Short Communication

Abstract: This manuscript deals with the Bent-fin Devil Ray Mobula thurstoni for its first time occurrence in the Andaman & Nicobar waters around the Indian EEZ which is a new locality record confirming the range extension of the species to the southeast of Bay of Bengal (the Andaman Sea). A female specimen of 318mm disc length and weighing 2.47Kg was caught by a multiday bottom trawler operated off North Bay and was landed at Junglighat fishing harbour, South Andaman. A detailed diagnostic description and morphometric measurements of M. thurstoni is provided. For the first time this species has been described from Indian waters and compared with the other related species, and so documenting its first occurrence in the Andaman & Nicobar waters.

Keywords: Devil Ray, Mobulidae, Myliobatoidae, range extension.

Abbreviations: WD - disk width, DL - disk length, AP - anterior projection, Rs - rostrum, PF - pelvic fin, HL - head length, SPL - spiracle length, ED - eye diameter, TL - tail length, CF - cephalic fin, OH - orbit height, CFL - cephalic fin length, CFW - cephalic fin width.

The mobulids are large sized zooplanktivorous elasmobranchs. They are distributed circumglobally in tropical, subtropical, and warm temperate waters (Croll et al. 2016). In earlier literature these beautiful creatures were portrayed as diabolical and ferocious brutes, even though they are harmless to human beings (Saenz- Arroyo et al. 2006). Although their existence has been documented since the 17th century (Willughby & Ray 1686), information on their biology and ecology is scanty (Coutrier et al. 2012; Croll et al. 2016). Meanwhile these species are well exploited worldwide (Croll et al. 2016) for their valuable gill rakers and face intense fishing pressure, even though they are listed as Near Threatened on the IUCN Red List (Walls et al. 2016).

The family Mobulidae includes two genera, Manta and Mobula with 11 identified species (Couturier et al. 2012). The genera are separated by the position of the mouth, which is located ventrally in Mobula and terminally in Manta (Townsend & Kyne 2010). The genus Mobula is presently represented by nine known species (Notarbartolo-di-Sciara 1988). The Pygmy Devil Ray M. eregoodootenkee (Bleeker, 1959), Atlantic Devil Ray M. hypostoma (Bancroft, 1831), Spinetail Devil Ray M. japanica (Muller & Henle, 1841), Short-fin Devil Ray M. kuhlii (Muller & Henle, 1841), Giant Devil Ray M. mobular (Bonnaterre 1788), Munk’s Devil Ray M. munkiana (Notarbartolo-di-Sciara 1988), Lesser Guinean Devil Ray M. rochebrunei (Vaillant, 1979), Chilean Devil Ray M. tarapacana (Philippi, 1893), and Bentfin Devil Ray M. thurstoni (Lloyd, 1908).

The close external resemblance of many species of mobulids has proven that their identification is problematic and has led to taxonomic ambiguities (Couturier et al. 2012). Lack of holotypes to accompany the original descriptions of M. eregoodootenkee, M. kuhlii, M. mobular, M. tarapacana and M. thurstoni make taxonomic work complicated and leads to misidentification of the new reports (Polack 2011). The large size and fast swimming abilities of most mobulid species make their study difficult even at well-established aggregation sites, otherwise killing or restraining of individuals are required (Couturier et al. 2012).

Reports of mobulids from Indian waters were mainly concentrated on Manta birostris. It had been reported from Saurashtra coast, Veraval (Sivaprakash 1965; Kunjipalu et al. 1981); Nachikuppam, Madras (James 1985); Karwar, (Telang & Harikantra 1998), Tuticorin (Armurgam 2002; Rajpakkiam et al. 1997); Honawar (Rajpakkiam et al. 2007); and Maharashtra (Rane 2002). In devil rays, M. diabolus is the dominant one and it has been reported from the Gulf of Mannar (Rajpakkiam et al. 1994); Vizhinjam, (Pillai 1998); Chennai (Baby 2010); Tuticorin (Zacharia & Kanthan 2010) and Andhra Pradesh (Satish kumar et al. 2013). Information on the biology and ecology of the other species of the devil rays are meagre, even though seven species have been reported from Indian waters (Akhilesh et al. 2014). Some of these reports require confirmation too. The present study confirms the occurrence of M. thurstoni in Indian waters and also describes the Near Threatened species (Walls et al. 2016) and its range extension to the southeastern Bay of Bengal (Andaman Sea).

Materials and Methods

The present specimen was collected from the fish landing centre at Junglighat, South Andaman District (Fig. 1; Image 1) on 17 August 2016, landed by a bottom trawler having operated at Junglighat. The specimen was caught from a depth of 70m off North Bay, South Andaman District. A single female specimen of M. thurstoni was landed along with other species of mobulids like Manta birostris and Mobula diabolus. The specimen was identified following Compagno & Last (1999), photographed, and morphometric and meristic measurements were recorded. All proportional measurements are expressed as percentage of disc width (WD). The specimen was dissected; gill rakers and the lower jaw were removed for comparison and identification. The gut content was removed carefully and transferred to 4% of formalin and later was examined under binocular microscope OLYMPUS CH20i (10X magnification). The specimen was preserved in 10% formalin in the museum of the zonal base of Fishery Survey of India, Port Blair (No: MUS.FSI.PB/EB/09/2016).

Results

Order: Mylobatiformes

Family: Mobulidae (Gill, 1893)

Genus: Mobula (Rafinesque, 1810)

Species: thurstoni (Lloyd, 1908)

Specimen examined: MUS.FSI.PB/EB/09/2016, female, 17.viii.2016, disk width (WD) 624mm, disc length (DL) 318mm, weight 2.470kg.

Diagnosis

The present specimen of M. thurstoni was a moderate size with a disc width (WD) of 624mm and disc length (DL) 318mm. It had a short head bearing short cephalic fins. The tip of the dorsal fin was white in colour, the spine was absent at the base of the tail. Cephalic fins were short extending from the tip of each fin to the corner of the mouth and less than the total disc width. The white ventral markings of the rostrum did not extend above the eyes. The tail base was found dorso-ventrally compressed and close to the dorsal fin. The pectoral fins’ anterior margin had a distinctive slight double curvature. The size of the spiracle was small, sub-circular and below the margin of the pectoral fin where it meets the body. The DL is 51% of WD, HL is 17.8% of WD, SPL is 0.1% of WD, ED is 2.2 % of WD, TL is 65.1% of WD and 65.1% to DL, CFL is 12.1 % of WD, CFW is 4.6 % of WD. AP is 34.6 % WD, Rs to PF is 51% to WD, Tip of CF to mouth is 11.4% WD, OH is 2.4% to WD and mouth width is 11.2% to WD. Details of morphometric measurements of the present specimen in comparison with other reports (Notabartolo-di-Sciara 1987; Mendonca et al. 2012; Mas et al. 2015) are given in Table 1.

Colour

The dorsal surface of the body was deep blue-black in colour. The anterior portion of the ventral surface was fully white and distal half with a silver-brown sheen. The dorsal side head had a dark band stretching across the head behind the eyes.

Discussion

Mobula thurstoni has a worldwide distribution and is reported from the Pacific, Atlantic and Indian Oceans (White et al. 2006; Marshall et al. 2009; Kashiwagi et al. 2011; Walls et al. 2016). Even though the mobulids are widely distributed in the tropical, subtropical, and warm temperate oceans this is the first time M. thurstoni has been described from the EEZ of India. Hence, their geographical distribution has been extended from Southeast Asian Seas to the EEZ of Andaman & Nicobar Islands, India. Akhilesh et al. (2014) reviewed the occurrence and distribution of chondrichthyans in the Indian waters and mentioned the occurrence of seven species of Mobula. But some species like M. mobular require confirmation, further there is no report on the morphometric and meristic counts of M. thurstoni.

M. thurstoni is usually pelagic or epipelagic in shallow, productive, neritic waters of <100m depth (Notarbartolo-di-Sciara 1988; Croll et al. 2016), although it is also caught in offshore pelagic waters (Mas et al. 2015) and around seamounts in the Mid-Atlantic Ridge (Mendonça et al. 2012). The present specimen was caught by a multiday trawler that operated at a depth range of 70–150 m along the North Bay, South Andaman. Segregation by size and sex is seasonal, with all size classes and sexes appearing together during the summer months (Notarbartolo-di-Sciara 1988).

Disc Width of the present specimen was 624mm and disc length was 318mm, which is 51% to disc width. The maximum WD recorded for M. thurstoni was 1,800mm (Notabartolo-di-Sciara 1987). The maximum disc width recorded for a mobulid from Indian waters was a M. diabolus recorded at 4500mm, and an adult specimen of M. japanica recorded at 2300–2380 mm (Rekha et al. 2015). Width of disc at maturity for male and female is M: >1,500mm and F: 1,538mm respectively for M. thurstoni (Notabartolo-di-Sciara 1987; White et al. 2006) hence the present specimen with a disc width (WD) of 624mm is a female juvenile.

The rear tips of pectoral fins can be used for identification of mobulids. In the case of M. kuhlli the anterior margin of pectoral fin is not undulated whereas, in M. thurstoni it is slightly convex in the anterior margin of the pectoral fin with a distinctive undulated profile (Compagno & Last 1999). In Myliobatidae, one row of laterally expanded medial plate-like teeth is present in both the jaws. In Mobulidae, teeth are usually found only in the lower jaw, but it was abnormally present in both jaws (Compagno & Last 1999). The examined M. thurstoni had hexagonal teeth with large rugosities on crown; teeth bands present on the both jaw having pavement-like mosaic arrangement teeth (Image 1G). In M. kuhlii, however, the teeth are transversely elongated (Compagno & Last 1999). M. thurstoni had a moderately long tail (about 60% of disc width in adults), in M. japanica the tail is longer than the disc when undamaged, in M. tarapacana the tail is much shorter than the disc (Compagno & Last 1999). The present specimen also consisted of a moderately long tail 65.1% in disc width and 127.7% to the disk length. Cephalic-fin length from fin tip to mouth corner is more than 16% of disc width in M. eregoodootenkee. Cephalic-fin length is less than 16% of disc width in M. thurstoni (Compagno & Last 1999). The present specimen had a short cephalic fin which is 9.1% to the total disc length and 12.1% to the disc width (Image 1D).

A sting is present at the tail base of M. japanica. The dorsal fin white-tipped and tail longer than disc when undamaged but in the case of M. thurstoni the caudal sting was absent with a white dorsal fin tip, but the tail was short as compared to M. japanica (Compagno & Last 1999). The examined specimen also had a white-tipped dorsal fin, depressed tail base without sting and a short tail (Image 1B,C ); in the case of M. kuhlii, it is quadrangular (Compagno & Last 1999).

Gill raker plates can be used as a good identification tool for mobulids because they show a distinctive appearance. The M. tarpacana was locally called “white” due to the white nature of the gills; the other species M. japanica and M. thurstoni were called “black” due to the black colour of the gill plates (Rekha et al. 2015). In the case of M. japanica the gill rakers are uniformly dark with a whitish tip (Photo Identification Guide Manta Trust) but in M. thurstoni it is uniformly dark up to mid region and the anterior portion is pale white in colour (Image 1G).

The stomach content of M. thurstoni caught from the Gulf of California were dominated by either euphausiids or mysid shrimps, although the two types of prey were never found together in the same stomach (Notarbartolo-di-Sciara 1988). It mainly feeds on Nyctiphanes simplex during summer, when it is most abundant, and on Mysidium spp. during winter ambiguities (Couturier et al. 2012). These observations suggest that M. thurstoni is able to adapt and shift diet, according to the food dominantly available in a particular location. The present specimen’s gut was half full with a major share of Lucifer hanseni and Nanocalanus sp.

The population and population trend for M. thurstoni is so far unknown (Couturier et al. 2012). The Bentfin Devil Ray is suspected to have declined by almost 30% over three generations throughout its global range, which combined with international trade value for their gill plates, increasing domestic demand for meat, low fecundity, high intrinsic sensitivity to overexploitation (White et al. 2006; Croll et al. 2016), and the likelihood that fishing effort will increase, may affect the mobula population. All these factors may further decimate the population of mobulids if stringent conservation measures are not undertaken.

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