Rhinobatos lionotus Norman 1926
Creators: name: Last, Peter R.
name: Séret, Bernard
name: Naylor, Gavin J. P.
Publisher: name: Zenodo
Year: 2019
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Elasmobranchii
Rajiformes
Rhinobatidae
Rhinobatos
Rhinobatos lionotus

description: Rhinobatos lionotus Norman 1926
Smoothback Guitarfish
(Figs 4, 5, 10–12; Table 2)
Rhinobatis schlegelii (nec Muller & Henle): Annandale, 1909, Memoirs Indian Museum 2, 15.
Rhinobatus lionotus Norman, 1926, Proc. Zool. Soc. London 1926 (62): 961, fig. 14.
Rhinobatos lionotus: Fowler, 1941, 310; Akhilesh et al. 2014, Checklist Chondrichthyans India: 115-118 (listed).
Material examined. Holotype, BMNH 1909.7.12.3 [ex ZSI], female 495 mm TL, mouth of Hooghly River, India, ~ 70 m depth; CSIRO H 7867-01, female 760 mm TL, Bay of Bengal, N of Andaman Is, Myanmar, 15°01.63’ N, 93°45.55’ E, bottom trawl, 76 m depth, RV Dr Fridtjof Nansen station 60, specimen 247, collected P. Psomadakis, 9 May 2015.
Diagnosis. A medium-sized species of the genus Rhinobatos (attaining at least 760 mm TL) distinguished by the following combination of characters: disc wedge-shaped, rather large, width 35–37% TL, length 1.2–1.3 times width; snout relatively short, length 2.5–2.7 times interspiracular distance, ~3.4 times interorbital width; orbit diameter 1.2–1.4 times spiracle length; nostrils moderately oblique, their length 1.3–1.5 times internarial distance; mouth width 6.3–6.9% TL; preoral length 5.5–5.9 times internarial distance; posterior nasal flaps broad; two spiracular folds, outermost fold subequal to marginally taller than inner fold; ridges of rostral cartilage almost parallel, widely separated, converging slightly anteriorly but not constricted medially; anterior cartilage weakly sickle shaped, almost truncate posteriorly; distance between fifth gill slits 2.5–2.7 times in ventral head length; prebranchial sensory pore system obvious, extending posteriorly to margin of first gill slit; postscapular sensory canal notched, with exposed lateral pores; weak thorn patches on supraorbit and scapular region, and row of rudimentary thorns (not conspicuous) along dorsal midline; denticles on dorsal fins densest anteriorly, more sparse posteriorly; dorsal fins small, height of first 7.7–8.1% TL; pelvic-fin inner margin subequal to or shorter than its base length; interdorsal distance 2.3–2.9 times first dorsal-fin base; dorsal caudal margin 1.9–2.0 times preventral margin; upper jaw with ~118 tooth rows; snout angle ~61°; 68–72 pectoral radials; ~164 post-synarcual centra; ~49 nasal lamellae; dorsal fins brownish anteriorly, dusky posteriorly and apically; dorsal disc of adults uniformly brown to greenish, with or without large, regularly spaced cloudy blotches (faint orange to dusky, more or less symmetrically arranged and of irregular sizes); ventral disc and tail pale; no large, black, teardrop marking on undersurface of snout.
Distribution. Off West Bengal in the Bay of Bengal (Hoogly River mouth) to the northwestern Andaman Sea (off Pathein Peninsula, Myanmar). Possibly more widespread in the Bay of Bengal. Benthic inshore on continental shelf to at least 76 m depth.
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Remarks. Rhinobatos lionotus remains poorly known and more material is needed to adequately define the species and determine its geographic range. However, data and images from the recently acquired specimen from Myanmar (kindly made available for study by P. Psomadakis), are congruent with Norman’s holotype and have helped immensely. Sequence, meristic and morphometric data suggest this species is most closely allied to R. annandalei and R. pengalli (Indonesia); all three species are characterised by a large, broad disc (width 38–44% TL in R. annandalei, 35–37% TL in R. lionotus, 36–37% TL in R. pengalli), broad mouth (width 7.1–8.0% TL in R. annandalei, 6.3–6.9% TL in R. lionotus, 6.0–6.7% TL in R. pengalli), and relatively low post-synarcual vertebral counts (~158 centra in R. annandalei, ~ 164 in R. lionotus, and 166–170 in R. pengalli). The two other Rhinobatos species in the Northern Indian Ocean, R. punctifer and R. ranongensis sp. nov., have a narrower disc (width 29– 34% TL) and mouth (width 5.4–6.0% TL), and more post-synarcual centra (172–184).
The newly examined material fit Norman’s descriptions of R. lionotus and R. annandalei well despite the absence of information on colour in preservation in the original descriptions. Rhinobatos lionotus differs from R. annandalei, in lacking symmetrically arranged white spots (being uniformly coloured or with faint or well-defined dark blotches) on the dorsal surface. However, squamation is particularly useful for distinguishing these species; Norman noticed the absence of a median row of short thorns in R. lionotus and their presence in R. annandalei (evident in his text figs. 13 and 14 and in our material). These species occur sympatrically at their type locality (Hooghly River mouth, Bay of Bengal) and presumably elsewhere.
Annandale (1909) confused Rhinobatos lionotus with Rhinobatos (as Rhinobatis) schlegelii based on several adult specimens taken from the type locality. A somewhat similar guitarfish, Rhinobatos schlegelii, is confined to the western North Pacific from Japan to Taiwan (Séret et al., 2016, Séret & Last, unpublished data). Annandale described the colour of these adult R. lionotus as uniform brownish grey to olive green, but noted that unborn young had ‘numerous faint white spots all over the dorsal surface ...’. His reference to ‘all over the dorsal surface’ suggests the spots were densely arranged (unlike the regular pattern of sparse spots in R. annandalei). Annandale also made no reference to white spots in adults, so if these unborn were young of R. lionotus, presumably the white spots are an ontogenetic feature that is lost as the fish grows. More material of the young of both species is needed to shed light on this issue.
description: Published as part of 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, pp. 257-287 in Zootaxa 4576 (2) on pages 265-270, DOI: 10.11646/zootaxa.4576.2.3, http://zenodo.org/record/2624894