Pelochelys cantorii Gray 1864, commonly referred to as the Asian giant softshell turtle or Cantor's giant softshell turtle, is a rare species of freshwater turtle, widespread in South and Southeast Asia (Das 2008). Pelochelys cantorii is considered to be the only softshell turtle species from India to often enter into the sea (Nair and Badrudeen 1975; Kar and Rao 1985; Hussain 2003) and its tolerance for brackish water is historically known (Boulenger 1890). It is listed as Endangered by the International Union for Conservation of Nature Red List (Asian Turtle Trade Working Group 2000) and in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). In India, the species has the highest protection status, being listed in Schedule I of the Indian Wildlife (Protection) Act, 1972.

Within the past few decades, populations of P. cantorii have declined and become fragmented throughout the species' range (Das 2008; Xiaoyou et al. 2019) as a result of various threats, such as exploitation for the consumption of eggs and meat, fishing activities, and habitat destruction resulting from illegal sand mining, urbanization, and water pollution (Das 2008; Mahapatra et al. 2009; Palot and Radhakrishnan 2011; Melvinselvan and Nibedita 2017; Behera et al. 2019b; Xiaoyou et al. 2019). Pelochelys cantorii has been reported from 6 Indian states: Kerala, Tamil Nadu, Karnataka, Andhra Pradesh, West Bengal, and Odisha. However, over the past 5 decades, there have been few reported sightings, most of which are anecdotal records from Peninsular India. This paucity of data hinders the assessment of species status in India and thus, the planning and implementation of conservation actions where they are needed most.

Here, we present a review of the distribution of P. cantorii in India based on presence data gathered from historical records, documented sightings, and surveys conducted in the Chandragiri River, Kasaragod District, Kerala. This compilation may be useful to better focus conservation efforts within the distribution range of P. cantorii in India.

Methods: Literature Review. — A comprehensive literature review was done to summarize the distribution of P. cantorii in India. We reviewed published scientific literature (books and articles), gray literature, popular media (newspaper and television network reports), and unpublished data through personal communication. Published distribution locations were mapped using QGIS v. 2.18.3. Chitra indica and P. cantorii are morphologically similar and the identification has been confused in few historical reports (Gray 1864; Das 2008; Das and Singh 2009); therefore, when records could not be verified for taxonomic accuracy, they were excluded from the data. For example, Jerdon's record of C. indica in 1853 from the Mahe estuary was suspected to be P. cantorii by Webb (1981); thus, we considered it questionable and did not include the report in our data. Similarly, Theobald's (1876) report of P. cantorii from Allahabad might also be a misidentification because it is a well-known part of the range of C. indica. Das (1986) found no evidence of the presence of P. cantorii in the Indian Sundarbans and Das's (1987) report from Sundarbans was mentioned to be of unknown reliability and therefore was not included. We included size of the individuals (carapace length [CL] in cm) when available; however, most of the reports do not mention how the length was measured (curved CL or straight CL).

Field Surveys. — The Chandragiri River is a west-flowing river that originates from the Patti Ghat Reserve Forest of Coorg District in the Karnataka part of the Western Ghats. The total length of the river is approximately 110 km originating in Karnataka State. Surveys were conducted in an approximately 63-km river stretch between 12.54°N and 12.47°N latitude and 75.31°E and 74.98°E longitude within the state of Kerala. In Kerala, the river is called Payaswini until it meets another tributary called Karicheri Puzha at Mundakai, where it reaches the town of Kasaragod pouring into the Arabian Sea. The district receives high rainfall with an annual average of 3350 mm during the Southwest Monsoon (June to August) and partly during the Northeast Monsoon from October to December (https://kasargod.nic.in/climate/; Diju and Thamban 2006).

Visual surveys along with semistructured interviews were conducted among the local communities in 40 wards (small administrative units of Grama Panchayat) situated along the Chandragiri River. Visual surveys were conducted between May and June 2019 (premonsoon) and between November 2019 and February 2020 (postmonsoon) between 0800 and 1300 hrs. We used a stratified sampling method, dividing the river into 3-km stretches and within each stretch, we opportunistically surveyed 5 sites based on the information provided by local key informants, how feasible it was to access the river, and the level of the water (Fig. 1). At each site, a 200-m line transect was plotted along the riverbank and visual scanning of the water surface as well as the riverbanks were carried out with the help of binoculars (Nikon A211, 10 × 50) by 2 observers. Locations of turtle sightings were recorded using a Global Positioning System receiver (Garmin eTrex x20). Four hundred seventy-six person-hours were spent conducting visual surveys on the Chandragiri River.

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Social surveys using informal interviews with the local fishers were conducted in the districts of Kozhikode and Kannur around the Kuttiyadi River and the Valapattanam River where the species was last reported. A network of local fishermen and other nature enthusiasts was built in the region. Using the built network, we identified local key informants through semistructured interviews conducted for the local people residing close to the Chandragiri River and established a local alert network in the study area to obtain information regarding the turtle species and to report any incidental catches via hooks and nets. Prior to each interview, the interviewee was asked to validate the identification of the species by describing and identifying the species from a small handbook of various softshell turtles. The interviewees, in most cases, were familiar with the 3 trionychid species known to inhabit the study area, namely, Lissemys punctata, Nilssonia leithii, and P. cantorii (Jain et al., unpubl. data, 2019–2020). Whenever possible, reports of turtles were attended by members of the team and records of these sightings and/or incidental catch were documented. When possible, measurements were taken using a plastic measuring tape of up to 1-mm accuracy. For each measured individual, the following measurements were taken: carapace length (CL; length of anterior to posterior edge of the curved carapace), carapace width (CW; maximum width between margins of carapace), plastron length (PL; length of proximal to distal end of plastron), plastron width (PW; maximum width between the lateral margins of the plastron), and tail length (TL). The rostrum length (RL; length of tip of snout to the eye), snout length (SL), snout width (SW), interorbital distance (ID; shortest distance between eyes), and maximum head width (HW) were measured with a Vernier caliper (Taşkavak 1998). Geographical coordinates were also recorded for the location where the turtle was caught or seen. All the sighting reports were mapped using Google Earth Pro and QGIS v. 2.18.3. (Fig. 2B).

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Results and Discussion. — Twenty-six independent records were collated from as early as the 18th century (Table 1; Fig. 2). Most of the earlier historical records reported the species under Pelochelys bibroni, which was later described to be restricted to southern New Guinea (Webb 1995). Much of this earlier confusion with historical nomenclature and misidentification of P. cantorii has been clarified by Webb (2003) and McCord and Pritchard (2002) and was taken into account while collating these records. We report 13 new records of P. cantorii from the state of Kerala mapped in Fig. 2B.

Table 1. Reports of Pelochelys cantorii from India arranged in chronological order. NA = data not available; TN = Tamil Nadu; AP = Andhra Pradesh; KL = Kerala; OD = Odisha; WB = West Bengal; UP = Uttar Pradesh; CL = carapace length.

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Seven of our records are unpublished and were collected through personal communication (Table 1). Through the local networks established around Kuttiyadi and Valapattanam River, a sighting fitting the description of P. cantorii was reported in an upstream reach of Chandragiri River by a local person. We report 6 new sighting records of P. cantorii from the Chandragiri River (Table 2; Fig. 2B). Three of the 6 reports (S2, S5, and S6 in Table 2; Fig. 3) were bycatch records provided by local key informants. Morphometric information collected for these 3 individuals is summarized in Table 3. We used McCord and Pritchard (2002) to distinguish juveniles, subadults, and adults based on size because of similar body size of P. cantorii and Chitra sp. (Pritchard 2001). A juvenile (S3) of P. cantorii was also reported by a key informant, which was described to have “olive carapace with dark irregular and indistinct spots and a pale white plastron.” Two individuals were sighted (S1), both classified as adults, in an isolated pool area with a maximum depth of 5.35 m at the time, increasing to roughly 13 m postmonsoon. One individual, seemingly a subadult (S4), was sighted from a stretch of river with slow current and with an approximate depth of 4 m.

Table 2. New sightings of Pelochelys cantorii recorded during this study from Chandragiri River in Kerala arranged in chronological order.

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Table 3. Morphometric measurements of Pelochelys cantorii recorded during the present study.

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The sightings were made in both estuarine and riverine habitats (Fig. 4A), showing the species' tolerance of saltwater. The most estuarine occurrence report made from the study area was approximately 3 km from the river mouth and the most inland report was made approximately 35 km from the river mouth (Table 2). The records of the subadult and juvenile individuals also confirm a potential breeding population of the species in the study area. From the available data, our records appear to be the first to report the occurrence of a juvenile P. cantorii in India. However, in some reports, size and ontogenic stage of the individual was not specified. Nair and Badrudeen's (1975) record of P. cantorii showed some discrepancies in the animal size where the body disc length and width (570 and 520 mm, respectively) were much larger compared with the carapace length and width (390 and 360 mm, respectively).

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Until 1981, the occurrence of P. cantorii on the west coast was doubtful (Webb 1981) and was only confirmed in 1986 (Moll and Vijaya 1986). Since then, there have been 16 reports from Kerala, which constitutes roughly 62% of all the reports from India. Presence of the species in Karnataka state is not well documented. Sirsi (2010) mentioned that an individual of P. cantorii housed in Pilikula Biological Park in Karnataka was rescued from the Netravati River, which can be further investigated to assess the presence of this species in the state of Karnataka. Considering the new records from Chandragiri River, presence of the species a little farther north in the Netravati River is highly likely.

During the surveys in the Chandragiri River, it was observed that this species can stay underwater for long periods of time, which might be the reason for the rare sightings of the species. Based on preliminary behavioral observation in May 2019 in an isolated deep pool area, the maximum interval noted between consecutive surfacing by the turtle was 80 min (conducted between 0800 and 1400 hrs). Long intervals between two consecutive sightings of an individual might be due to several environmental or physiological parameters; however, it also validates the species' elusive behavior. Therefore, more sophisticated and specific survey methods are needed for future studies on the species. Track marks along the riverbank were also recorded during the visual surveys; tracks were 40 cm wide and were deeply impressed (Fig. 4B), and we suspect they were made by P. cantorii.

The results from the surveys show that the Chandragiri River can potentially be one stronghold for the population of P. cantorii because it is the first instance in India where multiple sightings have been made in a single river. However, long-term monitoring of this population is needed to understand the species' behavior as well as breeding and nesting ecology and to identify crucial habitat areas to formulate a conservation plan for the survival of this species in the study area.