Sinobatis andamanensis, Last, Peter R. & Bussarawit, Somchai, 2016

Last, Peter R. & Bussarawit, Somchai, 2016, A new legskate, Sinobatis andamanensis (Rajiformes: Anacanthobatidae), from the Andaman Sea (northeastern Indian Ocean), Zootaxa 4168 (1), pp. 161-170 : 162-170

Sinobatis andamanensis sp. nov.

Figs. 1–7 ; Table 1

Holotype. PMBC 27938 (formerly 19478–5), adult male, 379 mm TL (193 mm DW), RV ‘ Chakratong Tongyai’, stn G8, off Phuket, Andaman Sea (8°00’ N, 97°06’ E), 508–518 m depth, 20 Nov 1999.

Paratypes. 5 specimens: PMBC 27939 (formerly 19478–1), adolescent male 328 mm TL, CSIRO H 8041-01 (19478–2), adolescent male 340 mm TL, PMBC 27940 (19478–3), adult male 351 mm TL, PMBC 27941 (19478– 4), juvenile male 276 mm TL, PMBC 27942 (19478–6), female 429 mm TL, all same data as holotype .

Diagnosis. Small Sinobatis with the following combination of characters: relatively narrow disc (width at anterior orbit 3.7–4.9 times mouth width); very long snout with slightly bulbous tip and a short, filament; moderately elongate tail (postcloacal length 127–157% of disc length); medium-sized eyes, orbit diameter 6.6–10.3 in horizontal snout and 8.2–12.8 in head lengths respectively; distal portion of tail not expanded laterally; moderate-sized pelvic fins, anterior lobe 14.3–16.1% TL; anterior pelvic-fin lobe narrow-based, width 2.7–5.0 in distance between pelvic-fin origins; caudal fin long-based and very low; 2 3–29 tooth rows in each jaw; teeth of adult males with short subconical cusps; pectoral-fin radials 66–72; abdominal vertebral centra 26, total centra to caudal fin origin ~125–131; dorsal and ventral surfaces both bluish when fresh, skin on dorsal surface darker and opaque, almost transparent ventrally.

Description. Disc extremely depressed, somewhat heart-shaped, strongly produced anteriorly; disc width of holotype 1.18 (male paratypes 1.08–1.17, female paratype 1.26) in its length from base of rostral filament, axis of maximum width at 58.6% (55.4–62.5%, 64.4%) disc length; lateral and inner margins broadly rounded. Tail moderately elongate, very slender and barely tapering anteriorly (almost uniform in largest female paratype); almost rounded in cross section at mid-length, then becoming noticeably depressed posteriorly; compressed and tapering to a point at tip; tail shorter than trunk, cloaca to tip of tail 1.23 (1.18–1.24, 1.46) in snout-cloaca (base of rostral filament), 1.16 (1.17–1.22, 1.25) in disc width, 1.36 (1.27–1.42, 1.57) in disc length; no lateral tail folds. Caudal fin small, very low, about equal to interspiracular width, upper lobe distinctly longer than lower lobe. No dorsal fins.

Head long, dorsal length 29.0% (27.9–29.9%, 34.7%) TL, ventral length 36.9% (35.8–36.8%, 41.1%) TL. Snout very long, its preorbital length 7.54 (6.95–7.87, 9.32) times interorbital distance, 23.1% (22.3–22.9%, 27.8%) TL; snout angle forward of spiracles ~67° (64–71°); tip slightly bulbous with a short, filamentous protuberance (more obviously thickened in largest female paratype). Orbits medium-sized, horizontal length about 7.81 (6.57–7.51, 10.28) in snout length, ~1.04 (0.84–1.04, 1.10) in interorbital distance. Spiracles small, mostly less than half length of orbits, subcircular to ovoid; combined orbit and spiracle length 4.70 (3.15–4.70%, 4.52%) TL. Mouth strongly convex in mature males, less so in female and adolescent male paratypes; upper jaw strongly concave near symphysis; width 3.75 (3.48–4.54, 5.36) in preoral length, 5.58 (5.32–6.18, 6.95) in ventral head length. Teeth of adult males with slightly elevated subcircular crowns; single cusp on each crown of holotype, short, broadly triangular; cusps typically shorter than diameter of crown, longest near symphysis, very short near corners of mouth; lingually directed near symphysis, directed almost laterally from about eight rows either side of symphysis; females and juveniles with oval crowns, each with a short posterior cusp. Nasal flaps small, lobe-like, weakly fringed, confined to anterior lateral margins of nostril; nasal curtain posterior margin with fine, mostly simple fleshy filaments, overlapping corners of mouth. Rostral cartilages compressed anteriorly, strongly curved (broken in holotype), tapering to snout apex.

Upper and lower surfaces of disc and tail entirely naked except for alar thorns in mature male; holotype with 3 rows (innermost row shortest) of delicate, strongly recurved, non-retractable alar thorns (16–17) on each pectoral fin; adult male paratype with 4 rows of alar thorns (16–18) on each pectoral fin; alar thorn patches located well rearward of apices of disc; length of alar patches about equal to combined lengths of eye and spiracle, width about equal to orbit length; mucous pores around spiracle and eye indistinct, white-edged and typically arranged in two rows along disc midline; no fleshy papillae on dorsal surface; dorsal skin thin, flabby, semi-translucent, not especially deciduous, skin similar ventrally but much more transparent.

Anterior margin of disc strongly concave, becoming more constricted about half way along snout, greatest width just behind spiracle (more posterior in largest female); interspiracular distance 5.26 (5.06–5.61, 6.58) in width at anterior margin of orbit; posterior pectoral-fin margins strongly convex, pectoral axil of holotype fused to dorsolateral margin of posterior pelvic-fin lobe slightly posterior to insertion of anterior lobe; in females and juvenile male, junction more posterior, pectoral fin fused mid-laterally on posterior pelvic-fin lobe. Anterior lobes of pelvic fins rather slender, leg-like, well separated from posterior lobes. Posterior lobes narrowly rectangular in males, length 1.13 (1.11–1.74, 1.19) in anterior lobe length; their inner margins fused with tail slightly forward of the distal tip in holotype (almost completely fused along entire length with base of tail in female and juvenile paratypes).

Claspers well developed (postcloacal length about ~15% TL when mature); broad initially, much thicker than base of tail, becoming more slender distally; glans about third length of clasper inner margin, everted in holotype; rhipidion present; shield narrow laterally; sentinel simple, slender, very elongate with flattened slightly, its tip strongly recurved; spike simple, slender (shorter than sentinel) with long scythe-shaped tip; sentinel capable of strong anteromedial rotation (almost forming 180° angle with spike when clasper everted); internal clasper cartilages not dissected.

Teeth in upper jaw ~28 (23–28, n=2); in lower jaw ~25 (~24, n=2). Vertebrae (n=3): monospondylous centra 26 (26), diplospondylous centra ~105 (99–105), total centra ~131 (125–131). Pectoral fin: propterygial radials: 22 (24–26), mesopterygial radials 19 (18–19), metapterygial radials 25–27 (27), total radials 66–68 (69–72). Pelvic fin in males (n=3): 3 (3) + 11 (11) radials.

Colour. Fresh types: Upper surface of disc uniformly dark blue; ventrally almost transparent pale bluish pink, darker around branchial region. Preserved holotype: Upper surface of disc and anterior tail medium brownish; posterior disc margin slightly darker; whitish patches where skin removed; distinctly darker brown on free rear tips of disc and on margins of posterior pelvic-fins lobe; alar thorn patch and orbital membrane darker than disc; orbitospiracular pores dark edged; claspers and anterior lobe of pelvic fin whitish; eyes bluish black, visible beneath skin dorsally. Ventral surface of disc and tail uniformly bluish white, skin very translucent.

Size. Attains at least 429 mm TL; two males mature at 351, 379 mm TL, still adolescent at 238, 340 mm TL.

Distribution. Continental slope in the Andaman Sea off Phuket (western Thailand) at 508–518 m depths. Probably more widely distributed in the eastern Indian Ocean, but likely to have remained undetected because of the limited sampling effort in this region.

Etymology. Epithet based on regional locality of the type series, the Andaman Sea. Suggested vernacular name: Andaman Legskate.

Remarks. A distinctive species of Sinobatis and the only legskate known from the northern Indian Ocean. It has an especially long and narrowly pointed snout with its preorbital length exceeding 23% TL (vs. typically less than 23% TL, but 28–29% TL in S. brevicauda ; Weigmann & Stehmann, 2016) and 7–9.5 times interorbital distance (vs. up to 6.7 times in other Sinobatis species). Within Sinobatis , it is somewhat similar to S. caerulea in having a bluish dorsal and ventral coloration when fresh and a long ventral head (length 36–41% TL in S. andamanensis and 38–42% TL in S. caerulea ). However, S. andamanensis appears to be a much smaller legskate (males mature at 186 and 193 mm DW vs. males still immature at 540 mm DW in S. caerulea ), the disc is narrower (width 50–56% vs. 57–63% TL), the interspiracular distance narrower (width 4.4–5.6% vs. 5.8–6.6% TL), and the anterior pelvic-fin base is less broad (width between fin bases 2.7–5 vs. 1.7–2.2 times width of anterior pelvic-fin lobe at its base).

Sinobatis andamanensis and S. bulbicauda each have a slightly expanded posterior tail, and differ from S. melanosoma which has a dark dorsal and ventral disc (rather than being darker dorsally than ventrally) and a ratlike tail without a lateral distal expansion. Sinobatis andamanensis differs from S. bulbicauda in having a relatively narrow disc (width at anterior orbit 3.7–4.9 vs. 5.2–7.8 times mouth width), narrower-based anterior pelvic-fin lobes (width 2.7–5.0 vs. 1.7–3.2 in distance between pelvic-fin origins), shorter and less-conical tooth cusps, and fewer vertebrae (total centra 125–131 vs. 148–171).

Figures

FIGURE 1. Sinobatis andamanensis sp. nov., holotype, PMBC 19556, 379 mm TL, adult male (preserved): A, dorsal surface; B, ventral surface.

FIGURE 2. Sinobatis andamanensis sp. nov., paratype, PMBC 19553, 340 mm TL, early adolescent male (preserved): dorsal disc.

FIGURE 3. Oronasal region and tooth bands of Sinobatis andamanensis sp. nov.: A, holotype, PMBC 19556, 379 mm TL, adult male (preserved); B, paratype, PMBC 19553, 340 mm TL, early adolescent male (preserved).

FIGURE 4. Sinobatis andamanensis sp. nov., holotype, PMBC 19556, 379 mm TL, adult male (preserved): ventral view of pelvic fins and claspers.

FIGURE 5. Mid-tail of Sinobatis andamanensis sp. nov., holotype, PMBC 19556, 379 mm TL, adult male (preserved): A, dorsal view; B, ventral view.

FIGURE 6. Radiograph of the head of Sinobatis andamanensis sp. nov., holotype, PMBC 19556, 379 mm TL, adult male.

FIGURE 7. Everted left clasper glans of Sinobatis andamanensis sp. nov., holotype, PMBC 19556, 379 mm TL, adult male (lateral view). Clasper components abbreviated as: cf—cleft, rh—rhipidion, sp—spike, and st—sentinel.

Tables

TABLE 1. Morphometric data for Sinobatis andamanensis sp. nov.: adult male holotype (PMBC 19556), female paratype (PMBC 19557), and ranges for the four male paratypes. Measurements expressed as percentages of total length.

	Holotype	Paratypes (male)	Paratype (female)
		Range (n=4)
Total length (mm)	379.0	276.0 351.0	429.0
Disc width	50.9	52.9 55.5	49.9
Disc length	59.9	57.2 61.8	63.0
Distance snout to pectoral-fin insertion	41.2	41.9 45.0	47.3
Distance snout to axis of maximum width	35.1	34.2 36.6	40.6
Disc width at anterior margin of orbits	26.0	25.5 28.3	29.2
Snout length (preorbital direct)	23.5	22.6 23.3	28.3
Snout length (preorbital horizontal)	23.1	22.3 22.9	27.8
Prespiracular length	26.2	26.7 27.3	31.5
Head length (dorsal)	29.0	27.9 29.9	34.7
Orbit diameter	3.0	3.0 3.4	2.7
Orbit and spiracle length	4.7	3.2 4.7	4.5
Spiracle length	1.2	1.3 1.7	1.6
Distance between orbits	3.1	2.9 3.3	3.0
Distance between spiracles	4.9	4.9 5.6	4.4
Distance snout to cloaca	54.1	52.1 54.7	58.4
Distance cloaca to caudal fin tip	44.1	43.6 45.9	40.0
Preoral length	24.8	23.9 26.3	31.7
Prenasal length	22.5	21.5 23.2	28.6
Head length (ventral)	36.9	35.8 36.8	41.1
Mouth width	6.6	5.8 6.9	5.9
Distance between nostrils	5.4	4.7 5.0	4.6
Distance nare to mouth	3.0	3.0 3.1	2.7
Nasal curtain length	3.4	2.8 3.6	2.8
Nasal curtain total width	7.3	5.9 7.4	6.4
Nasal curtain lobe width	1.7	1.7 1.8	1.6
Width first gill opening	1.3	1.3 1.6	1.5
Width third gill opening	1.4	1.2 1.5	1.3
Width fifth gill opening	0.9	0.9 1.1	0.8
Distance between first gill openings	12.7	11.8 12.6	12.7
Distance between fifth gill openings	6.0	6.2 6.8	7.6
Clasper length (post cloaca)	14.9	7.0 14.5	0.0
Length anterior pelvic-fin lobe	15.6	15.1 16.1	14.3
Length posterior pelvic-fin lobe	13.9	9.2 13.6	12.0
Pelvic-fin base width	5.5	5.1 6.6	6.8
Width anterior pelvic-fin lobe	1.9	1.3 1.9	1.6
Tail width (axil of pelvic fins)	1.3	0.9 1.4	0.8
Tail width (pelvic-fin tips)	1.0	0.8 1.0	0.8
Tail height (pelvic-fin tips)	0.8	0.7 0.8	0.7
Tail width (midlength)	0.5	0.5 0.5	0.4
Width across posterior pelvic-fin lobes	6.0	4.5 6.7	7.4