Introduction
The family Pseudotriakidae Gill, 1893, which comprises three genera globally and at least five species, is a small group of poorly known, deep-sea sharks, typically characterized by their soft, slender bodies, bell-shaped snout, and tricuspid dermal denticles. Whilst one species, Pseudotriakis microdon de Brito Capello, 1868, is known from the Atlantic and Indo-Pacific, the four other species in the family occur in the western Indian Ocean and Indo-West Pacific (Akhilesh et al., Reference Akhilesh, Hashim, Bineesh, Shanis and Ganga2010; Ebert et al., Reference Ebert, Akhilesh, Grandcourt and Khan2017a). The genus Planonasus was introduced by Weigmann et al. (Reference Weigmann, Stehmann and Thiel2013) based on the description of P. parini, collected from depths of 560–1,120 m off the Socotra Archipelago in the Gulf of Aden. Subsequently, Planonasus indicus was described by Ebert et al. (Reference Ebert, Akhilesh and Weigmann2018) from specimens captured along the southwestern coast of India and Sri Lanka, between 200 and 1,000 m depth. Both species are morphologically similar and were initially assumed to be rare, due to the limited number of known specimens, but it should be recognized that their habitats are generally under-sampled.
The present study reports the first confirmed record of Planonasus parini within the Indian Exclusive Economic Zone (EEZ), based on two specimens collected from the southwestern coast of India. This finding significantly extends the known range of the species eastward into the Laccadive Sea. In addition, multiple specimens of P. indicus were also encountered as bycatch in a deep-sea shrimp trawl fishery in the same region. These records provide additional distributional and morphological data for both species and highlight their continued occurrence in deep-slope habitats of the Laccadive Sea.
Materials and methods
Specimens of P. parini and P. indicus examined in this study were collected from landings at Sakthikulangara Fishing Harbour, Kollam, Kerala (8°52′48″ N, 76°34′32″ E), during exploratory fishery surveys from December 2024 to March 2025. These specimens were derived from the bycatch of deep-sea shrimp trawling operations conducted by commercial vessels operating within the Indian EEZ of the Laccadive Sea, at depths ranging from 200 to 600 meters. Following landing, specimens were transported under ice to the laboratory for detailed analysis. Each individual was carefully rinsed to remove external debris, and high-resolution photographs were taken by a Nikon D3500 camera to document the morphological features. A muscle tissue sample (∼100 mg) was excised from the left lateral flank of each specimen and preserved in 95% ethanol for molecular investigations. Measurements were taken from the preserved materials using a digital Vernier calliper to the nearest 0.1 mm. Sex determination was conducted by examining the pelvic fins, where the presence of claspers indicated males. Species identification and comparative analysis were carried out using diagnostic morphological characters, with morphometric and meristic measurements recorded and verified following the methodologies and taxonomic references provided by Weigmann et al. (Reference Weigmann, Stehmann and Thiel2013) and Ebert et al. (Reference Ebert, Akhilesh and Weigmann2018). The specimens were then preserved in 10% formalin and were registered and deposited in the National Zoological Collections of Marine Biology Regional Centre, Zoological Survey of India, Chennai, Tamil Nadu, India.
Total genomic DNA was extracted from lateral muscle tissue (∼100 mg) using the Qiagen DNeasy Tissue Kit, following the manufacturer’s instructions. A partial fragment of the mitochondrial cytochrome c oxidase subunit I gene was amplified using the universal fish primers FishF1 (5′-TCAACCAACCACAAAGACATTGGCAC-3′) and FishR1 (5′-TAGACTTCTGGGTGGCCAAAGAATCA-3′) (Ward et al., Reference Ward, Zemlak, Innes, Last and Hebert2005). PCR reactions were carried out in a 25 µL volume containing 1.5 U of Taq DNA polymerase, 1 × PCR buffer, 1.5 mM MgCl₂, 0.2 mM of each dNTP, 5 pmol of each primer, and 2 µL (25–50 ng) of genomic DNA. Amplification was performed using an Eppendorf thermal cycler with the following profile: initial denaturation at 94 °C for 5 min; 35 cycles of denaturation at 94 °C for 30 s, annealing at 50 °C for 30 s, and extension at 72 °C for 1 min; followed by a final extension at 72 °C for 10 min. PCR products were visualized on 1% agarose gels and purified using the Qiagen PCR Purification Kit. Bidirectional sequencing was conducted on an ABI 377 Genetic Analyzer. Forward and reverse sequences were aligned and edited using BioEdit v7.0.5 (Hall, Reference Hall1999) and were submitted to NCBI GenBank.
Results
Systematic position
Order Carcharhiniformes
Family Pseudotriakidae
Genus Planonasus Weigmann, Stehmann & Thiel, 2013
Species Planonasus parini Weigmann, Stehmann & Thiel, 2013
Dwarf false catshark
(Figures 1, 2, 3a, 3c, 4b, 4d, 5a, 8b, 8d, 8f; Table 1)
Dorsal view of P. parini (a) ZSI/MBRC/F/3350, immature male, 521 mm TL; (b) ZSI/MBRC/F/3349, immature female, 477 mm TL.
Lateral view of P. parini (a) ZSI/MBRC/F/3350, immature male, 521 mm TL; (b) ZSI/MBRC/F/3349, immature female, 477 mm TL.
Ventral head view of (a) P. parini, ZSI/MBRC/F/3350, immature male, 521 mm TL, (b) P. indicus, ZSI/MBRC/F/3342, female, 631 mm TL; oral papillae of (c) P. parini, ZSI/MBRC/F/3350, immature male, 521 mm TL, (d) P. indicus, ZSI/MBRC/F/3342, female, 631 mm TL.
Upper teeth (a) P. indicus, ZSI/MBRC/F/3342, female, 631 mm TL, (b) P. parini, ZSI/MBRC/F/3350, immature male, 521 mm TL; lower teeth (c) P. indicus, ZSI/MBRC/F/3342, female, 631 mm TL, (d) P. parini, ZSI/MBRC/F/3350, immature male, 521 mm TL.
Morphometric characters of P. indicus and P. parini, expressed as a percentage of total length (% TL)
Table 1 Long description
Morphometric measurements for P. parini and P. indicus are listed as ranges, expressed as percentages of total length, with columns comparing a present study to earlier published samples for each species. Total length ranges indicate P. indicus specimens are larger overall, spanning about 559 to 640, while P. parini spans about 339 to 521. Precaudal length is slightly higher in P. indicus, around 79.2 to 79.7 percent, compared with roughly 74.6 to 76.1 percent in P. parini. Head and trunk proportions overlap but tend to be broader in P. indicus, with head width at mid gill slits about 13.3 to 16.1 percent versus about 10.2 to 13.3 percent in P. parini, and trunk width about 9.9 to 12.4 percent versus about 8.5 to 10.7 percent. Several fin and spacing measures show overlap between species, such as interdorsal space near 12 to 15 percent and many dorsal, pelvic, and anal fin lengths in the high single digits to mid teens. Some entries are missing for certain columns, including caudal peduncle width and girth in the present-study columns, and clasper measurements appear only in the earlier P. parini dataset. Interpretation should be cautious because sample sizes are small in all columns and some ranges are based on only two to four specimens.
Specimens
Two specimens: ZSI/MBRC/F/3349, immature female, 477 mm total length (TL), trawled off Kollam at 8°14′00.0″ N, 76°35′00.0″ E on 20 December 2024; ZSI/MBRC/F/3350, immature male, 521 mm TL, collected on 27 March 2025 from operations conducted at 8°56′00.0″ N, 75°46′00.0″ E. Both specimens were landed at Sakthikulangara Fishing Harbour, Kerala.
Diagnosis
A small deep-water shark distinguished by the presence of numerous oral papillae on the roof, tongue, and floor of the mouth (Figure 3a); absence of nictitating eyelids; dusky body colouration with a distinct white marking on the free rear tip of the first dorsal fin; broad internarial space (1.4–1.8 times nostril width); long, bell-shaped snout; and large, nearly vertical spiracles. Dorsal fins triangular, with the second dorsal fin considerably taller than the first. Dermal denticles tricuspid, loosely arranged (Figure 8b, 8d, 8f). Dentition includes ∼110 upper and ∼120 lower tooth rows, with comb-like lateral teeth in both jaws (Figure 4b, 4d); the lower teeth bear 3–4 cusps (Figure 4d). Morphometric values of the Indian specimen fall within the range of the holotype and non-type specimens from the Socotra Islands, including precaudal length (76.0–76.1% TL), head length (21.4–24.6% TL), and interdorsal space (11.9% TL).
Description
A species of Planonasus distinguished by the presence of numerous small but distinct oral papillae on the tongue, roof, and floor of the mouth (vs. absent in P. indicus); a markedly more depressed and slender body form with reduced head height (4.9% TL); narrower internarial width (3.6–3.8% TL); and narrow abdominal region (abdominal width at D1 base 4.0% TL). Upper labial furrows relatively long (1.1–1.21% TL); and trunk width at pectoral base reduced (7.9% TL). Head moderately long (head length 21.4–24.6% TL); mouth width 10.3–10.7% TL. First dorsal fin base long (12.91% TL), with slightly rounded apex; second dorsal fin similar in shape, slightly shorter; and pelvic midpoint to first dorsal fin base spacing short (11.1–11.2% TL). Body tapering, with compressed trunk and caudal peduncle. Lateral trunk denticles loosely spaced, weakly overlapping, semi-erect and directed posteriorly, each with a slender lanceolate crown bearing a pointed median cusp and minute lateral cusplets, becoming more elongate, slender, and widely spaced below the first dorsal fin (Figure 8d). Denticles on the dorsal head surface and ventral snout differ from those of the trunk, being teardrop- to leaf-shaped, posteriorly inclined, and marked by three longitudinal ridges. Denticles on the dorsal surface of the caudal peduncle are more closely packed, with a comparatively slenderer median cusp than in P. indicus (Figure 8f).
Colouration. Body dusky greyish brown with sides darker, ventral side also dusky greyish. First dorsal fin free rear tip edges marked white, which becomes slightly pale after preservation (Figure 5a). Mouth and head also greyish with nostril edges dark grey.
Lateral view of first dorsal fin (a) P. parini, ZSI/MBRC/F/3350, 521 mm TL with white spot on the free rear tip; (b) P. indicus, ZSI/MBRC/F/3342, gravid female, 631 mm TL.
Distribution
Previously known only from the northwestern Indian Ocean off the Socotra Archipelago and the Gulf of Aden. This record represents the first confirmed occurrence of P. parini within the Indian EEZ, off Kollam, southwest coast of India, significantly extending its known distribution into the Laccadive Sea (Figure 6).
Map showing the type locality of P. parini near Socotra Island in the Arabian Sea (red triangle), type locality of P. indicus at Cochin Fisheries Harbour, Kochi, India, and paratype from Muttur, Trincomalee Harbour, Sri Lanka (yellow star) and the new distributional records for P. parini from trawling sites off the southwest coast of India (orange circles), where both P. parini and P. indicus were collected together.
Planonasus indicus Ebert, Akhilesh & Weigmann, 2018 (Figures 3b, 3d, 4a, 4c, 5b, 7, 8a, 8c, 8e, 9; Table 1)
Lateral view of P. indicus. (a) ZSI/MBRC/F/3342, gravid female, 631 mm TL, (b) ZSI/MBRC/F/3341, female, 598 mm TL, (c) ZSI/MBRC/F/3341, female, 598 mm TL (d) ZSI/MBRC/F/3340, female, 559 mm TL.
Dermal denticles of P. indicus, ZSI/MBRC/F/3342, gravid female, 631 mm TL. (a) Dorsal head, (c) lateral trunk, (e) caudal peduncle (dorsal); P. parini ZSI/MBRC/F/3349, immature female, 477 mm TL. (b) Dorsal head, (d) lateral trunk, (f) caudal peduncle (anterior margin).
Biology observation of P. indicus, ZSI/MBRC/F/3341, female, 598 mm TL. (a) Stomach contents and (b) a small pup encased inside the egg case.
Specimens
Four specimens were collected as bycatch from deep-sea shrimp trawling operations off Kollam, southwest coast of India, at depths of 378–500 m (approx. 8°56′00.0″ N, 75°46′00.0″ E). ZSI/MBRC/F/3340, female, 559 mm TL, collected on 8 April 2024; ZSI/MBRC/F/3341, female, 598 mm TL; ZSI/MBRC/F/3342, female, 631 mm TL; ZSI/MBRC/F/3343, female, 582 mm TL; latter three were specimens collected on 27 March 2025. All specimens were landed at Sakthikulangara Fishing Harbour, Kerala.
Diagnosis
A species of Planonasus separable from P. parini by the absence of oral papillae on the tongue, palate, and floor of the mouth (vs. present in P. parini) (Figure 3b); uniform brownish body without a white marking on the apex of the first dorsal fin (vs. white dorsal tip present). Spiracles are proportionally longer (1.3–1.5% TL vs. 1.4% TL in P. parini), and head depth is greater (head height 7.2–10.8% TL vs. 4.9% TL). The species possesses more numerous lower jaw tooth rows (153–156 vs. ∼115–120). Upper labial furrows are shorter (0.5–1.1% TL vs. 1.1–1.2% TL). Precaudal length (80.1–80.4% TL) and prepelvic length (56.2–57.0% TL) are longer than in P. parini (76.0–76.1% TL and 53.9–54.8% TL, respectively). The anal fin is proportionally larger, with a base length of 6.9–7.2% TL (vs. 6.2% TL).
Description
Body slender, elongate, and soft. Head broad and moderately depressed; head length 24.1–25.8% TL, head height 7.2–10.8% TL. Snout broadly bell-shaped in dorsal view. Abdomen stout. Internarial width broad (3.5–6.1% TL); nostrils relatively wide (2.5–3.0% TL). Preorbital length 6.8–7.9% TL; preoral 6.5–6.9% TL. Eyes moderately large, orbit length 4.1–4.2% TL. Spiracles subcircular, large (1.3–1.5% TL long; 1.2–1.4% TL high). Mouth width 10.9–12.6% TL; upper labial furrows short (0.5–1.1% TL). Interorbital space 6.8–7.1% TL. Trunk robust, with pectoral base width 9.9–12.1% TL. Width at abdomen 8.0–8.6% TL. Prebranchial length 20.2% TL. Prepelvic length 56.2–57.0% TL; preanal 63.9–68.1% TL; precaudal 80.1–80.4% TL. First dorsal fin angular, moderately large; base length 10.7–13.9% TL; height 5.1–5.3% TL. Second dorsal fin smaller, but similarly shaped; base 12.6–14.9% TL; height 6.3–6.5% TL. Distance between dorsal fins 13.4–13.5% TL. Pelvic fins low, with base 4.9–5.8% TL. Distance from pelvic midpoint to first dorsal base 10.8–11.9% TL. P2–anal space 5.0–6.6% TL. Anal fin large, with base 6.9–7.2% TL; anterior margin 7.0–7.6% TL; height 3.0–3.2% TL. Caudal fin with long dorsal margin (19.3–20.1% TL); upper postventral margin 10.5–9.6% TL. Terminal lobe moderately long (4.1–4.8% TL). Caudal peduncle height 3.1% TL. Pectoral fins broad; anterior margin 10.7–10.9% TL; height 9.9–10.1% TL. Pelvic fins short and subrounded. Dentition includes ∼110 upper and 153–156 lower tooth rows; teeth with narrow, comb-like lateral cusps, the lower teeth predominantly bearing 4–5 cusps (Figure 4c). Denticles on the dorsal surface of the head and ventral snout differ from those of the trunk, being teardrop- to diamond-shaped and bearing a distinct median ridge. Denticles along the anterior margin of the dorsal caudal fin are slightly enlarged, up to twice the size of adjacent caudal denticles (Figure 8a, 8c, 8e).
Colouration. Body dark blackish brown, without any prominent markings; ventral side similar to dorsal; first dorsal fin without white rear tip (Figure 5b). Colouration in preservation uniform brownish-grey dorsally and paler ventrally, with all fins lacking pale or white markings.
Distribution
The holotype of P. indicus was recorded from deep waters off the southwestern coast of India near Kochi, Kerala, and one paratype from Trincomalee Harbour, Sri Lanka. The collection of the current specimens off Kollam, India, further confirms its occurrence in this area (Figure 6).
Biological observations of P. indicus. An adult female specimen measuring 631 mm TL and weighing 789 g was found to be pregnant, carrying two developing embryos encased within membranous egg cases (Figure 9b). This observation provides further evidence of the species’ reproductive mode, consistent with other members of the family Pseudotriakidae. Stomach content analysis revealed the presence of six shrimps (Figure 9a), with a stomach weight of 0.41 grams. Individual prey items weighed between 0.002 and 0.13 g. The shrimps were identified as Aristeus alcocki, suggesting a benthic feeding preference and potential trophic association with deep-sea crustacean communities.
Discussion
Two specimens of P. parini were collected from deep-water trawl operations within the Indian EEZ, marking the first confirmed occurrence of this species in Indian waters. Diagnostic features, including the presence of oral papillae and characteristic colour patterns, were consistent with the original species description by Weigmann et al. (Reference Weigmann, Stehmann and Thiel2013). Although tissue samples were obtained from two specimens of P. parini, the extracted DNA was of poor quality, and repeated amplification attempts failed to produce reliable sequences. The detection of P. parini within the Indian EEZ significantly extends its known range eastward into the Laccadive Sea, highlighting the need for further exploration of deep-sea habitats in this region. The species remains classified as ‘Least Concern’ by the IUCN Red List assessment due to its deep-water habitat, which reduces exposure to commercial fishing (Ebert et al., 2017).
Contrary to earlier assumptions of rarity, this study recorded multiple specimens of P. indicus from the southwestern Indian EEZ, particularly from commercial deep-sea shrimp trawling operations. Detailed morphological and molecular analysis confirmed these specimens as P. indicus, aligning with the diagnostic criteria outlined by Ebert et al. (Reference Ebert, Akhilesh and Weigmann2018). Genetic sequences derived from five specimens for the first time were submitted to NCBI GenBank under accession numbers PV716013-15 and PX460997-98, thereby strengthening the molecular dataset available for the genus. One of the five P. indicus sequences was derived from a damaged juvenile specimen (ca. 150 mm TL), which was unsuitable for detailed morphological examination. Additional support for its broader distribution in the region comes from Fernando et al. (Reference Fernando, Bown, Tanna, Gobiraj, Ralicki, Jockusch and Caira2019), who reported the occurrence of P. indicus in Sri Lankan waters through targeted sampling of the deep-sea longline fishery in Mutur, Trincomalee Harbour, Eastern Province. A specimen (SL-107) collected as bycatch at a depth of 640 m (350 fathoms) was subsequently designated as a paratype, providing additional evidence of P. indicus in the region, reinforcing its presence beyond the original holotype from India described by Ebert et al. (Reference Ebert, Akhilesh and Weigmann2018). Previously, Ebert et al. (2017) had reported the specimen as P. parini, but further re-examination confirmed it as P. indicus, consistent with the diagnostic criteria outlined in its formal description, further supporting the occurrence of P. indicus in Sri Lankan waters (Fernando et al., Reference Fernando, Bown, Tanna, Gobiraj, Ralicki, Jockusch and Caira2019). The IUCN currently lists Planonasus indicus as Data Deficient (Finucci et al., Reference Finucci, Bineesh, Cheok, Dharmadi and Sherman2020), due to insufficient information on its population size, distribution, ecology, and interaction with fisheries. Although historically regarded as rare, the species may be more frequently encountered than previously assumed, particularly through incidental capture in deep-water trawl fisheries targeting shrimps along the southwestern coast of India. These operations, conducted between depths of 200–1,000 m, have expanded offshore in recent years due to declining coastal resources (Akhilesh et al., Reference Akhilesh, Ganga, Pillai, Vivekanandan, Bineesh, Shanis and Hashim2011), thereby increasing the likelihood of capturing deep-sea elasmobranchs such as P. indicus. Despite its low commercial value and presumed discard status, ongoing exposure to such fisheries may pose conservation concerns. The present records of both P. indicus and P. parini from shrimp trawl bycatch suggest that these species may be more widespread and vulnerable to fishing pressure than previously documented. The discovery of P. parini within the Indian EEZ and the recurrent capture of P. indicus together underscore the need for systematic monitoring of deep-sea fisheries and targeted biological studies. These findings highlight the importance of bycatch monitoring and targeted field surveys to better understand the abundance, distribution, and population dynamics of both species, which are necessary steps towards informed management and potential reassessment of their threat status.
Acknowledgements
We sincerely thank the Director and Officer-in-Charge of the Marine Biology Regional Centre, Zoological Survey of India, for their invaluable institutional support and guidance. P.R. thanks the University of Madras, Chennai, for the support towards his doctoral research. We are also grateful to the fishermen of Kollam for their valuable support and information, which greatly contributed to the success of this research.
Author contributions
P.R.: Conceptualization (lead); data curation (equal); formal analysis (lead); investigation (lead); methodology (lead); visualization (equal); writing – original draft (lead); writing – review and editing (equal); S.B.: Data curation (supporting); investigation (supporting); visualization (supporting); writing – review and editing (supporting); A.K.: Data curation (supporting); methodology (supporting); validation (supporting); writing – review and editing (supporting); V.P.A.: Data curation (supporting); formal analysis (supporting); investigation (supporting); visualization (supporting); writing – review and editing (supporting); K.K.B.: Conceptualization (lead); supervision (lead); project administration (lead); resources (lead); validation (lead); writing – review and editing (lead).
Declaration of interest
The authors declare that they have no competing financial or non-financial interests that could have appeared to influence the work reported in this paper.
Data availability statement
The datasets generated and/or analysed during the current study are available from the corresponding author upon reasonable request.
Ethics and approach
The specimens used in this study were obtained from routine commercial fisheries landings. As no experimental procedures were conducted on live animals, ethical approval was not required.
Funding statement
We express our thanks to the Deep Ocean Mission programme, Ministry of Earth Sciences, Govt. of India, for the funding support (Project No. MoES/DOM/GIA/PAMC/01/2023).
