Rhinobatos ranongensis, Last & Séret & Naylor, 2019

Last, Peter R., Séret, Bernard & Naylor, Gavin J. P., 2019, Description of Rhinobatos ranongensis sp. nov. (Rhinopristiformes: Rhinobatidae) from the Andaman Sea and Bay of Bengal with a review of its northern Indian Ocean congeners, Zootaxa 4576 (2), pp. 257-287 : 275-285

Rhinobatos ranongensis sp. nov.

Ranong Guitarfish

( Figs 8–14 ; Table 2)

urn:lsid:zoobank.org:act:B80CB45D-2AC1-4BF3-8160-DA2D88BA17C9

Holotype. CSIRO H 7861-02 ( GN 17091 ), adult male 645 mm TL, Andaman Sea , W of Maliwun, Myanmar, 10°20.97 N, 97°46.14 E, bottom trawl, 69 m depth, RV Dr Fridtjof Nansen station 173, specimen 754, collected P. Psomadakis, 25 May 2015.

Paratypes. 4 specimens. ANSP 171546 , adult male 637 mm TL, Ranong fish landing, collected from Andaman Sea , off Myanmar or western Thailand, ~ 30–40 m, 1994. H 8403-01 (formerly SA005 ), juvenile male 494 mm TL, Ranong fish landing, collected S. Arunrugstichai, 17 Jul 2014 ; H 8404-01 (formerly SA043 ), juvenile male 425 mm TL, H 8404-02 (formerly SA045 ), female 392 mm TL, Ranong fish landing, collected S. Arunrugstichai, 22 Jan 2015 .

Other material. 5 specimens (not retained). GN 16774, female 511 mm TL, 16775, immature male 503 mm TL (tissues only), Ranong fish landing, collected Andaman Sea , off Myanmar or western Thailand, W. White, 2014; GN 17066 (tissue and image), female 690 mm TL, Bay of Bengal , Myanmar, RV Dr Fridtjof Nansen, station 126, collected P. Psomadakis; GN 17070 (tissue and image), female 530 mm TL, Bay of Bengal, Myanmar, RV Dr Fridtjof Nansen, station 132, collected P. Psomadakis; GN 17105, tissue, 350 mm TL, Bay of Bengal, Myanmar, RV Dr Fridtjof Nansen, station 125, collected P. Psomadakis .

Diagnosis. A medium-sized species of the genus Rhinobatos (attaining at least 645 mm TL) distinguished by the following combination of characters in adult male: disc wedge-shaped, relatively narrow, width 31–33% TL, length 1.3–1.4 times width; snout moderately elongate, length 3.0–3.2 times interspiracular distance, 4.1–4.4 times interorbital width; orbit and spiracular openings small, orbit diameter ~1.5 times spiracle length; nostrils weakly oblique, their length 1.3–1.6 times internarial distance; mouth narrow, 5.4–6.0% TL; preoral length 6.3–6.9 times internarial distance; posterior nasal flaps broad; two spiracular folds, outermost fold slightly taller than inner fold; ridges of rostral cartilage almost parallel, converging anteriorly but not constricted medially; anterior cartilage broadly sickle shaped, truncate posteriorly; gill slits and their interspaces narrow, distance between fifth gill slits 3.0–3.2 times in ventral head length; prebranchial sensory pore system obvious, extending to just behind margin of first gill slit; postscapular sensory canal indistinct, deeply embedded, not forming shallow groove; thorn patches on supraorbit and scapular region, and row along dorsal midline rudimentary, not conspicuous; denticles on dorsal fins densest anteriorly, present but more sparse posteriorly; dorsal fins small, height of first 5.4–6.2% TL; pelvic-fin inner margin subequal to its base length; interdorsal distance 2.9–3.3 times first dorsal-fin base; dorsal caudal margin 2.0–2.4 times preventral margin; upper jaw with 68–82 tooth rows; snout angle 59–64°; 64–68 pectoral radials; 172–176 post-synarcual centra; 47–48 nasal lamellae; dorsal fins brownish anteriorly, dusky posteriorly; dorsal disc of adults largely uniformly brownish, without spots or obvious blotches; no large, black, teardrop marking on undersurface of snout.

Description. Disc of holotype wedge shaped, bluntly angular anteriorly, angle anterior to eyes 59° (60–61° in two male paratypes, 64° in female paratype); anterior margins slightly undulated, apex broadly rounded, more narrowly rounded posteriorly; length 1.35 (1.28–1.29, 1.28) times width. Pelvic fins elongate, short-based, base length 1.01 (0.93–1.16, 1.06) length of inner margin; length 1.84 (1.80–2.03, 1.91) times their base length, 2.61 (2.53–2.83, 2.71) times width; anterior margin straight to weakly concave, apex broadly rounded, posterior margin weakly convex. Tail elongate, slender, tapering gradually; in cross-section nearly flat ventrally, rounded dorsally; length from anterior cloaca to tail tip 1.50 (1.50–1.52, 1.36) times precloacal length, 1.46 (1.47–1.50, 1.35) times disc length, 5.89 (5.94–6.59, 6.07) times body width at pelvic-fin insertions; tail width 2.19 (2.52–2.59, 2.56) times depth at pelvic-fin insertions, 2.43 (2.64–2.82, 2.56) times at first dorsal-fin origin, 1.94 (2.02–2.15, 2.02) times at second dorsal-fin origin. Dermal fold ventrolateral on tail, originating near free rear tip of pelvic fin, reaching just behind ventral caudal-fin origin; fold well developed, maximum width within interdorsal space about a fifth width of spiracle.

Head elongate, ventral length 27.3 (26.6–27.4, 28.3)% TL; snout moderately elongate and bluntly pointed; preoral snout length 3.37 (3.17–3.27, 3.16) times mouth width, 6.92 (6.92, 6.32) times internarial distance, 1.39 (1.38–1.48, 1.46) times dorsal caudal-fin margin, 5.26 times distance from nostril to margin of disc; snout length (direct) 3.17 (3.00–3.11, 3.07) times interspiracular length, 4.36 (4.08–4.16, 4.16) times orbit diameter, 4.43 (4.10– 4.37, 4.22) times interorbital width; interorbital space almost flat, relatively broad; eye dorsolateral, not elevated or protruding; orbit relatively small, diameter 1.48 (1.56, 1.31) times spiracle length, 1.01 (1.00–1.05, 1.01) times interorbital width. Spiracle narrowly bean-shaped, moderately large; two compressed spiracular folds on posterior margin, innermost fold slightly shorter than outer fold, distance between bases of folds less than half length of inner fold.

Nostril moderately large, oblique, nasal flaps well developed; anterior aperture suboval, width exceeding length; nostril length 3.08 (3.21–3.53, 2.98) times anterior aperture width, 1.53 (1.50–1.73, 1.47) times anterior nasal-flap base length, 1.11 (1.03–1.13, 1.01) times distance from nostril to edge of disc, 1.45 (1.40–1.42, 1.23) times internarial width. Anterior nasal flap relatively well developed with long, slender process anteriorly; flap base 1.61 (1.63–1.82, 1.71) times its width at process, 2.00 (1.85–2.35, 2.02) times anterior aperture width; insertion in internarial space slightly mesial to nostril margin, its distance from nostril about equal to half width of anterior nasal aperture; distance between their insertions 3.82 (3.42–3.63, 3.34) in distance between lateral margins of anterior apertures, 1.12 (1.02–1.08, 1.12) in internarial width; process of flap more than twice as long as wide at its base, overlapping posteromesial edge of posterolateral nasal flap and determining inner margin of anterior aperture. Posterolateral nasal flap lobe-like, broadest medially, length 4.64 (5.02–5.25, 5.28) times width; originating lateral to lateral extremity of anterior nasal aperture, extending posteromedially as a free fold almost to mesial tip of posterior flap. Posterior nasal flap broadly lobe-like, base length 2.25 (2.20–2.41, 2.58) times its width, not reaching innermost end of nostril, inserted well forward of its posterior tip; width 0.80 (0.72–0.96, 0.66) of anterior aperture width, 1.40 (1.30–1.66, 1.24) times posterolateral nasal-flap width. Nasal lamellae 47 (47–48).

Mouth moderately broad, width 1.41 (1.51–1.53, 1.62) times nostril length, 7.39 (7.07–7.18, 7.06) in precloacal length; positioned slightly forward of hind margin of orbit; jaws not greatly thickened. Upper jaw weakly convex, upper lip arched slightly, no preoral groove; lower lip pronounced, separated from post-oral groove by a few ridges of strongly corrugated skin; short lateral grooves around corners of mouth. Teeth small, blunt, crowns rhomboidal; teeth quincuncial, 80 (68–69 in paratypes) rows in upper jaw and ~73 rows in lower jaw of holotype; upper and lower jaw teeth similar in shape and size. Gill openings s-shaped; length of third gill slit 2.23 (1.92–2.12, 2.05) in nostril length, 5.02 (4.76–5.02, 5.29) in distance between fifth gill slits; distance between first gill slits 1.43 (1.38–1.44, 1.40) times distance between fifth gill slits; distance between fifth gill slits 3.28 (3.30–3.54, 3.17) times internarial distance, 1.60 (1.56–1.62, 1.59) times mouth width, 3.16 (2.96–3.11, 2.97) in ventral head length.

Dorsal fins small, relatively low, raked slightly, relatively narrow, triangular, apices narrowly rounded to almost angular; anterior margins weakly convex, posterior margins weakly concave to nearly straight; free rear tips forming right angle, not produced; first dorsal-fin slightly taller than second; length of first dorsal fin 1.10 (1.08– 1.18, 1.21) times its height, base length 1.72 (1.49–1.59, 1.60) times inner margin length; second dorsal-fin length 1.20 (1.34–1.38, 1.42) times its height, base length 2.05 (1.94–1.97, 2.11) times inner margin length. First dorsalfin origin well behind pelvic-fin rear tip, interspace 0.74 (0.91–0.97, 0.94) times interdorsal distance; interdorsal space relatively short, 2.50 (2.48–2.63, 2.50) times second dorsal-fin height, 3.26 (2.94–3.19, 2.89) times base length of first dorsal-fin, 1.78 (1.46–1.63, 1.49) times interspace between second dorsal-fin insertion and upper origin of caudal fin, 1.41 (1.30–1.50, 1.46) times tail width at first dorsal-fin origin. Caudal fin small, dorsal caudal margin 2.41 (2.04–2.43, 2.12) times preventral margin length. Clasper of adult male holotype slender, short, inner length 13.8% TL; tip acute, glans weakly expanded.

Dermal denticles minute, close-set, covering entire body and fins; thorns and tubercles absent; dorsal surface with narrow series of barely enlarged, minute denticles around orbit, those along midline, and on scapular region seed-shaped with irregularly notched bases (barely discernable without magnification); around orbit, enlarged denticles most pronounced anteriorly at preorbit and posteriorly, with posterior patch extending over upper spiracle margin; along midline, in a single widely spaced row and weakly represented between dorsal fins; dorsal surface of claspers naked at tip and near pelvic-fin insertions. Ventral surface uniformly covered with minute denticles, including upper lip edges, near insertion of anterior nasal flap, and most of claspers; nasal region below nasal flaps and tail beside pelvic fins partly naked; a dense covering of small denticles over nasal lamellae.

Prebranchial sensory pore system relatively narrow, extending posteriorly to level of first gill slit. Postscapular sensory canal indistinct, deeply embedded, not forming a shallow groove.

Rostral cartilage in holotype long and broad, its shaft not increasing in width in a posterior direction; rostral node broadly expanded and elongate, rounded apically, not angular, axis at widest part of node 13.3% of length of rostral cartilage from snout tip; precerebral fontanelle broad and convex posteriorly with a small median notch, pointed anteriorly at rostral node; rostral cartilage length about 64% of length of neurocranium, ventral edges of rostral cartilage united; nasal capsules large, their transverse axes anterolaterally directed; maximum width across capsules 1.29 times nasobasal length of cranium (base of rostrum to occipital condyles); nasal capsule width subequal to length; basal plate minimum width 4.56 times in nasobasal length; cranial roof lacking a posterior fenestra behind precerebral cavity; anterior cartilage broadly sickle shaped, truncate posteriorly.

Pectoral skeleton with 31 (30–33, n=3) propterygial, 7 (6–9) mesopterygial, 2 (1–2) neopterygial, 26 (24–25) metapterygial, 66 (64–68) total radials; anterior radials of propterygium extending forward of nasal capsules by about 21% of rostral length. Total pelvic radials 1+25 (1+24); first greatly enlarged, on puboischiadic bar; clasper calcified. Vertebral column with 185 (185–191) total centra (synarcual and free), 172 (174–176) post-synarcual centra; 13 (11–15) synarcual centra; 25 (25–27) monospondylous centra, most centra with ribs; 102 (108–111) diplospondylous precaudal centra, about 45 (38–43) diplospondylous caudal centra.

Colour. In preservative: Holotype mainly plain brown dorsally, lacking obvious pattern of spots or blotches; rostral cartilage strongly demarcated from paler yellowish lateral snout; dorsal disc and pelvic-fin apices and posterior margins, caudal fin, anterior parts of dorsal fins, and clasper tip paler yellowish; posterior parts of dorsal fins dusky; fontanelle of rostral cartilage and preorbit paler than areas adjacent. Ventral surface creamish, snout tip lacking dark blotches; small, symmetrical black patches present beneath anterior nasal and lateral nasal flaps; posterior margins of pectoral and pelvic fins with irregular dusky margins; small asymmetrical dark blotches beneath gill slits. Fresh (based on holotype and discarded specimen, RV Dr Fridtjof Nansen station 132): Dorsal surface of body plain, medium brownish without obvious spots or blotches (sometimes faint); paler parts of snout and on outer parts of disc, and pelvic, dorsal and caudal fins evident but much less obvious than in preserved holotype; posterior parts of dorsal fins distinctly more dusky than anterior parts. Ventral surface largely uniformly white; no dark tip on snout apex; snout beside rostral cartilage and apices and posterior margins of disc and pelvic fins darker, yellowish green.

Size. The holotype (645 mm TL) and largest paratype (637 mm TL), are both adult males; smaller males 494 mm and 503 mm TL were immature. The largest female measured was 511 mm TL and was most probably at a late adolescent or early mature stage of development.

Etymology. Epithet based on the location, Ranong fish landing (western Thailand), from where the first specimen was collected 25 years ago, and subsequently most of the types.

Molecular analysis. The Maximum Likelihood analysis of the aligned NADH2 sequences suggests that Rhinobatos ranongensis represents a lineage that is distinct from 13 other species of Rhinobatos ( Fig. 15 ). Rhinobatos ranongensis appears to be most closely related to R. jimbaranensis (Bali, Indonesia), and these two species, in turn, are sister to a group containing R. sainsbury (northwestern Australia) and a recently described guitarfish from Papua New Guinea, R. manai . The immediate sister group to this R. ranongensis , R. jimbaranensis , R. sainsburyi , R. manai clade is made up of two other recently described species from Borneo, R. borneensis , and the Philippines, R. whitei . This group is in turn sister to a distinct clade containing three morphologically different guitarfishes, R. lionotus , R. penggali and R. annandalei . These clades are in turn sister to highly divergent clade containing two subgroups: R. punctifer and R. holcorhynchus (western Indian Ocean) and R. schlegelii and R. hynnicephelus (western North Pacific). The type species of the genus Rhinobatos , R. Rhinobatos , from the eastern Atlantic and Mediterranean Sea, is basal to these clades with species from the Indo-Pacific While this inference appears robust, we caution that it is based on a single mitochondrial marker. Inclusion of multiple nuclear markers could well affect both the estimated relationships among the species and the associated branch lengths on the tree.

Remarks. In the northern Indian Ocean, Rhinobatos ranongensis is most similar morphometrically to the allopatric R. punctifer (western sector); amongst the morphs of R. punctifer it most closely resembles the plain morph in coloration (which appears to be most common off Pakistan). The adult male holotype of R. ranongensis differs from adult males of R. punctifer (n=6) in several key measurements: longer proportions about the head (e.g. ventral head length 27.3% TL vs. 25.7–26.6% TL in R. punctifer , snout length 15.4% TL vs. 13.1–14.4% TL, prenarial distance 14.3% TL vs. 11.6–12.7% TL); slightly smaller proportions in the orbito-spiracular region (e.g. orbit and spiracle length 4.6% TL vs. 4.8–5.1% TL, interspiracular width 4.9% TL vs. 5.0–5.3% TL); smaller fins (e.g. first dorsal-fin length 6.5% TL vs. 6.6–7.6% TL, first dorsal-fin height 5.9% TL vs. 6.7–8.0% TL, first dorsalfin base length 4.2% TL vs. 4.4–5.4% TL, second dorsal-fin length 6.6% TL vs. 6.6–7.2% TL, caudal-fin dorsal margin length 13.1% TL vs. 14.1–15.5% TL), and possibly a marginally thinner trunk (body depth at first dorsalfin origin 4.0% TL vs. 4.1–4.5% TL, at second dorsal-fin origin 2.5% TL vs. 2.6–2.8% TL). These differences are similarly reflected in values for the smaller paratypes of R. ranongensis .

Rhinobatos ranongensis is readily distinguishable from other northern Indian Ocean species, R. annandalei and R. lionotus , with which it is sympatric in the eastern sector, in having a much narrower (maximum disc width 30.5–33.3% TL for types vs. 38.0–44.1% TL R. annandalei and 34.4–36.9% TL in R. lionotus ) and longer disc (length 40.1–42.7% TL vs. 44.9–47.8% TL R. annandalei and 44.1–44.4% TL in R. lionotus ). Rhinobatos ranongensis also lacks the symmetrically arranged pattern of white spots evident in R. annandalei .

As discussed above, based on molecular data ( Fig. 15 ), R. ranongensis appears to be most closely allied to R. jimbaranensis from Indonesia. Rhinobatos ranongensis has smaller measurements around the head (e.g. ventral head length 26.6–28.3% TL in types vs. 28.3–29.8% TL in R. jimbaranensis , snout length 15.2–15.8% TL vs. 17.0–17.8% TL, prenarial distance 13.9–14.8% TL vs. 15.8–16.8% TL); a lower first dorsal fin (height 5.4–6.2% TL vs. 6.4–7.3% TL), slightly shorter disc (length 40.1–42.7% TL vs. 42.8–44.8% TL), slightly larger nostrils (length 3.6–3.8% TL vs. 3.5% TL in 5 specimens), and larger gill openings (length of third 1.7–1.9% TL vs. 1.3– 1.5% TL). R. jimbaranensis is covered with faint rusty brown blotches on the dorsal surface when fresh (vs. plain).

Of the other allopatric species clustering close to R. ranongensis in Fig. 15 , R sainsburyi often has a pronounced blotched pattern on the dorsal surface, slightly shorter snout (length 13.1–14.7% TL vs. 15.2–15.8% TL in R. ranongensis ) and taller dorsal fins (first dorsal-fin height 7.6–7.9% TL vs. 5.4–6.2% TL, second dorsal-fin height 6.6–7.2% TL vs. 4.8–5.5% TL); R. manai has a distinctive dorsal colour pattern of small of rusty brown spots and whitish spots with greyish edges (absent in R. ranongensis ); R. whitei is very similar morphometrically (differing slightly in relative eye size and mouth width) but is usually blotched with whitish spots on the dorsal surface; and R. borneensis , which is also very similar morphometrically, has taller dorsal fins in adult males (first dorsal-fin height 7.1–7.3% TL vs. 5.4–6.2% TL, second dorsal-fin height 5.9–6.0% TL vs. 4.8–5.5% TL) and the dorsal surface is covered with orange-brown blotches when fresh.

The newly described Rhinobatos austini ( Ebert & Gon 2017) and R. holcorhynchus , a sympatric congener from the southwestern Indian Ocean, both have a prominent teardrop-shaped, dark blotch on the ventral surface of their snout (only otherwise possessed by R. schlegelii in the Indo-Pacific in young and often obscure in adults, Ebert & Gon, 2017). Rhinobatos austini also has a distinctive pattern of paired spots with some forming darker transverse bands across its back, more nasal lamellae (61–63 vs. 47–48 in R. ranongensis ), and slightly more vertebral centra (193–198 vs. 185–191 total segments). The head cartilages and chondrocranium differ significantly between the two species with R. ranongensis having a much larger and more elongate rostral node, the cranial roof lacking a posterior fenestra, anterior cartilage more broadly sickle shaped, and stronger jaw cartilages.

Rhinobatos nudidorsalis , a poorly known guitarfish from the central Indian Ocean, is similar in body shape but can be distinguished from R. ranongensis in its squamation; the tail and unpaired fins of R. nudidorsalis are almost entirely naked and there is a small, oval-shaped, denticle patch across a naked, upper caudal peduncle and caudalfin lobe.

Figures

FIGURE 8. Dorsal views of Rhinobatos ranongensis sp. nov.: A. adult male holotype, 645 mm TL (CSIRO H 7861-02), freshly caught; B. adult male paratype, 637 mm TL (ANSP 171546), preserved.

FIGURE 9. Rhinobatos ranongensis sp. nov.: RV Dr Fridtjof Nansen station 132 (fresh, not retained), female 530 mm TL, Bay of Bengal, Myanmar, A. dorsal view, B. ventral view.

FIGURE 10. View of dorsal head (preserved): A. Rhinobatos ranongensis sp. nov., adult male holotype 645 mm TL (CSIRO H 7861-02); B. Rhinobatos punctifer, female 570 mm TL (MNHN 2014-0156); C. Rhinobatos annandalei, female 870 mm TL (CSIRO H 7866-01); and D. Rhinobatos lionotus, female 760 mm TL (CSIRO H 7867-01).

FIGURE 11. Views of oronasal region (preserved): A. Rhinobatos ranongensis sp. nov., adult male holotype 645 mm TL (CSIRO H 7861-02); B. Rhinobatos punctifer, female 570 mm TL (MNHN 2014-0156); C. Rhinobatos annandalei, female 870 mm TL (CSIRO H 7866-01); and D. Rhinobatos lionotus, female 760 mm TL (CSIRO H 7867-01).

FIGURE 12. Dorsal view of skin at scapulocoracoids of: A. Rhinobatos ranongensis sp. nov., adult male holotype 645 mm TL (CSIRO H 7861-02); B. Rhinobatos punctifer, adult male 643 mm TL (MNHN 2014-0157); C. Rhinobatos annandalei female 870 mm TL (CSIRO H 7867-01); and D. Rhinobatos lionotus female 760 mm TL (CSIRO H 7866-01).

FIGURE 13. Radiograph of the chondrocranium of Rhinobatos ranongensis sp.nov., preserved adult male holotype CSIRO H 7861-02, 645 mm TL.

FIGURE 14. Lateral views of the unpaired fins of Rhinobatos ranongensis sp. nov., preserved adult male holotype, 645 mm TL (CSIRO H 7861-02): A. first dorsal fin; B. second dorsal fin; and C. caudal fin.

FIGURE 15. Maximum Likelihood (GTR+I+G) tree derived from aligned mitochondrial NADH2 sequences (1044 sites).