Acroteriobatus leucospilus ( Norman, 1926)

Weigmann, Simon, Ebert, David A. & Séret, Bernard, 2021, Resolution of the Acroteriobatus leucospilus species complex, with a redescription of A. leucospilus (Norman, 1926) and descriptions of two new western Indian Ocean species of Acroteriobatus (Rhinopristiformes, Rhinobatidae), Marine Biodiversity 51, pp. 1-30 : 19-25

Acroteriobatus leucospilus ( Norman, 1926)

(Blue-spotted guitarfish)

( Figs 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , and 37, Table 3)

Acroteriobatus leucospilus ― Ebert and van Hees (2015: 146); Weigmann (2016: 920); Séret et al. (2016: 81); Séret and de Carvalho (in press); Séret (in press)

Rhinobatus annulatus (not Müller & Henle)― Smith (1961: 64 [in part], not pl. 3)

Rhinobatus blochii (not Müller & Henle)― Regan (1908: 242) (in part); Gilchrist & Thompson (1911: 55); Gilchrist & Thompson (1916: 284) (in part); Smith (1961: not 64, pl. 3)

Rhinobatus blochi ― Barnard (1925: 61) (in part)

Rhinobatos leucospilus ― Fowler (1941: 313) and subsequent authors

Rhinobatus leucospilus ― Norman (1926: 966); Barnard (1927: 1014–1015)

Syntypes BMNH 1905.6.8.12, subadult male, 416 mm TL, and BMNH 1920.7.23.1, juvenile male, 248 mm TL, Durban, KwaZulu-Natal, South Africa, southwestern Indian Ocean .

Non-types South Africa: SAIAB 189087 , 223 mm TL (after preservation), juvenile male, Tugela Bank, KwaZuluNatal , South Africa, 29° 7.5′ S, 31° 45′ E, 15 March 2009; SAIAB 34588 , 690 mm TL (after preservation), gravid female, KwaZulu-Natal, South Africa, 23 October 1982; SAIAB 11142 (formerly ORI B 90), 960 mm TL (after preservation), adult female, Tugela Bank, KwaZulu-Natal, South Africa; SAIAB 11143 (formerly ORI B 112), 650 mm TL (after preservation), adult female, Tugela Bank, KwaZulu-Natal, South Africa; SAIAB 11159 (formerly ORI B 799), 295 mm TL (after preservation), juvenile female, Tugela Bank, KwaZulu-Natal, South Africa.

Mozambique: SAIAB 40000 , 430 mm TL (after preservation), subadult male, Maputo Bay , Mozambique, 25.96666717° S, 32.5833320618° E, 29 May 1992; SAIAB 55691 , 445 mm TL (after preservation), juvenile female, Moebase fishing village, Mozambique, 17.04999923° S, 39.7333335876° E, 12 July 1997; SAIAB 12829 , 515 mm TL (after preservation), juvenile female, Inhaca Island , Mozambique, 26.01666641° S, 32.96666717° E, October 1953.

Tanzania: SAIAB 12162 , 629 mm TL (after preservation), female, Zanzibar Island, Tanzania, 6.166666984° S, 39.1833343506° E.

Diagnosis

A medium-sized Acroteriobatus species distinguished by the following combination of characters: dorsal surface smooth, without prominent thorns or tubercles, except for slightly enlarged granular denticles partially around orbital rims and rather regularly distributed along midline from nape to near first dorsal-fin origin; absent between dorsal fins and upper caudal fin. Snout semi-translucent with elongated bluish spots giving stripe-nosed appearance, numerous small bluish spots covering snout, pectoral, pelvic, dorsal, and caudalfins but not central disc on a sandy brown background with darker brown spots of varying sizes covering the disc, tail, fin bases, and fins, sometimes giving the caudal peduncle a striped appearance; outer edges of pectoral- and pelvic-fin margins blue, lateral tail folds white or striped blue and brown; ventral surface white. Nasal lamellae 37–41; upper jaw tooth row count ~60–75; 187–192 post-synarcual centra; 200–205 total vertebral segments; 63–70 total pectoral skeleton radials.

Description

The description is based on the two syntypes (BMNH 1905.6.8.12 and BMNH 1920.7.23.1), as well as nine nontype specimens. Where relevant, ratios are based on horizontal measurements unless otherwise stated. Detailed morphometric measurements and meristics are given in Table 3.

External morphology ( Figs. 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , and 36) Disc wedge-shaped, narrowly angular anteriorly, snout angle before eyes moderately acute, 68.0–80.5°; anterior margin straight, outer pectoral margin broadly rounded, posterior margin convex, rear tip broadly rounded; disc length 1.15– 1.24 times its width. Pelvic fins elongate, short-based, base length 0.89–2.04 times inner margin length; pelvic-fin total length 1.44–2.12 times base length, 2.19–3.06 times width; anterior margin straight, becoming weakly convex towards broadly rounded apex, posterior margin weakly convex, free rear tip narrowly rounded. Tail elongate, slightly constricted in anterior part between pelvic-fin origin and about level pelvic-fin free rear tip, broadest somewhat anterior to first dorsal-fin origin, tapering posteriorly; very strongly depressed dorsoventrally, in cross section nearly flat below, more rounded above; tail length from anterior cloaca 1.35–1.62 times precloacal length, 1.31–1.66 timesdisc length, and 5.20– 6.03 times body width at pelvic-fin insertions; body width 2.17–2.67 times depth at pelvic-fin insertions, 2.39–3.26 at first dorsal-fin origin, 1.74–2.44 at second dorsal-fin origin. Dermal fold lateral along tail, originating about level pelvicfin free rear tip and reaching onto base of caudal fin just behind ventral origin of caudal fin; fold well developed, maximum width at interdorsal space more than half width of posterior nasal flap and more than length of longest spiracular fold ( Figs. 28 , 29 , 30 , and 31).

Head moderately elongate, ventral length 22.9–28.3% TL, dorsal length 24.0–29.2% TL; snout rather short, bluntly pointed, preoral length 2.22–2.76 times mouth width, 4.28– 5.08 times internarial distance, 1.09–1.54 times dorsal caudalfin margin, 2.59–3.46 times distance from nostril to margin of disc; direct preorbital snout length 2.17–2.6 times interspiracular distance, 3.3–5.36 times orbit diameter, 2.65– 3.67 times skeletal interorbital space; interorbital space weakly concave, rather narrow; eyes moderately large, slightly elevated, but not protruding, orbit diameter 1.07–2.0 times spiracle length, 0.58–1.11 times skeletal interorbital space. Spiracles relatively large, narrowly bean-shaped; two prominent and strongly compressed folds on upper posterior margin, length of inner spiracular fold 0.5–0.75 times that of outer fold; distance between bases of folds 0.25–1.0 times length of shortest fold ( Figs. 32 and 33 ).

Nostril moderately large, oblique, all nasal flaps well developed; anterior aperture subcircular; nostril length 1.7–3.0 times anterior aperture width, 0.75–1.13 times anterior nasalflap base length, 0.5–0.94 times distance from nostril to edge of disc, 0.81–1.31 times internarial distance. Anterior nasalflap base greatly extended, penetrating horizontally into internarial space, mesial edges barely separated (by about width of posterior nasal flap); innermost extension of flap not narrow; flap base length 1.88–2.86 times its width at process, 1.88–3.33 times anterior aperture width; minimum distance between mesial insertions of flaps 6.75–26.5 in greatest distance across nostrils anteriorly, 2.63–9.5 in minimum internarial distance; process of flap about twice as long as wide, narrowing distally to blunt pointed tip, and adjoining with posterolateral nasal flap and anterior aperture posterior margin. Posterolateral nasal flap well developed, lobe-like, broadest medially, length 2.0–5.5 times its width; originating just behind lateral extremityof anterior nasal aperture, extending posteromesially as a free fold almost to the level of insertion of posterior nasal flap. Posterior nasal flap well developed, lobe-like, base length 1.6–2.5 times its width, its inner edge nearly reaching innermost margin of nostril; width 0.5– 1.0 times anterior aperture width, 0.92–2.0 times posterolateral nasal-flap width ( Figs. 34 and 35 ).

Mouth width 1.59–2.24 times nostril length, 5.94–7.25 in precloacal length; positioned beneath posterior margin of orbit. Upper jaw slightly convex, upper lip broadly arched; lower lip not pronounced, not separated from oral groove by ridges of strongly corrugated skin; corners of mouth with several strong, short lateral grooves. Teeth arranged in quincunx, small, close-set, rhombic, broad-based with short, bluntly rounded cusp; upper and lower teeth similar in shape and size; upper and lower jaws each with ~60–75 tooth rows. Gill openings strongly wavy; length of third gill slit 1.63–3.4 in nostril length, 4.33–7.0 in distance between fifth gill slits; distance between first gill slits 1.31–1.46 times distance between fifth gill slits; distance between fifth gill slits 2.54–3.24 times internarial distance, 1.3–1.62 times mouth width, 0.32– 0.38 of ventral head length ( Fig. 35 ).

Dorsal fins erect, relatively tall, dissimilar in size and shape; first dorsal-fin anterior margin weakly convex, curving rearwards towards rounded apex; posterior margin weakly convex near tip, then becoming weakly concave to nearly straight; second dorsal-fin anterior margin weakly convex, curving towards pointed or slightly rounded apex; posterior margin nearly straight; free rear tips rounded of both dorsal fins, almost forming right angle, not produced; first dorsal fin height subequal to second, length of first 0.79–1.15 times height, its base length 1.67–2.8 times inner margin length; second dorsal-fin length 0.84–1.21 times its height, base length 1.5–3.3 times inner margin length ( Fig. 36a, b ). First dorsal-fin origin well posterior to pelvic-fin free rear tips, interspace 1.04–1.99 times interdorsal distance; interdorsal space relatively short, 1.32–2.19 times second dorsal-fin height, 2.13–3.17 times base of first dorsal fin, 0.98–1.33 times tail width at origin of first dorsal fin, 1.29–1.9 times interspace between second dorsal-fin insertion and upper origin of caudal fin. Caudal fin relatively small; dorsal caudal margin weakly convex, length 1.03–1.99 times preventral margin length; ventral lobe broadly rounded; posterior margin weakly concave; fin tip acutely pointed ( Fig. 36c ).

Dermal denticles mostly minute, close-set, covering entire body and fins; surfaces mostly smooth to the touch, except for ~7 slightly enlarged granular denticles on each orbital rim and ~3–4 on the inner edge of spiracle; up to ~50 enlarged, coarse denticles rather regularly distributed along midline from nape to near first dorsal-fin origin, absent along midline between dorsal fins and upper caudal origin; enlarged denticles absent on snout tip, anterior and lateral margins of disc, pelvic fins, and bases of dorsal fins; nasal flaps and lamellae naked. Crowns of denticles mostly oval, except largest denticles with slightly erect, bluntly acute cusp, base stellate. Juveniles have proportionally larger denticles than larger specimens and the crowns of the denticles have acutely pointed cusps. The minute dermal denticles covering entire body and fins are also proportionally in juveniles, giving the skin a velvet touch. Thorns always absent. Prebranchial sensory pore patch distinct, extending to about the level of third gill slit. Postscapular sensory canal rather indistinct, strongly undulated anteriorly, terminating near pectoral-fin insertions; sensory pores minute, canal not forming a shallow groove ( Fig. 29 ).

Rostral cartilage broad, its shaft just slightly increasing in width posteriorly from rostral node; rostral node rounded at apex, not angular, broadly expanded, relatively short, axis at maximum node width 34.4–41.2% of length of rostral cartilage from tip; anterior fontanelle relatively broad posteriorly, tapering gradually towards rostral node, dorsolateral edges of cartilage surrounding fontanelle (rostral ridges on surface of snout) well separated posteriorly, not constricting medially; rostral cartilage 54.2–58.1% of neurocranium length, ventral edges of rostral cartilage united; nasal capsules large, with their transverse axes directed anterolaterally; width of cranium across nasal capsules 1.15–1.27 times nasobasal length (base of rostrum to occipital condyles); width of nasal capsule 1.3– 1.45 times its length; basal plate relatively broad, its minimum width 3.35–3.61 times in nasobasal length; cranial roof with small, oval-shaped fenestra, with its anterior edge located behind precerebral cavity by a distance ~1.5 times its length; anterior cartilage triangular, narrow, posteriorly wedgeshaped, without an anterior lobe extending past nasal capsules; preorbital processes well developed; postorbital processes moderately large, not bifurcate; greatest width across processes 1.78–1.97 times in nasobasal length ( Fig. 37 ).

Nasal lamellae 37–41. Skeletal meristics based on the two syntypes and non-type specimens SAIAB 189087, SAIAB 34588, and SAIAB 55691: pectoral skeleton with 29–32 propterygial, 8–9 mesopterygial, 1–2 neopterygial, 22–27 metapterygial, amounting to 63–70 total radials. Total pelvic-fin radials 25–30. Total vertebral segment (synarcual and free) counts 200–205; post-synarcual centra 187–192; precaudal centra (excluding synarcual centra) 148–151; synarcual segments 13–15; monospondylous precaudal centra 39–41; diplosondylous precaudal centra 109–110; diplosondylous caudal centra 37–42. Monospondylous to diplosondylous centra transition posterior to pelvic girdle.

Coloration

Prior to preservation: dorsal surface sandy brown with a pronounced and colorful, organized pattern of numerous small and large bluish circular spots, with a darker ring encircling a lighter bluish center, and small and large brownish spots ( Fig. 31 ); the bluish spots cover the snout, pectoral, pelvic, dorsal, and caudal fins but not the central disc; the brownish spots cover the disc, tail, fin bases, and fins, sometimes giving the caudal peduncle a striped appearance ( Fig. 31 ); snout semi-translucent with elongated bluish spots giving it a stripe-nosed appearance ( Fig. 31 ). Outer edges of pectoral-and pelvic-fin margins blue, lateral tail folds white or striped blue and brown ( Fig. 31 ). Ventral surface white. After preservation: dorsal surface similar but bluish spots less pronounced and brownish spots mostly faded and not evident ( Figs. 28 and 30 ). Ventral surface also similar but creamy rather than white as typical for preserved specimens ( Fig. 29 ).

Size

The 11 examined specimens ranged in size from 223 to 960 mm TL. Acroteriobatus leucospilus has been reported to reach 1200 mm TL ( Compagno et al. 1989; Séret et al. 2016; Weigmann 2016), but no voucher specimens of such size are known and the maximum size might possibly be based on the larger A. andysabini sp. nov. instead. The largest verified specimen is the adult female of 960 mm TL examined in the present study. The 416 mm TL male syntype BMNH 1905.6.8.12 and the 430 mm TL non-type male SAIAB 40000 are both subadult, and the 690 mm TL non-type female SAIAB 34588 is gravid. Following Wallace (1967), a male of 402 mm TL was juvenile, oneof 415 mm TL was subadult, and specimens of more than 565 mm TL were mature, with gravid females ranging from 564 to 921 mm TL. Considering the rather small maturing size of about 415–565 mm TL, a maximum size above 1000 mm TL appears unlikely. Wallace (1967) indicates a size of 93 mm TL and a weight of 1/ 4 lb for the smallest embryo seen and 250 mm TL for the smallest juvenile on record (allegedly also weighing 1/ 4 lb). Based on the maximum verified size of 960 mm TL, A. leucospilus is a medium-sized species of the genus.

Distribution

Acroteriobatus leucospilus so far has been recorded from off the central Transkei coast (Eastern Cape Province) to the northern KwaZulu-Natal Province ( South Africa) and Mozambique. Ebert et al. (2021), based on the examined material, extended its range to Zanzibar and Dar es Salaam, Tanzania ( Fig. 13 ). It is known from 0 to 100 m depth ( Compagno et al. 1989; Séret et al. 2016; Weigmann 2016) but seems to be most abundant in depths of around 20 fathoms or 37 m ( Wallace 1967).

Figures

Fig. 28 Acroteriobatus leucospilus, BMNH 1905.6.8.12, subadult male syntype, 416 mm TL, in total dorsal view. The photograph was taken and kindly provided by Kevin Webb

Fig. 29 Acroteriobatus leucospilus, BMNH 1905.6.8.12, subadult male syntype, 416 mm TL, in total ventral view. The photograph was taken and kindly provided by Kevin Webb

Fig. 30 Acroteriobatus leucospilus, SAIAB 34588, gravid female, 690 mm TL, in total dorsal view. Photograph courtesy Marsha Englebrecht ©

Fig. 31 Acroteriobatus leucospilus, underwater photograph showing the vivid live coloration. The photograph was taken and kindly provided by Dennis King

Fig. 32 Acroteriobatus leucospilus, BMNH 1905.6.8.12, subadult male syntype, 416 mm TL, head in dorsal view. Scale bar: 2 cm. The photograph was taken and kindly provided by Kevin Webb

Fig. 33 Acroteriobatus leucospilus, SAIAB 34588, gravid female, 690 mm TL, close-up of orbital and spiracular region. Photograph courtesy Marsha Englebrecht ©

Fig. 34 Acroteriobatus leucospilus, BMNH 1905.6.8.12, subadult male syntype, 416 mm TL, head in ventral view. Scale bar: 2 cm. The photograph was taken and kindly provided by Kevin Webb

Fig. 35 Acroteriobatus leucospilus, SAIAB 34588, gravid female, 690 mm TL, close-up of mouth-nasal region. Photograph courtesy Marsha Englebrecht ©

Fig. 36 Acroteriobatus leucospilus, SAIAB 34588, gravid female, 690 mm TL; first dorsal (a), second dorsal (b), and caudal (c) fins in lateral views. Photograph courtesy Marsha Englebrecht ©

Fig. 37 Acroteriobatus leucospilus, BMNH 1905.6.8.12, subadult male syntype, 416 mm TL, radiograph of cranium and snout in dorsal view. The radiograph was taken and kindly provided by James Maclaine and reworked by Nemo Martin

Fig. 13 Map of the southwestern Indian Ocean depicting the catch locations of the examined specimens of all three species of the Acroteriobatus leucospilus species complex. Holotype (white star) and paratypes (white circles) of A. andysabini sp. nov., holotype (gray star) and paratypes (gray star and gray circle) of A. stehmanni sp. nov., and syntypes (black triangles) and other specimens (black squares) of A. leucospilus