Introduction
The genus Labidocera Lubbock, Reference Lubbock1853 is the largest in the family Pontellidae, at present comprising 63 species, 5 subspecies, and 1 variety, following Labidocera baliensis Mulyadi, 2021 and L. gagensis Mulyadi, 2021 (Walter and Boxshall, Reference Walter and Boxshall2025). The Labidocera was classified into four species complexes (based on morphological characters), the L. wilsoni Fleminger & Tan, Reference Fleminger and Tan1966, L. darwini Lubbock, Reference Lubbock1853, L. detruncata (Dana, Reference Dana1849), and L. kroyeri (Brady, Reference Brady1883), and some unassigned species (Fleminger, Reference Fleminger1967, Reference Fleminger1986; Fleminger et al., Reference Fleminger, Othman and Greenwood1982; Hirabayashi and Ohtsuka, Reference Hirabayashi and Ohtsuka2014; Mulyadi, 1997, 2002, 2014). The L. detruncata species complex is by far the most species rich species complex within the genus, comprising 17 nominal species (Mulyadi, 2021) widely distributed across the coastal waters of tropical and subtropical (i) Indo-West Pacific (L. detruncata (Dana, Reference Dana1849), L. pavo Giesbrecht, Reference Giesbrecht1889, L. cervi Krämer, Reference Krämer1895, L. bataviae Scott A., Reference Scott1909, L. madurae Scott A., Reference Scott1909, L. gangetica Sewell, Reference Sewell1934, L. caudata Nicholls, Reference Nicholls1944, L. sinilobata Shen & Lee, Reference Shen and Lee1963, L. tasmanica Taw, Reference Taw1974, L. farrani Greenwood & Othman, Reference Greenwood and Othman1979, L. jaafari Othman, Reference Othman1986, L. churaumi Hirabayashi & Ohtsuka, Reference Hirabayashi and Ohtsuka2014, L. kaimanaensis Mulyadi, 2014, and L. baliensis Mulyadi, 2021); (ii) Atlantic waters (L. nerii (Krøyer, Reference Krøyer1849) and L. orsinii Giesbrecht, Reference Giesbrecht1889); and (iii) Red Sea (L. boxshalli El-Sherbiny & Ueda, Reference El-Sherbiny and Ueda2010) (El-Sherbiny and Ueda, Reference El-Sherbiny and Ueda2010; Fleminger, Reference Fleminger1967; Hirabayashi and Ohtsuka, Reference Hirabayashi and Ohtsuka2014; Mulyadi, 2002, 2014). Further subdivision of the species complex into five species groups, viz., L. gangetica, L. cervi, L. detruncata, L. pavo, and L. madurae was proposed by Hirabayashi and Ohtsuka (Reference Hirabayashi and Ohtsuka2014). Despite, the Indian water L. detruncata species complex includes five species representing all species groups sensu Hirabayashi and Ohtsuka (Reference Hirabayashi and Ohtsuka2014), such as L. bataviae (L. pavo species group), L. gangetica, L. detruncata, L. madurae, and L. pavo, except L. cervi species group. Among them, the Indian species L. gangetica has been considered endemic to northeastern Bay of Bengal and was not included in the species groups of the Indo-West Pacific L. detruncata species complex (Mulyadi, 2002, 2014). Furthermore, the species had hitherto been incompletely described, with only key diagnostic features illustrated from the initial collection, and has not been reported or redescribed since the original description (Sewell, Reference Sewell1912, Reference Sewell1934). During a study on the community structure of zooplankton in West Bengal coastal waters, L. gangetica, a species that had been poorly understood and partially described until now, was recognised and subsequently collected from Hooghly River. The morphological re-evaluation of adult L. gangetica raises uncertainty about the validity of the species L. jaafari, as it aligns entirely with L. gangetica in all morphological features, suggesting they are probably synonymous. Herein, we discuss the taxonomic status of L. jaafari and present a complementary morphological description of L. gangetica, including its swimming legs and mouthparts, marking a century since its initial collection.
Materials and methods
Zooplankton samples were collected at the mouth of Hooghly River (21°46ʹ42.99″N, 88° 4ʹ8.16″E) on the northwestern coast of Sagar Island, Bay of Bengal, India, by subsurface, horizontal tows of a plankton net (200-μm mesh and 0.6-m diameter) at day light (09:30–10:30 h) on 7th June 2023. The temperature of the sampling site was ca. 29.5°C and salinity of 19.6. All samples were fixed and preserved in a 4% formaldehyde–seawater solution. Specimens of L. gangetica were sorted from the original samples, dissected with needles in 10% glycerol/distilled water, and examined under a stereomicroscope and inverted microscope. Illustrations were prepared using a drawing tube and by tracing photomicrographs using computer software package GIMP 2.8. Additionally, the samples were prepared for scanning electron microscopy by dehydrating them through a series of ethanol gradients, followed by drying with hexamethyldisilazane. The dried samples were then coated with a gold-palladium layer and analysed using a scanning electron microscope (TESCAN Vega 3) to show selected morphological features in detail. The morphological terminology followed by Huys and Boxshall (Reference Huys and Boxshall1991). Prosome length (distance between the anterior to mid-posterior margin) and width, urosome length (distance between the mid-posterior margin of prosome to posterior margin of caudal ramus excluding the distal setae), and width and length of the urosomites were measured with a calibrated ocular micrometer and image analysis software. The armature formula of legs 1–4 was proposed by Sewell (Reference Sewell1949). Voucher specimens were deposited in the Zoological Survey of India, Marine Biology Regional Centre, Chennai (Reg. No. ZSI/MBRC/CP.01).
Results
Systematic account
Subclass Copepoda Milne Edwards, Reference Milne Edwards1840
Order Calanoida Sars G.O., Reference Sars1903
Family Pontellidae Dana, 1852–Reference Dana1852-1853
Genus Labidocera Lubbock, Reference Lubbock1853
Labidocera gangetica Sewell, Reference Sewell1934 (Figures 1–5)
Labidocera gangetica. Female: (A) habitus, dorsal view; (B) antennule; (C) antenna; (D) mandible palp and gnathobase; (E) maxillule; (F) maxilla; (G) maxilliped.
Previous descriptions
Labidocera euchaeta Giesbrecht, Stage I, Dimorph I, Sewell, Reference Sewell1912, p. 339, pl. xviii, Figures 1–9.
Labidocera euchaeta Giesbrecht, forma major, Sewell, Reference Sewell1932, p. 361.
Labidocera gangetica Sewell, Reference Sewell1934, pp. 79–80.
Labidocera jaafari Othman, Reference Othman1986, pp. 193–201, Figures 1–3 (probable synonym).
Description
Material examined: 8 ♀♀, total length 2.34–2.57 mm, mean length 2.46 ± 0.07 mm (mean ± SD, n = 8), prosome width 0.69–1.06 mm and, prosome (1.87–2.05 mm) and urosome (0.42–0.52 mm) length in approximate proportion of 4:1; prosome length to width ratio 2.7:1.
Adult female: Cephalic profile rounded in dorsal and lateral view, without lateral hooks (Figures 1A, 2A). A distinct indentation occurs on both lateral and dorsal profiles about midlength of the cephalosome. Paired dorsal eyes with cuticular lenses. Rostrum bifid and directed posteroventrally (Figure 2B). Prosome about 4.3 times as long as urosome. Fourth and fifth pedigerous somites fused with posterior corner of last pediger produced posteriorly into symmetrical, strong spiniform processes, reaching nearly two-thirds of genital double-somite. Urosome 2 segmented, genital double-somite smooth, longer than wide, nearly symmetrical, genital operculum located on the left ventrolateral side, anal somite very short, symmetrical and smooth (Figure 2C, D). Caudal rami symmetrical, each ramus with 5 plumose setae and 1 rudimentary naked seta.
Labidocera gangetica. Female: (A) habitus, dorsal view; (B) rostrum and ventral lens; (C) urosome, dorsal view; (D) urosome, ventral view; (E) leg 5, posterior view.
Antennule (Figure 1B) symmetrical, 23-segmented, reaching mid-length of genital double-somite. Posterior margins of segments 2–14 (ancestral segments II–XVII) fringed with fine hairs; ancestral segments II–IV and XXVII–XXVIII completely fused and VII–IX incompletely fused. Armature as follows: I, 2 + ae (aesthetasc); II–IV, 4 + ae; V, 2 + ae: VI, 2; VII, 2 + ae; VIII–IX, 4 + ae; X, 2; XI, 2 + ae; XII, 2; XIII, 2 + ae; XIV, 2 + ae; XV, 2 + ae; XVI, 2 + ae; XVII, 2 + ae; XVIII, 2 + ae; XIX, 2 + ae; XX, 2 + ae; XXI, 2 + ae; XXII, 1; XXIII, 1; XXIV, 2; XXV, 2 + ae; XXVI–XXVIII, 6 + ae. Antenna (Figure 1C) biramous; coxa with 1 plumose seta and basis with 2 setae proximally and 2 setae distally; basis and endopod 1 completely fused to form elongate allobasis. Compound distal endopodal segment with 9 and 7 setae on proximal and distal lobes, respectively; exopod 5-segmented, setation formula 0, 1, 2, 2, 3. Mandible (Figure 1D) with wide gnathobase, mandibular palp biramous, basis robust, armed with 4 inner setae. Endopod 2-segmented, first segment armed with 1 short and 3 long setae, second segment with 1 dorsal and 6 long terminal setae. Exopod 5-segmented, with setation formula 0, 1, 1, 1, 3. Mandibular gnathobase with distal edge bearing 8 teeth comprising from ventral margin 1 apical, 1 subapical, 3 compound medial, and 3 basal; medial teeth with bifurcated cusps, each armed with a spine on posterior surface; dorsal end of gnathobase with 1 seta. Maxillule (Figure 1E) praecoxal arthrite with 15 setal elements, 4 on posterior surface; coxal endite with 3 setae (2 long and 1 short) and coxal epipodite with 9 setae; basis with 4 setae on proximal endite and 3 setae on distal endite fused to endopod, 1 large seta on basal exite; endopod with 4 lateral and 5 apical setae, exopod with 11 setae. Maxilla (Figure 1F) with first praecoxal endite bearing 6 setae, second with 3 setae; coxa with 3 setae each on proximal and distal endites. Basis with 3 setae; endopod 3-segmented, setal formula of endopod: 1, 1, 4. Maxilliped (Figure 1G) with praecoxa and coxa fused, three syncoxal endites well developed, with setal formula 2, 3, 3; endite setae strong, spinulose. Basis without seta and fringed with medial row of spiniform processes. Endopod 5-segmented bearing distal seta with setal formula 2, 2, 1, 1, 3.
Swimming legs 1–4 (Figure 3A–D) biramous with 3-segmented exopods and 2-segmented endopods; Coxae with plumose inner seta. Spine (in Roman) and seta (in Arabic numeral) formula of legs 1–4 as follows:
Leg 5 (Figures 2E, 3E) uniramous, slightly asymmetrical; coxa and intercoxal sclerite fused. Basis left slightly longer than right, with posterior plumose seta and a minute seta proximally. Exopod 1 – segmented, left slightly longer than right, ends with 4 blunt, finger-like spiniform processes and 1 lateral spiniform process at midlength of outer margin.
Labidocera gangetica. Female: legs 1–4, anterior view, (A) leg 1; (B) leg 2; (C) leg 3; (D) leg 4; (E) leg 5, posterior view.
Material examined: 7 ♂♂, total length 2.18–2.33 mm, mean length 2.25 ± 0.05 mm (mean ± SD, n = 7), prosome width 0.62-0.65 mm and, prosome (1.70–1.82 mm) and urosome (0.48–0.56 mm) length in approximate proportion of 3.5:1; prosome length to width ratio 2.8:1.
Adult male: General body shape as in female, without lateral hooks and distinct indentation of the cephalosome (Figures 4A,B, 5A). Paired dorsal eyes with cuticular lenses and rostrum bifid and directed posteroventrally as in female. Prosome about 3.8 times as long as urosome. Fourth and fifth pedigerous somites fused with posterior corner of last pediger produced posteriorly into symmetrical, strong spiniform processes, reaching nearly two-thirds of the urosomites 2. Urosome 5 segmented, proportional lengths of urosomal somites 1–5 and caudal ramus as follows: 19: 24: 20: 8: 4: 25 (=100). Genital somite slightly asymmetrical with weakly swollen on the left side; other urosomites, anal somite and caudal rami symmetrical, each armed with 5 plumose setae and small dorsal seta (Figure 5B).
Labidocera gangetica. Male: (A) habitus, dorsal view; (B) habitus, lateral view; (C) right antennule, posterior view; (D) right antennule, ancestral segments XIX–XXVIII; (E) leg 5, posterior view; (F) right leg 5 exopodal segment, anterior view.
Labidocera gangetica. Male: (A) habitus, dorsal view; (B) urosome, ventral view; (C) right antennule, ancestral segment XX; (D) right antennule, ancestral segments XXI–XXVIII; (E) leg 5, posterior view; (F, G) left leg 5 second exopodal segment, posterior view; (H) right leg 5 exopodal segment, anterior view.
Appendages similar to those of female except right antennule (Figures 4C,D, 5C,D) and leg 5 (Figures 4E,F, 5E–H). Right antennule geniculate, indistinctly 17-segmented, posterior margin of ancestral segment V–IX fringed with fine hairs (Figure 4C). Armature as follows: I, 1; II–IV, 4 + ae (aesthetasc); V, 2 + ae; VI, 2 + ae; VII, 2 + ae; VIII–IX, 4; X–XI, 4 + ae; XII–XIV, 6 + 2ae; XV–XVI, 5 + 2ae; XVII, 2 + ae; XVIII, 2 + ae; XIX, 1 + ae; XX, 1 + ae; XXI- XXIII, 3 + ae; XXIV, 2; XXV, 2 + ae; XXVI–XXVIII, 6 + ae. Segment XIX with lateral round projection and hooked process proximally; segment XX with enlarged toothed process; compound segments XXI to XXIII with toothed ridge possessing comb-like denticles; segment XXIV with lamelliform process extended beyond the next segment (Figures 4C,D, 5D).
Leg 5 uniramous and asymmetrical with right leg longer than left. Left leg: coxa very short, basis 2.5 times the length of the coxa, with a plumose seta on posterior surface near base. Exopod 2-segmented; first exopodal segment as long as basis, rectangular, with pointed process near distolateral corner. Second exopodal segment 0.6 length of the first with hirsute inner border, bulb-shaped with 1 lateral spine mid-length of the segment, 3 apical spinous processes, with different length proportions (Figures 4E, 5F,H). Right leg: coxa short, basis twice the length of the coxa, basis with a plumose seta on proximal posterior surface. Exopod 2-segmented, forming chela; first exopodal segment elongate and subcylindrical, with thumb narrow, stretched sigmoid and pointed process. First exopod segment with semicircular lamellar processes posteriorly near base of thumb and an adjacent seta. First exopod segment with 2 minute setae, and posteriorly near semicircular lamellar processes and anteriorly at one-third length of the segment and 1 distal seta near base of the finger. Second exopodal segment elongate and medially curved, with 4 slender marginal setae, 2 near the curvature, 1 distal and 1 at the tip of the finger (Figures 4E,F, 5E,G).
Distribution: Rangoon (now Yangon) River estuary, Hooghly River (Diamond Harbour), Achipur and Piyali River at Uttarbhag (West Bengal), Kuala Selangor and Pulau Besar in the Straits of Malacca, Matang estuary in west coast of Peninsular Malaysia and Hooghly River mouth in northeast Bay of Bengal of India (Present study).
Remarks: Labidocera gangetica belongs to the L. gangetica species group, as defined by Hirabayashi and Ohtsuka (Reference Hirabayashi and Ohtsuka2014). It is distinguished from other members of the L. detruncata species complex by the absence of endopods on the female leg 5, the slender thumb and finger of the male right leg 5, the presence of a semicircular process near the base of the thumb on the male right leg 5, and an inner protrusion on the terminal segment of the male left leg 5. The mere illustrations of diagnostic features by the original description (Sewell, Reference Sewell1912), whereas lack of updated redescriptions and complete description of L. gangetica, have led to its limited consideration in biogeographic assessments within the Indo-West Pacific community (Greenwood and Othman, Reference Greenwood and Othman1979; Mulyadi, 2002, 2014). However, the complete description of the adult forms by the present study revealed an identical resemblance to that of L. jaafari Othman, Reference Othman1986. As a result, the possibility of synonymy of L. jaafari and L. gangetica described from the Kuala Selangor and Pulau Besar in the Straits of Malacca is proposed by comparing the description and illustrations of Othman (Reference Othman1986) with adult specimens of L. gangetica from the Hooghly River, the locality of the original description (Sewell, Reference Sewell1934). The overall structure of the body plan, particularly the structure and morphological features of the leg 5 of both sexes, are completely identical. The similarity of L. jaafari to the original description of L. gangetica and the present examples is evident in the following features: (i) The size of the adult copepods, although slightly smaller than the original description (2.52–2.94 mm), is comparable with present examples (female total length: 2.31–2.55 mm vs 2.34–2.57 mm; male total length: 2.14–2.29 mm vs 2.18–2.33 mm). (ii) A distinct indentation occurs on both the lateral and dorsal profiles about mid-length of the cephalosome. (iii) The posterior prosome is symmetrical with strong spiniform processes, reaching nearly two-third of genital double-somite. (iv) The width-length ratio of the female genital compound somite is approximately 0.7, which aligns with both L. gangetica and L. jaafari (inferred from Othman, Reference Othman1986). (v) The architecture of the leg 5 of both sexes, especially the structure of exopods in female is characterised by 4 finger-like spiniform processes (Figures 2E, 3E) and the length ratio of the exopods is approximately 1.1 consistent with both the species. (vi) The male right leg 5 distal segment with slender, elongated and medially curved finger. (vii) The thumb is narrow, stretched sigmoid and pointed process, as observed from the illustrations of Othman (Reference Othman1986). (viii) A semicircular process at the thumb base, and a protrusion on the terminal segment of the left leg (Figure 5E–H). However, there appears to be a minor variation between the original description, present examples and L. jaafari. The original illustration of the female leg 5 in L. gangetica (see Figure 5, Sewell, Reference Sewell1912) appears with a small tubercle on the distal edge of the inner side of the basis. Similarly, the location and length proportions of the spinous processes on the terminal segment of male left leg 5 showed a slight difference from the original depiction (Sewell, Reference Sewell1912) but closely matching with L. jaafari. Sewell’s original description of both sexes was brief, and the illustrations were in the conventional style of the period. While the basis of the female leg 5 is broader than the exopod in both L. gangetica and L. jaafari, the presence or absence of this tubercle may not represent a reliable diagnostic character. Nevertheless, Sewell (Reference Sewell1912) did not explicitly indicate neither the tubercle nor the length proportions of the spinous processes of the terminal segment in the original description, instead focused only on the absence of the endopod in the female and three terminal processes in the male leg 5. Moreover, the location and length proportions of the spinous processes on the left leg 5 terminal segment agree well with those of L. jaafari Ohtman, 1986 (Figures 4E, 5H). In addition, L. gangetica has symmetrical caudal rami, whereas Othman (Reference Othman1986) noted that the right ramus is marginally longer but slightly thinner than the left in L. jaafari. However, no clear pattern of variability or asymmetry was evident from Othman’s illustrations (see Figure 1A, D, Othman, Reference Othman1986). Similarly, the second exopodal segment of the male right leg 5 has 4 slender marginal setae, whereas Othman (Reference Othman1986) noted only 3 marginal setae in L. jaafari. Additionally, the present study observed the absence of rostral lens, but a ventral lens is present, consistent with the original description (Sewell, Reference Sewell1912), while, this feature was not mentioned in L. jaafari (Othman, Reference Othman1986).
The original description of L. gangetica (including L. jaafari Othman, Reference Othman1986) is incomplete, as the mouthparts and legs of the species have never been illustrated, including those of L. sinilobata, the other member of the L. gangetica species group (Chen and Zhang, Reference Chen and Zhang1965; Jeong et al., Reference Jeong, Soh and Chae2022; Razouls et al., 2005-Reference Walter and Boxshall2025; Shen and Lee, Reference Shen and Lee1963). A detailed examination of the mandible, maxillule, maxilla, and maxilliped of L. gangetica reveals characteristics consistent with other members of the detruncata species complex, particularly L. churaumi Hirabayashi & Ohtsuka, Reference Hirabayashi and Ohtsuka2014, and L. boxshalli El-Sherbiny & Ueda, Reference El-Sherbiny and Ueda2010. The segmental setae and setation formula of the antennal endopod and exopod in L. gangetica are similar to L. churaumi, although they differ from L. boxshalli. Additionally, the endopod of the mandibular palp, maxillule, and maxilla in L. gangetica resemble those of both L. churaumi and L. boxshalli. In contrast, L. churaumi has a two-segmented exopod on the mandibular palp, whereas L. gangetica possesses a five-segmented exopod, a feature similar to that of L. boxshalli.
Discussion
The copepod genus Labidocera predominantly inhabits surface waters (0–30 cm layer) in tropical to warm temperate regions (El-Sherbiny and Ueda, Reference El-Sherbiny and Ueda2010; Jeong et al., Reference Jeong, Suh, Jeong, Yoon and Soh2009). Their species assemblages are widely used for major zoogeographic divisions and water masses indication (Fleminger, Reference Fleminger1967, Reference Fleminger1986; Fleminger et al., Reference Fleminger, Othman and Greenwood1982; Mulyadi, 2002). In Indian waters, 15 species of Labidocera have been reported from neritic, estuarine, and island coastal waters (Anandavelu, Reference Anandavelu2016; Anandavelu et al., Reference Anandavelu, Robin, Purvaja, Ganguly, Hariharan, Raghuraman, Prasad and Ramesh2020; Roy, Reference Roy2018; Silas and Pillai, Reference Silas and Pillai1973). These species are classified into three species complexes, viz., L. darwinii, L. detruncata, and L. kroyeri, along with an unassigned group. (i) The L. darwinii species complex includes L. acutifrons (Dana, Reference Dana1849). (ii) The L. detruncata species complex includes L. bataviae Scott A., Reference Scott1909, L. detruncata (Dana, Reference Dana1849), L. gangetica Sewell, Reference Sewell1934, L. madurae Scott A., Reference Scott1909 and L. pavo Giesbrecht, Reference Giesbrecht1889. (iii) The L. kroyeri species complex includes L. bengalensis Krishnaswamy, Reference Krishnaswamy1952, L. euchaeta Giesbrecht, Reference Giesbrecht1889, L. gallensis Thompson I.C. & Scott A., Reference Thompson and Scott1903, L. kroyeri kroyeri (Brady, Reference Brady1883), L. kroyeri stylifera Thompson I.C. & Scott A., Reference Thompson and Scott1903, L. minuta Giesbrecht, Reference Giesbrecht1889, and L. pectinata Thompson I.C. & Scott A., Reference Thompson and Scott1903. Additionally, the unassigned-group includes L. acuta (Dana, Reference Dana1849) and L. laevidentata (Brady, Reference Brady1883). Among these groups, the L. detruncata species complex is notable for being both the second most diverse group in Indian waters and richest within the genus. Mulyadi (2002) defined the Indo-West Pacific L. detruncata species group within the L. detruncata species complex. Subsequently, Hirabayashi and Ohtsuka (Reference Hirabayashi and Ohtsuka2014) proposed further subdivision into five species groups, viz., L. gangetica, L. cervi, L. detruncata, L. pavo, and L. madurae, on the basis of variation in the sexual dimorphism and the unique structure of the terminal exopodal segment of the male left leg 5, which served as a robust synapomorphy. In parallel, Mulyadi (2014) subdivided the members of the Indo-Australian L. detruncata species group into five species-subgroups: Labidocera detruncata cervi, L. d. detruncata, L. d. farrani, L. d. sinilobata, and L. d. kaimanaensis, based on the characteristic features of females within the L. detruncata group. Despite, the assignment of species within the subdivisions are varied among authors, the L. gangetica species group sensu Hirabayashi and Ohtsuka (Reference Hirabayashi and Ohtsuka2014) and L. d. sinilobata subgroup sensu Mulyadi (2014) both include L. gangetica, L. sinilobata, and L. jaafari, based on their distinct defining characteristics (Hirabayashi and Ohtsuka, Reference Hirabayashi and Ohtsuka2014).
Labidocera gangetica Sewell, Reference Sewell1934 was named by referring to L. euchaeta, Stage I, as described by Sewell (Reference Sewell1912) from the collections by the R.I.M.S. Investigator in the Rangoon River estuary. This species was subsequently confirmed from collections in the Hooghly River at Diamond Harbour, Achipur, and in the Piali River at Uttarbhag (Sewell, Reference Sewell1934). Although Sewell (Reference Sewell1912) observed significant morphological differences, he considered it to be L. euchaeta Giesbrecht, forma major (Sewell, Reference Sewell1932), until validating it as a distinct species two decades later (Sewell, Reference Sewell1934). Although the diagnostic features of L. gangetica were well-described, the incomplete illustrations and lack of updated redescriptions (Sewell, Reference Sewell1912, Reference Sewell1934) may have contributed to the species being overlooked or its limited recognition in the subsequent taxonomic studies. Meanwhile, Othman (Reference Othman1986) described L. jaafari from the Straits of Malacca and compared it only with L. sinilobata, failing to consider the conspecific species, possibly unaware of the description of L. gangetica. The estuarine species L. gangetica has been considered as Indian species endemic to northeastern Bay of Bengal. It was assigned to neither the L. d. sinilobata subgroup nor the broader Indo-West Pacific L. detruncata species group, both proposed by Mulyadi (2002, 2014). This omission may be reasonably attributed to assumptions about the absence of L. gangetica in Australian waters, the lack of complete descriptions of adults of both sexes, and/or limited knowledge of the species distribution beyond its type locality (Sewell, Reference Sewell1912, Reference Sewell1934). However, Hirabayashi and Ohtsuka (Reference Hirabayashi and Ohtsuka2014) reinstated the inclusion of L. gangetica within the Indo-West Pacific L. detruncata species complex. Subsequently, the re-evaluation and complete description of L. gangetica adults in the present study raise questions about the validity of L. jaafari, due to the morphological similarity. Based on the characteristics described and illustrated by Othman (Reference Othman1986), L. jaafari is morphologically identical to specimens of L. gangetica from the Hooghly River and the original descriptions. While the original specimens were slightly larger than those described here, such differences likely fall within the range of seasonal length variation influenced by environmental factors, such as temperature, food availability, or growth patterns (Greenwood and Othman, Reference Greenwood and Othman1979). In contrast, the size of L. jaafari agrees strongly with the present forms of L. gangetica. Based on detailed morphological comparisons and geographical facts, it is probable that L. jaafari is conspecific with the L. gangetica, therefore, be envisaged as its junior synonym. Nevertheless, their classification needs further revision with integrated morphological and molecular biogeographical approach that uses specimens from various localities of their previous records to elucidate the specific similarity or variability. Furthermore, this study reinstates the definition of the L. gangetica species group sensu Hirabayashi and Ohtsuka (Reference Hirabayashi and Ohtsuka2014), rather than the L. d. sinilobata subgroup sensu Mulyadi (2014), due to the priority of L. gangetica over L. sinilobata and accommodate both species in the subgroup (Srinui and Ohtsuka, Reference Srinui and Ohtsuka2015).
The population of L. gangetica, previously considered a relict species confined to northeastern Bay of Bengal and geographically segregated from the Indo-West Pacific Labidocera species populations, has now been reconsidered. Similarly, L. jaafari has been regarded as restricted to Malaysian waters (Boonyanusith et al., Reference Boonyanusith, Wongkamhaeng and Azman2024; Chew and Chong, Reference Chew and Chong2016; Chew et al., Reference Chew, Chong, Ooi and Sasekumar2015; Mulyadi, 2021). However, the probable synonymisation of L. jaafari with L. gangetica in the present study suggests an extended distributional range of L. gangetica from the major estuaries of northeastern Bay of Bengal (Sundarbans) and Myanmar (Yangon River estuary, Andaman Sea) to in the Malacca Strait, the Malaysian waters (Chew et al., Reference Chew, Chong, Ooi and Sasekumar2015; Mulyadi, 2021). These regions are geographically connected through historical geological events, including sea-level fluctuations during the Pleistocene and the formation of the Sundaland continental shelf (Fleminger, Reference Fleminger1986; Voris, Reference Voris2000).
Labidocera sinilobata Shen Lee, Reference Shen and Lee1963, originally described from the Southern China (Zaikong River), later from estuaries of East and South China Sea (Chen and Zhang, Reference Chen and Zhang1965; Liu et al., Reference Liu, Wang, Zhang, Cai and Zhang2006; Rui et al., Reference Rui, Xiao, Ren, Jingxin and Peng2024; Sun et al., Reference Sun, Liu, Zhang, Wang and Shao2016; Zhang et al., Reference Zhang, Xu, Gao and Chen2016), Yellow Sea (Zheng et al., Reference Zheng, Li, Sj and Chen1982), and Korean waters (Jeong et al., Reference Jeong, Soh and Chae2022). The species had been restricted to the West Pacific region, with its range extending to the Indonesian waters (Murniati and Mulyadi, Reference Murniati and Mulyadi2018), Java Sea and the Flores Sea (Hirabayashi and Ohtsuka, Reference Hirabayashi and Ohtsuka2014; Mulyadi, 2021) (Figure 6). The restricted distributions within this species group suggest the potential for parallel speciation driven by geological vicariance events during the past climate change in this region (Fleminger, Reference Fleminger1986; Hirabayashi and Ohtsuka, Reference Hirabayashi and Ohtsuka2014; Srinui and Ohtsuka, Reference Srinui and Ohtsuka2015). Furthermore, the co-occurrence of L. euchaeta and L. gangetica in the Hooghly River mouth, northeast Bay of Bengal, and L. jaafari (= L. gangetica) in the Matang estuary, west coast of Peninsular Malaysia (Chew et al., Reference Chew, Chong, Ooi and Sasekumar2015), indicates that both species are inhabitants of brackish and estuarine waters, occupying comparable ecological niche. Labidocera gangetica has been recorded at salinity ranging from 7.9 to 8.5 in the Hooghly River and extending upstream to salinity as low as 2.23 at Achipur (Sewell, Reference Sewell1934). In the present study, the species was found at the mouth of the Hooghly River at the salinity of 19.6, indicating its euryhaline nature. This suggests a distinct tendency for the species to inhabit the large tropical rivers of the Indo-West Pacific (Sewell, Reference Sewell1934), at least in the major estuaries of Bay of Bengal (e.g. Mahanadi, Godavari and Krishna Estuary). Furthermore, the lack of taxonomic exploration and biodiversity assessment of keystone zooplankton communities (e.g. copepods and tintinnids) has rendered many species largely being overlooked, resulting to a significant gap in understanding their taxonomy, species assemblages, and biodiversity in Indian waters (Anandavelu et al., Reference Anandavelu, Asha Devi, Singh and Chowdhury2024). Consequently, this has resulted to nearly a century-long delay in achieving a comprehensive description and elucidation of the morphological structure of this species, as highlighted in the current study. Therefore, an intensive faunistic survey of marine planktonic invertebrates around the Indian waters, especially focusing on the estuarine, coastal and open ocean waters, and island ecosystems is essential for understanding biodiversity and zoogeographic pattern, which will not only fill existing taxonomic knowledge gaps but also provide valuable insights for climatological studies and the impacts of environmental changes on marine ecosystems.
Biogeographic distribution of Labidocera gangetica species group (L. gangetica, L. jaafari, and L. sinilobata) based on the present and previous data.
Data availability
The specimens were deposited in Zoological Survey of India, Marine Biology Regional Centre, Chennai (Reg. No.: ZSI/MBRC/CP.01).
Acknowledgements
The authors are grateful to the Director, National Centre for Sustainable Coastal Management, Ministry of Environment, Forest and Climate Change, for providing access to the Scanning Electron Microscope facility. The first author sincerely acknowledges C. R. Asha Devi, Scientist F, Centre for Marine Living Resources and Ecology, Ministry of Earth Sciences, for the support during the preparation of this article. The authors also thank Dr. Ganesh Basumatary, University of Calcutta, for his support with sample collection during this study.
Author contributions
I. Anandavelu: Conceptualisation, Species identification, Methodology, Writing – original draft, Writing – review & editing. R. Jayabarathi: Formal analysis, Writing – original draft, Writing – review & editing. G. Hariharan: Methodology, Map generation, Writing – review & editing. G. Padmavati: Conceptualisation, Writing – review & editing.
Funding
The author(s) received no financial support for the research.
Competing interests
The authors declare no conflict of interests.
