelling of cold, high-saline waters to the surface. During the inactive phase (October to May), the quantity of suspended sediment is low, and the fluid mud is present in a more settled form.
Results
Systematic account
Spionidae Grube, 1850
Trochochaeta Levinsen, Reference Levinsen1884
Trochochaeta chakara Abdul Razaque, Abdul Jaleel & Radashevsky sp. nov.
http://zoobank.org/urn:lsid:zoobank.org:act:3B64A3EA-4858-43F3-BF69-10992BA6E835
Adult morphology of T. chakara sp. nov. (CMLRE IO/OV/POL/00891). (A–D) Anterior ends, frontal view, showing two pairs of eyes on prostomium, palps missing. Abbreviations: ca – caruncle; no1 – notopodial postchaetal lamella of chaetiger 1. Scale bars: (A–D) = 50 µm.
Trochochaeta watsoni: Achari (1968: 100–102, fig. 1). Not Disoma watsoni Fauvel, 1916.
Disoma orissae: Damodaran (1973: 56). Not D. orissae Fauvel, 1932.
Trochochaeta orissae: Sivadas et al. (2010: 204; 2013: 9; 2016: supplement table 1). Sukumaran et al. (Reference Sukumaran, Vijapure, Mulik and Ridha2021: supplement table 1). Not D. orissae Fauvel, 1932.
Material examined. India, Kerala, Alappuzha mud bank, mud, coll. Abdul Razaque M.A.: st. 3, 9.3539°N, 76.3148°E, 8 m, 23 Nov 2021, CMLRE IO/OV/POL/00891 (paratype), 16 Dec 2021, CMLRE IO/OV/POL/00892 (paratype), 1 Mar 2022, CMLRE IO/OV/POL/00893 (holotype); st. 1, 9.3551°N, 76.3402°E, 3 m, 1 Mar 2022, CMLRE IO/OV/POL/00894 (2 paratypes); st. 2, 9.3547°N, 76.3351°E, 5 m, 23 Nov 2021, CMLRE IO/OV/POL/00895 (2 paratypes); st. 4, 9.3521°N, 76.2797°E, 15 m, and 16 Dec 2021, CMLRE IO/OV/POL/00896 (2 paratypes).
Etymology. The species name refers to the type locality of T. chakara, Chakara (calm or dead shore in Malayalam language of the Kerala state of India), unique seasonal mud banks along the coast of Alappuzha district of Kerala. During the southwest monsoon, these mud banks dampen wave action, making sea much calmer than in the surrounding coastal areas. Therefore, in Kerala, they are locally known as Chakara, meaning calm or dead shore, as well as active fishing and fish landing zone.
Diagnosis. Adults up to 24 mm long, 1.5 mm wide for 110 chaetigers. Anterior part of body slightly flattened, posterior part almost cylindrical. Prostomium anteriorly entire. Caruncle to end of chaetiger 1. Occipital antenna absent. Two pairs of red eyes present. Chaetiger 1 with notochaetae almost twice as short as neurochaetae, and notopodial postchaetal lamellae almost twice as long as neuropodial lamellae. Heavy falcate spines in neuropodia of chaetigers 2 and 3, those in chaetiger 3 much stronger and darker. Capillary chaetae in notopodia of chaetigers 1, 3–10. Frayed, flattened, hirsute, distally pointed heavy spines in neuropodia of chaetigers 4–13. From chaetiger 14 onwards, neuropodia with hirsute capillaries. Lateral interneuropodial membranes from chaetiger 14 onwards. One pair of ventral papillae on each chaetiger from chaetigers 14–16 onwards. Paired bundles of acicular spines in notopodia from chaetigers 50–52. Pygidium with up to six pairs of short cirri. Dioecious. Oocytes with thick honey-combed envelope with four cortical alveoli.
Juvenile morphology. Smallest incomplete specimen about 0.4 mm wide, with seven anterior chaetigers widest, and succeeding chaetigers gradually narrowing. Chaetigers 3 and 4 separated by deep lateral notch. Capillary chaetae in notopodia of chaetigers 1–9 (absent on chaetiger 2) appearing same as in larger specimens, absent on succeeding chaetiger. Heavy falcate spines in neuropodia of chaetigers 2 and 3 fewer and less pigmented than in larger specimens. Small lateral membranes, ventro-lateral pads, and ventral papillae present as in larger specimens. Pygidium with two pairs of cirri.
Adult morphology. Adults up to 24 mm long, 1.5 mm wide on chaetiger 7, with 110 chaetigers. Complete specimens rare (Figure 3A); mainly anterior fragments present in samples. Pigmentation absent on body and palps, but large noto- and neuropodial postchaetal lamellae and ventro-lateral pads below neuropodia on anterior chaetigers light rose in fixed specimens. In large individuals, 13 anterior chaetigers widest (Figures 3B, 3C, 4D), flattened dorso-ventrally; succeeding chaetigers gradually narrowing, oval to rounded in cross-section.
Adult morphology of T. chakara sp. nov. (CMLRE IO/OV/POL/00892). (A) Early juvenile, general view, palps missing. (B) Juvenile, anterior end, ventral view. (C) Chaetigers 7–15, ventral view. (D) Middle chaetigers, right side, ventral view. Abbreviations: ch3, ch13 – chaetigers 3, 13; me – neuropodial membrane; ne – neuropodial postchaetal lamellae; ne8, ne13 – neuropodial postchaetal lamellae of chaetigers 8, 13; vp – ventro-lateral pads of chaetigers 10 and 11. Scale bars: (A, C) = 200 µm; (B) = 500 µm; (D) = 100 µm.
Adult morphology of T. chakara sp. nov. (CMLRE IO/OV/POL/00893). (A) Middle chaetigers, left lateral view, showing ventral papillae. (B) Posterior end, dorsal view. (C) Pygidium, dorsal view. (D–F) Anterior end, ventral view. (G) Anterior end, left lateral view. (H) Neuropodia and ventro-lateral pads of chaetigers 5–9, left ventro-lateral view. Abbreviations: ch3–ch13 – chaetigers 3–13; ne1–ne7 – neuropodial postchaetal lamellae of chaetigers 1–7; pa – ventral papillae; pr – prostomium; py – pygidium; vp5, vp9 – ventro-lateral pads of chaetigers 5, 9. Scale bars: (A) = 200 µm; (B) = 500 µm; (C, F–H) = 100 µm; (D, E) = 200 µm.
Prostomium narrow, widest in middle part, anteriorly curved downwards (Figures 2A–2D, 4D–4F, 5A), rounded to truncate, with four small knobs (Figure 9B, 9C), posteriorly extending to end of chaetiger 1 as a high narrow caruncle (Figure 9B). Peristomium reduced to narrow ventral lip enclosing mouth laterally and posteriorly (Figure 10B, 10C). Two pairs of small red eyes arranged trapezoidally, comprising one pair of median eyes and one pair of lateral eyes situated anteriorly (ventrally) and set wider apart (Figures 2A–2D, 5A). Occipital antenna absent in all specimens (Figures 2A, 2C, 9A, 9B). Nuchal organs U-shaped ciliary bands lateral to caruncle (Figure 9B). Palps arising from dorsal parts (residues) of peristomium, between prostomium and notopodia of first chaetiger, as long as 10–20 chaetigers, with frontal longitudinal groove lined with fine cilia.
Adult morphology of T. chakara sp. nov. (CMLRE IO/OV/POL/00893). (A) Head and chaetigers 1 and 2, frontal view, palps missing. (B) Chaetiger 36, ventral view. (C) Frayed, flattened, distally pointed, and hirsute heavy spine from neuropodium of chaetiger 7. (D, E) Aristate neurochaetae of chaetiger 36. (F) Hirsute neurochaetae of chaetiger 36. (G) Middle chaetigers, left lateral view, showing paired ventral papillae. Abbreviations: me – lateral membrane; ne – neuropodial postchaetal lamella; pa – ventral papillae. Scale bars: (A) = 100 µm; (B) = 50 µm; (C) = 20 µm; (D) = 10 µm; (E) = 5 µm; (F) = 10 µm; (G) = 200 µm.
Chaetiger 1 with smooth, straight capillaries and large lanceolate subulate postchaetal lamellae in both rami directed forwards (upwards in fixed specimens) (Figures 2C, 4F, 5A, 6A, 9B–9D, 10B, 10C). Notochaetae thinner, fewer and almost twice as short as neurochaetae (Figures 5A, 6A, 9B–9D). Numerous neurochaetae arranged in a fan-shaped row and directed antero-laterally and upwards extending beyond prostomium (Figures 4E, 4F, 10B, 10C). Notopodial postchaetal lamellae almost twice as long as neuropodial lamellae.
Adult morphology of T. chakara sp. nov. (CMLRE IO/OV/POL/00893). (A) Parapodia of chaetigers 1 and 2. (B) Parapodium of chaetiger 3. (C) Parapodium of chaetiger 4. (D) Neurochaetae of chaetiger 4. (E) Neurochaetae of chaetiger 7. (F, G) Neurochaetae of chaetiger 6. (H) Neurochaetae of chaetiger 8. (I) Notopodium of chaetiger 10. (J) Neuropodium of chaetiger 10. Abbreviations: in – inferior tuft of neurochaetae; ne1, ne2, ne10 – neuropodia of chaetigers 1, 2, 10; no1, no3, no4, no10 – notopodia of chaetigers 1, 3, 4. Scale bars: (A–C, I, J) = 50 µm; (D, E) = 20 µm; (F–H) = 5 µm.
Chaetiger 2 with neurochaetae and short conical lamellae in both rami; notochaetae absent (Figures 4E, 4F, 6A, 9B, 9D, 9E). Neurochaetae on each side comprising up to six thin spines with curved tips arranged in a fan-shaped vertical row and alternating with equal number of slender capillaries. Capillary chaetae arising from body wall slightly posterior to falcate spines, then passing between spines and distally appearing as a row slightly anterior to spines (Figure 9E). Notopodial postchaetal lamellae situated close to neuropodial lamellae and both shifted ventrally, appearing lower than neuropodial lamellae on chaetigers 1 and 3 (Figures 5A, 10B). Flattened semioval pads situated obliquely below neuropodial lamellae (Figure 10B, 10C).
Chaetiger 3 with smooth slender capillaries in notopodia, two kinds of chaetae in neuropodia and short conical lamellae in both rami (Figures 4D, 4E, 6B, 9B, 9F, 10A, 10B). Neurochaetae comprising up to seven falcate spines arranged in a fan-shaped vertical row and alternating with equal number of short slender capillaries arising slightly posterior to spines (Figures 6B, 9F). Falcate spines dark brown, smooth and heavy, much larger and darker than spines of chaetiger 2, deeply embedded into body, with distal protruding part slightly curved (Figures 4E–4G, 6B). Prominent flattened, rounded pads below neuropodial lamellae smaller than those on chaetiger 2 (Figure 10A, 10B).
Chaetiger 4 with smooth slender capillaries in notopodia, three kinds of chaetae in neuropodia and fleshy, rounded postchaetal lamellae in both rami (Figure 4G), separated from chaetiger 3 by deep lateral notches (Figures 3B, 10A, 10B). Neurochaetae comprising thin hirsute spines in anterior row and smooth slender capillaries in posterior row and in inferior group (Figure 6C, 6D). Small, rounded pads present below and slightly anterior to neuropodia (Figure 10A, 10B).
Chaetigers 5 and 6 with slender hirsute capillaries in notopodia and fleshy, rounded notopodial postchaetal lamellae (Figure 4G, 4H). On succeeding chaetigers, notopodial capillaries gradually reducing in number and postchaetal lamellae gradually diminishing in size (Figures 6I, 9A); both capillaries and lamellae absent after chaetiger 10, except small papillate lamellae present on chaetiger 11 in some individuals (Figure 9K). Neuropodia of chaetigers 5–13 large (Figure 7D), with two kinds of numerous chaetae protruding from prominent conical neuropodial lobes flanked posteriorly by flattened postchaetal lamellae and below by large fleshy, rounded pads (Figure 10A, 10D–10G). Neurochaetae of chaetigers 5–13 comprising two vertical rows of thick frayed, flattened, distally pointed, and hirsute spines (Figures 5C, 6E–6G, 7A–7C), up to nine in each row, posterior row of long capillaries with conspicuously hirsute convex side of distal end, and inferior tuft of two to five slightly shorter and thinner capillaries (Figures 4H, 6H, 6J, 8D–8F, 9G–9I, 9K). Neuropodial spines heaviest and darkest on chaetigers 6–10, gradually becoming smaller, thinner, lighter and fewer on succeeding chaetigers. Neuropodial postchaetal lamellae rounded on chaetiger 13, reduced on succeeding chaetigers (Figure 10E–10G). Ventro-lateral pads largest, subspherical from chaetigers 7–8 to chaetigers 10–11, gradually reducing in size on chaetigers 12 and 13, absent on succeeding chaetigers (Figure 10A, 10G).
Adult morphology of T. chakara sp. nov. (CMLRE IO/OV/POL/00893). (A, B) Neurochaetae of chaetiger 11. (C) Neurochaetae of chaetiger 12. (D) Neuropodium of chaetiger 12. (E) Neuropodium of chaetiger 15. (F) Neuropodium of chaetiger 25. (G) Aristate chaetae from neuropodium of chaetiger 25. (H) Hirsute chaetae from neuropodium of chaetiger 27. (I) Aristate chaetae from neuropodium of chaetiger 27. (J) Hirsute chaetae from a posterior neuropodium. Abbreviation: ar – aristate chaeta. Scale bars: (A, F) = 20 µm; (B, G–J) = 5 µm; (C–E) = 50 µm.
Adult morphology of T. chakara sp. nov. (CMLRE IO/OV/POL/00893). (A) Chaetigers 35–37, ventral view. (B) Chaetigers 10–16, ventral view. (C) Posterior end, ventral view. (D) Neurochaetae of chaetiger 6. (E) Neurochaetae of chaetiger 7. (F) Neurochaetae of chaetiger 9. (G) Neurochaetae of chaetiger 14. (H) Neurochaetae of chaetiger 15. (D–H) – ventral view. (I) Notopodial spines from a posterior chaetiger. Abbreviations: ch13–ch37 – chaetigers 13–37; in – inferior capillaries; me – lateral membrane; ne – neuropodial postchaetal lamellae; py – pygidial cirri; sp – spines in posterior notopodia. Scale bars: (A, C) = 100 µm; (B) = 200 µm; (D–I) = 30 µm; (J) = 100 µm.
Adult morphology of T. chakara sp. nov. (CMLRE IO/OV/POL/00894). (A–C) Anterior end, dorsal view, palps missing. (D) Notopodia of chaetigers 1 and 2, right side, dorsal view. (E) Neuropodia of chaetigers 1 and 2, right side, ventral view. (F) Chaetiger 3 and notopodia of chaetigers 1 and 2, left side, dorsal view. (G) Chaetigers 7–9, right side, dorsal view. (H) Chaetiger 7 neuropodium, left side, dorsal view. (I) Chaetiger 9 neuropodium, right side, ventral view. (J) Chaetiger 8 notopodium, right side, dorsal view. (K) Chaetigers 10, 11, right side, dorsal view. Abbreviations: ca – caruncle; ch3–ch13 – chaetigers 3–13; ne1–ne11 – neuropodial postchaetal lamellae/neurochaetae of chaetigers 1–11; no1–no11 – notopodial postchaetal lamellae/notochaetae of chaetigers 1–11; nu – nuchal organ; pr – prostomium; vp2 – ventro-lateral pad of chaetiger 2. Scale bars: (A) = 200 µm; (B) = 100 µm; (C–F, H–J) = 20 µm; (G, K) = 10 µm.
Adult morphology of T. chakara sp. nov. (CMLRE IO/OV/POL/00895). (A–C) Anterior end, ventral view. (D) Neuropodia and ventro-lateral pads of chaetigers 8–10, right side, ventral view. (E) Neuropodia and ventro-lateral pads of chaetigers 10–13, right side, ventral view. (F) Ventro-lateral pads of chaetigers 12 and 13, right side, ventral view. (G) Neuropodia of chaetigers 13–15, right side, ventral view. Abbreviations: ch3–ch15 – chaetigers 3–15; ne1–ne13 – neuropodial postchaetal lamellae of chaetigers 1–13; no2 – notopodial postchaetal lamella of chaetiger 2; pr – prostomium; vp2–vp11 – ventro-lateral pads of chaetigers 2–11. Scale bars: (A–C) = 100 µm; (D, E) = 50 µm; (F, G) = 20 µm.
From chaetiger 14 onwards, neuropodia small, with hirsute and aristate capillaries protruding from low conical lobe (Figure 7F–7J); neuropodial postchaetal lamellae reduced to small triangular process situated on distal posterior side of conical lobe and directed aside (Figures 3B–3D, 5B, 7E, 8A, 8B, 8H, 8J); notopodia lacking. In succeeding chaetigers number of chaetae per neuropodium gradually reducing.
From chaetigers 50–52 onwards, bundles of six to nine acicular spines appearing deeply embedded in body wall (Figures 4B, 8J); notopodial lobes and postchaetal lamellae absent. Spines almost straight, with sharply pointed distal part and blunt basal tip (Figure 8I). Spines gradually decreasing in number posteriorly and absent in 10–15 posterior most chaetigers.
Lateral interneuropodial membranes from chaetiger 14 through most of body, decreasing in size around chaetigers 50–55 and absent on some posterior chaetigers.
Small, paired papillae present on ventral side from chaetigers 14–16 onwards, one pair per chaetiger (Figures 4A, 5G), absent on some posterior chaetigers.
Pygidium small, with two pairs of short cirri in small juveniles (Figure 3A) and up to six pairs of cirri in large individuals (Figures 4B, 4C, 8C).
Narrow esophagus extending through 10–20 anterior chaetigers and terminating with simple voluminous, thin-walled gizzard-like structure in posterior 3–4 chaetigers.
Habitat. Trochochaeta chakara sp. nov. was found in abundance on the Alappuzha mud bank during all period of study from November 2021 to October 2022, both in monsoon and non-monsoon periods. The population density reached in some places 250 ind·m−2. The sediment within the population was composed primarily of clay (73.35 ± 13.76%), followed by silt (24.90 ± 12.36%) and sand (1.79 ± 1.91%) throughout the year. Organic carbon ranged from 2.21 to 2.88%, with a mean value of 2.64 ± 0.16%.
Reproduction. Trochochaeta chakara sp. nov. is likely dioecious. The oocytes were up to 87 µm in diameter, with thick envelope having external honey-combed surface and four vesicles (cortical alveoli) associated with inner surface (Figure 11A–11C).
Oogenesis and oocyte morphology of T. chakara sp. nov. (CMLRE IO/OV/POL/00896). (A, B) Female fertile chaetigers, showing oocytes developing inside ovaries. (C) Individual oocyte artificially released from an ovary. Abbreviations: al, cortical alveolus; n, nucleus; nu, nucleolus. Scale bars: (A) = 100 µm; (B) = 50 µm; (C) = 30 µm.
Discussion
Fauvel (Reference Fauvel1932: 176) described D. orissae based on a single anterior fragment ‘whose hind part is unfortunately soft and twisted, is about 6 mm. long, 1 mm. broad in the thorax, 0.6 mm. in the abdomen, with about 25 segments’. Fauvel (Reference Fauvel1932: 174–175) noted the following features of this fragment: prostomium extending posteriorly as a caruncle to the end of chaetiger 1; ‘small erect, tapering median tentacle’ present in the middle of the prostomium; chaetiger 1 with long smooth capillary chaetae and ‘large lanceolate subulate’ postchaetal lamellae (called cirri in his description) in both rami; chaetiger 2 without notochaetae but with capillary chaetae and ‘stouter shorter bristles with blunt curved tips’; chaetiger 3 with capillary chaetae in noto- and neuropodia and ‘a vertical row of 7 stout yellow acicular setae with a blunt, bent tip’ in neuropodia; and chaetigers 5–8 with capillary chaetae in both rami and ‘ventral bristles’ large, yellow, ‘stout, doubly curved, nearly sickle-shaped with a broad wing, showing a tendency to split into fine spines’. Fauvel (Reference Fauvel1932: 175) also noted that the fourth and the succeeding chaetigers had ‘thick rounded’ dorsal and ventral cirri (=postchaetal lamellae); ‘The dorsal setae disappear about the 11th foot. From the 9th foot backwards, the dorsal cirri become filiform and the ventral ones are modified about the 12th–13th feet. Beyond the 11th foot long filiform ventral papillae make their appearance, a single one at first under each foot, but increasing to 2, 3 or 5’. Fauvel’s (Reference Fauvel1932: 176) description of chaetae in the ninth and subsequent chaetigers is ambiguous and incomplete: ‘From the 9th foot backwards, the ventral setae are of two kinds: (1) stout, straight bodkin-shaped, and (2) very slender capillary’. It can no longer be completed, as the type specimen of D. orissae has apparently been lost. Our request to the Zoological Survey of India, Calcutta, where this specimen was supposed to be deposited, gave a negative reply, indicating that this specimen is absent there (Officer-in-Charge, in litt., 6 June 2024).
Both Pettibone (Reference Pettibone1976) and Mackie (Reference Mackie and Morton1990) noted similarity between T. diverapoda and T. orissae in having a prominent occipital antenna on the prostomium, heavy spines in the neuropodia of chaetigers 2 and 3, and chaetiger 2 without notochaetae. Moreover, adults of both species have dorsal capillary chaetae until chaetiger 10, and up to five ventral papillae on each chaetiger from chaetiger 12. At the same time, Pettibone (Reference Pettibone1976: 5) distinguished T. orissae as having a caruncle extending on chaetiger 2, thoracic postchaetal lamellae rounded, and abdominal acicular neurochaetae without aristae, instead of a caruncle extending on chaetiger 3, thoracic postchaetal lamellae subrectangular, and abdominal acicular neurochaetae with aristae. Mackie (Reference Mackie and Morton1990: 342) noted that ‘The significance of these differences is unclear and conceivably could simply reflect the smaller size (estimated 1.3 mm wide) and treatment/preservation of the specimen [T. orissae]’. Indeed, the described specimens of T. diverapoda were up to 3 mm wide and could have more developed morphology than the smaller specimen of T. orissae. The length of caruncle, the arrangement of chaetae, and the number of thoracic chaetigers in Trochochaeta are known to be age/size dependent (Radashevsky Reference Radashevsky2012; Radashevsky et al. Reference Radashevsky, Peixoto and Peixoto2018).
The Trochochaeta specimens collected in the present study from the Alappuzha mud bank appear similar to specimens described by Achari (Reference Achari1968) and different from species known to date. They are similar to T. cirrifera in the absence of occipital antenna but differ by the rounded instead of cirriform notopodial postchaetal lamellae on anterior chaetigers. Trochochaeta from Alappuzha are similar to T. diverapoda/orissae in having notopodial capillaries until chaetiger 10. They differ, however, by the absence of occipital antenna on the prostomium. Moreover, they have interneuropodial membranes from chaetiger 14 and single pairs of ventral papillae from chaetigers 14–16, whereas in T. diverapoda/orissae interneuropodial membranes and up to five ventral papillae on each chaetiger are present from chaetiger 12. Due to these differences, Trochochaeta from the Alappuzha mud bank are described above as a new species, T. chakara sp. nov.
Conclusion
Even though the first Trochochaeta specimen was described in India almost a century ago, and Trochochaeta have been widely reported in environmental studies in Indian waters in recent decades, the taxonomic identity of these worms remained unclear. In this study, we have shown that worms from southwestern India, which for a long time were erroneously identified as T. watsoni or T. orissae, actually belong to a new species, which we have described here as T. chakara sp. nov. However, ambiguities remain, and further studies are needed. At the moment, we temporarily consider T. orissae to be a valid species, pending confirmation in a further study. The morphology of this species, which was originally described based on a single anterior fragment and has never been re-described since, should be supplemented based on specimens from the type locality or nearby. Records of T. cirrifera from Indian waters should be verified, and the morphology of this species should also be supplemented. Like T. orissae, T. cirrifera was originally described based on a single anterior fragment and has never been re-described since.
Acknowledgements
Our sincere thanks to the Director of the CSIR – National Institute of Oceanography (CSIR–NIO), Goa, India, and to the Scientist-in-Charge at the CSIR – NIO Regional Centre, Kochi, India, for their invaluable support. The first author gratefully acknowledges the Council of Scientific and Industrial Research (CSIR), India, for providing a research fellowship. We would like to thank Vishnu M. L. for his help in the dissection and taxonomic identification processes. We also thank our colleagues at CSIR – NIO for the successful execution of the field sampling. This is NIO contribution No. 7564
Author contributions
M. A. Abdul Razaque: conceptualization, investigation, visualization, formal analysis, writing original draft; K. U. Abdul Jaleel: conceptualization, supervision, investigation, visualization, manuscript editing and review; Vasily I. Radashevsky: conceptualization, supervision, investigation, visualization, writing the final version of the manuscript; Saraswathi Ragesh: methodology, visualization; Narayan Panda: methodology, visualization; and Soniya Sukumaran: manuscript editing and review.
Funding
The authors thank the Ministry of Earth Sciences, Government of India, for financial support (Grant No. MoES/36/OOIS/Extra/22/2016).
Competing interests
The authors declare none.
Data availability
The authors confirm that the data supporting the findings of this study are available within the article.
