Acroteriobatus stehmanni, Weigmann & Ebert & Séret, 2021

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 : 10-18

Acroteriobatus stehmanni sp. nov. Weigmann, Ebert & Séret

http://zoobank.org/ 8C6C09A5-4110-4ADF-A879-024A31EC8E3A

( Socotra blue-spotted guitarfish)

( Figs. 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , and 27, Table 2)

Acroteriobatus leucospilus ― Manilo & Bogorodsky (2003: S93)?

The holotype and seven paratypes are deposited in the Zoological Museum Hamburg ( ZMH), and two paratypes are deposited in the National Museum of Natural History, National Academy of Sciences of Ukraine ( NASU), Kyiv (Kiev) ( HMNH NASU).

Holotype ZMH 25553, adult male, 597 mm TL fresh, 597 mm TL 70% ethanol preserved, off Socotra Islands , 12°39′ N, 53°27′ E – 12°36′ N, 53°20′ 2″ E, 41–43 m depth, RV Vityaz, cruise 17, station 2567, 30 m bottom trawl, trawl # 7, on the bottom from 8:30 to 9: 27 p. m., 28 October 1988, collected by Matthias F.W. Stehmann.

Paratypes (9) ZMH 25554, adultmale, 564 mm TL fresh, 562 mm TL 70% ethanol preserved, data the same as holotype ZMH 25553; ZMH 25555, adult male, 546 mm TL fresh, 547 mm TL 70% ethanol preserved, data the same as holotype ZMH 25553; ZMH 25556, very early subadult male, 378 mm TL fresh, 376 mm TL 70% ethanol preserved, data the same as holotype ZMH 25553; ZMH 25557, adultfemale, 602 mm TL fresh, 603 mm TL 70% ethanol preserved, data the same as holotype ZMH 25553; ZMH 25558, adultfemale, 594 mm TL fresh, 591 mm TL 70% ethanol preserved, data the same as holotype ZMH 25553; ZMH 25559, adult male, 622 mm TL fresh, 613 mm TL 70% ethanol preserved, off Socotra Islands, 12°04′ 48″ N, 53°12′ 36″ E – 12°09′ 12″ N, 53°10′ 6″ E, 36–40 m depth, RV Vityaz, cruise 17, station 2829, 29 m shrimp trawl, trawl # 100, onthe bottom from 10:23 to 11: 30 p. m., 15 January 1989, collected by Matthias F.W. Stehmann ; ZMH 25560, juvenile male, 208 mm TL fresh, 201.4 mm TL 70% ethanol preserved, data the same as paratype ZMH 25559; HMNH NASU 103836 , adult male, 567 mm TL fresh, 568 mm TL 4% formaldehyde preserved, data the same as holotype ZMH 25553; HMNH NASU 103838 , female, caudal-tip missing, 523 mm TL fresh, 518 mm TL 4% formaldehyde preserved, data the same as holotype ZMH 25553.

Diagnosis

Asmall Acroteriobatus species distinguished bythe 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 or to somewhat anterior to first dorsal-fin origin; absent between dorsal fins and upper caudal fin. Snout semi-translucent with a few, somewhat elongated bluish-gray spots only giving it a very reduced stripe-nosed appearance, patterning with small bluish-gray circular spots generally sparse and confined to symmetrical patterns on snout tip, posterior pectoral-fin margins, a pair of tiny spots on midbody behind occipital joint, and few spots on posterior pelvic-fin margins; indistinct brown spots on the body and dorsal and caudal fins; ventral surface white except for a blackish blotch and two tiny black spots on ventral snout tip in smallest juvenile paratype. Nasal lamellae 43–48; upper jaw tooth row count ~64–78; 181–186 post-synarcual centra; 194–200 total vertebral segments; 63– 67 total pectoral skeleton radials.

Description of the holotype

Values of the seven ZMH paratypes in parentheses; more complex differences are described separately. Where relevant, ratios are based on horizontal measurements unless otherwise stated. Detailed morphometric measurements and meristics of the holotype and seven ZMH paratypes are given in Table 2.

External morphology ( Figs. 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , and 27) Disc wedge-shaped, narrowly angular anteriorly, snout angle before eyes moderately acute, 72° (71– 77°); anterior margin straight, outer pectoral margin broadly rounded, posterior margin convex, rear tip broadly rounded; disc length 1.3 (1.25–1.32) times its width. Pelvic fins elongate, short-based, base length 1.0 (0.81–1.47) times inner margin length; pelvic-fin total length 1.86 (1.56–2.18) times base length, 2.93 (2.52–3.34) times width; anteriormargin 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 1.61 (1.29–1.69) times precloacal length, 1.62 (1.25–1.69) timesdisc length, and 6.34 (5.41–6.82) times body width at pelvic-fin insertions; body width 2.29 (2.09– 2.46) times depth at pelvic-fin insertions, 2.63 (2.57–2.91) at first dorsal-fin origin, 2.18 (2.08–2.27) at second dorsal-fin origin. Dermal fold lateral along tail, originating about level pelvic-fin freerear tip and reaching ontobase of caudalfin 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. 14 , 15 , 16 , 17 , and 18).

Head relatively short, direct ventral length 24.4 (23.6–25.3 except for 29.7 in smallest juvenile paratype)% TL, dorsal length 17.8 (17.2–22.8)% TL; snout rather short, bluntly pointed, preoral length 2.94 (2.63–3.02) times mouth width, 6.9 (5.46–6.75) times internarial distance, 1.32 (1.26–1.38) times dorsal caudal-fin margin, 4.66 (3.37–4.5) times distance from nostril to margin of disc; direct preorbital snout length 2.78 (2.59–2.72) times interspiracular distance, 3.76 (3.4– 3.73) times orbit diameter, 4.32 (3.83–4.23) times skeletal interorbital space; interorbital space weakly concave, rather narrow; eyes moderately large, slightly elevated, but not protruding, orbit diameter 1.8 (1.68–2.26) times spiracle length, 1.15 (1.03–1.23) 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 0.73 (0.59–0.75) times that of outer fold; distance between bases of folds 0.85 (0.95–1.83) times length of shortest fold ( Figs. 19 and 20 ).

Nostril moderately large, oblique, all nasal flaps well developed; anterior aperture subcircular; nostril length 3.35 (2.67–2.98) times anterior aperture width, 1.16 (0.91–1.15) times anterior nasal-flap base length, 1.01 (0.6–0.93) times distance from nostril to edge of disc, 1.49 (0.98–1.39) times internarial distance. Anterior nasal-flap 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 1.87 (1.82–2.19) times its width at process, 2.9 (2.47–3.21) times anterior aperture width; minimum distance between mesial insertions of flaps 8.24 (7.39–10.4) in greatest distance across nostrils anteriorly, 2.29 (2.56–3.17) 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 3.9 (2.3–4.29) times its width; originating just behind lateral extremity of 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 2.16 (1.49–2.46) times its width, its inner edge nearly reaching innermost margin of nostril; width 0.88 (0.65–1.11) times anterior aperture width, 1.26 (0.88–1.48) times posterolateral nasal-flap width ( Figs. 21 and 22 ).

Mouth width 1.57 (1.56–1.95) times nostril length, 7.11 (6.51–7.53) 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; ~72 (~64–78) tooth rows in upper and ~70 (~64–76) in lower jaws. Gill openings strongly wavy; length of third gill slit 2.68 (2.08–2.73) in nostril length, 6.00 (5.92–6.45) in distance between fifth gill slits; distance betweenfirst gill slits 1.42 (1.39–1.48) timesdistance between fifth gill slits; distance between fifth gill slits 3.34 (2.79–3.53) times internarial distance, 1.43 (1.4–1.6) times mouth width, 0.32 (0.31–0.36) of ventral head length ( Fig. 22 ).

Dorsal fins erect, relatively tall, dissimilar in size and shape; first dorsal-fin anterior margin weakly convex, curving rearwards towards pointed (pointed to 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 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 1.0 (0.91–1.15) times height, its base length 2.76 (1.61–2.86) times inner margin length; second dorsal-fin length 1.12 (0.96–1.19) times its height, base length 3.05 (2.01–3.5) times inner margin length ( Fig. 23a, b ). First dorsal-fin origin well posterior to pelvic-fin free rear tips, interspace 1.29 (1.2–1.3) times interdorsal distance; interdorsal space relatively short, 2.05 (1.61–2.17) times second dorsal-fin height, 2.54 (2.3–2.82) times base of first dorsal fin, 1.34 (1.08–1.44) times tail width at origin of first dorsal fin, 1.85 (1.71–2.05) times interspace between second dorsal-fin insertion and upper origin of caudal fin. Caudal fin relatively small; dorsal caudal margin weakly convex, length 1.5 (1.45–1.73) times preventral margin length; ventral lobe broadly rounded; posterior margin weakly concave; fin tip acutely pointed ( Fig. 23c ).

Dermal denticles mostly minute, close-set, covering entire body and fins; surfaces mostly smooth to the touch, except for 4 (4–5) slightly enlarged granular denticles on each anterior orbital rim, 1 (1–2) on each inner posterior rim, and 2 (1–2) on the inner edge of each spiracle ( Figs. 20 and 24a–d ); 53 (41– ~60) enlarged, coarse denticles rather regularly distributed along midline from nape to first dorsal-fin origin (to origin also in most paratypes but to ~ 60 mm before origin in adult male paratype ZMH 25554 and to ~ 10 mm before origin in juvenile male paratype ZMH 25560; about 10 posteriormost denticles only weakly pronounced in both female ZMH paratypes) ( Figs. 24e, i and 25 ); enlarged denticles absent along midline between dorsal fins and upper caudal origin, 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 ( Fig. 24a–e ). Crowns of denticles in juvenile paratype ZMH 25560 with acutely pointed cusp and a minute cusplet on each side of the cusp ( Fig. 24f–i ). Furthermore, the denticles of this juvenile paratype are proportionally larger compared to larger specimens and those in midline are set much denser ( Fig. 24i vs. Fig. 24e and Fig. 25b vs. Fig. 25a ). With respect to the minute dermal denticles covering entire body and fins, those anterior to orbit are tricuspidate in this juvenile paratype with three well-developed, acutely pointed cusps ( Fig. 24f ). Thorns absent in holotype and paratypes. Prebranchial sensory pore patch distinct, extending to about the level of third gill slit. Post-scapular sensory canal distinct, weakly undulated anteriorly, terminating near pectoral-fin insertions; sensory pores minute, canal not forming a shallow groove ( Fig. 15 ).

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 31.2 (30.7–34.0)% 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 separatedposteriorly, notconstricting medially; rostral cartilage 60.5 (57.4–61.3)% of neurocranium length, ventral edges of rostral cartilage united; nasal capsules large, with their transverse axes directed anterolaterally; width of cranium across nasal capsules 1.25 (1.15–1.35) times nasobasal length (base of rostrum to occipital condyles); width of nasal capsule 1.48 (1.39–1.68) times its length; basal plate relatively broad, its minimum width 4.11 (3.41–4.2) 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 wedge-shaped, without an anterior lobe extending past nasal capsules; preorbital processes well developed; postorbital processes moderately large, not bifurcate; greatest width across processes 2.02 (1.66–2.11) times in nasobasal length ( Fig. 26 ).

Nasallamellae 47 left, 48 right (43–48) (left/right orientation based on dorsal view). Pectoral skeleton with 31 left, 30 right (29–32) propterygial, 8 (8–10) mesopterygial, 1 left, 2 right (1– 2) neopterygial, 25 (23–25) metapterygial, amounting to 65 (63–67) total radials. Total pelvic-fin radials 28 (27–31). Total vertebral segment (synarcual and free) counts 194 (195–200); post-synarcual centra 181 (181–186); precaudal centra (excluding synarcual centra) 146 (145–148); synarcual segments 13 (12–14); monospondylous precaudal centra 39 (38–40); diplosondylous precaudal centra 107 (105– 109); diplosondylous caudal centra 35 (36–39). Monospondylous to diplosondylous centra transition posterior to pelvic girdle.

Coloration

Prior to preservation: dorsal surface light to medium brown with a greenish tinge; the patterning with small bluish-gray circular spots is sparse, and such spots are confined to symmetrical patterns on the snout tip, posterior pectoral-fin margins, a pair of tiny spots on midbody behind occipital joint, and few spots on posterior pelvic-fin margins. The spots have rather inconspicuous and very thin brownish margins. Indistinct small (on anterior disc) to larger (on posterior disc and tail) dark brown spots can be found in varying degrees of conspicuousness, sometimes very indistinct, but always arranged in symmetrical patterns, extending also onto dorsal and caudal fins ( Figs. 16 and 17 ). Snout semi-translucent with a few, somewhat elongated bluish-gray spots only giving it a very reduced stripe-nosedappearance ( Figs. 16 and 17 ). Outer edges of pelvic-fin (and partially posterior pectoral-fin) margins, as well as lateral tail folds conspicuously white ( Figs. 16 and 17 ). Ventral surface uniformly white. After preservation: bluish-gray spots mostly not visible anymore, at most few detectable on posterior pectoral-fin margins; brownish spots more or less faded, some specimens, including one of the females with fresh coloration described above, show almost no dark spots anymore. Outer edges of pelvic-fin (and partially posterior pectoral-fin) margins, as well as lateral tail folds creamy instead of white as typical for preserved specimens ( Fig. 14 ). Ventral surface uniformly creamy as well ( Fig. 14 ); smallest juvenile paratype ZMH 25560 with prominent blackish blotch on underside of snout tip plus two tiny black spots on outer edges of anterior snout about half way from snout tip to nostrils ( Fig. 27 ).

Size

The new species is described from 10 specimens (seven males and three females), ranging in size from 208 to 622 mm TL (fresh) and from 201 to 613 mm TL (preserved). Adult males range from 546 to 622 mm TL (fresh), a 378 mm TL male is a very early subadult specimen, and a 208 mm TL male is juvenile. Accordingly, males of the new species apparently start to mature at around 378 mm TL. The two female paratypes of 594 mm and 602 mm TL are presumed to be adult, considering that they are distinctly larger than the smallest adult male paratypes. Based on all known specimens, the new species is assumed to be a small Acroteriobatus species reaching only about 622 mm TL.

Distribution

Acroteriobatus stehmanni sp. nov. is presently only known from the coastal waters surrounding the Socotra Islands and may be endemic to this region ( Fig. 13 ). It is known from 36 to 43 m depth.

Etymology

The species is named in honorof Dr. Matthias F.W. Stehmann for his invaluable contributions to chondrichthyan taxonomy, particularly with respect to skates. He taught SW and BS chondrichthyan taxonomy, collected all type specimens of the new species, and kindly provided fresh photographs of two of them.

Figures

Fig. 14 Acroteriobatus stehmanni sp. nov., ZMH 25553, adult male holotype, 597 mm TL, in total dorsal view. Scale bar: 5 cm

Fig. 15 Acroteriobatus stehmanni sp. nov., ZMH 25553, adult male holotype, 597 mm TL, in total ventral view. Scale bar: 5 cm

Fig. 16 Acroteriobatus stehmanni sp. nov., ZMH 25557, adult female paratype, 602 mm TL fresh, in total dorsal view taken directly after catching.Scale bar: 5 cm. The photograph was taken and kindly provided by Matthias Stehmann

Fig. 17 Acroteriobatus stehmanni sp. nov., ZMH 25558, adult female paratype, 594 mm TL fresh, in total dorsal view taken directly after catching.Scale bar: 5 cm. The photograph was taken and kindly provided by Matthias Stehmann

Fig. 18 Acroteriobatus stehmanni sp. nov., ZMH 25560, juvenile male paratype, 201.4 mm TL, in total dorsal view. Scale bar: 5 cm

Fig. 19 Acroteriobatus stehmanni sp. nov., ZMH 25553, adult male holotype, 597 mm TL, head in dorsal view. Scale bar: 5 cm

Fig. 20 Acroteriobatus stehmanni sp. nov., ZMH 25553, adult male holotype, 597 mm TL, close-up of orbital and spiracular region. Scale bar: 2 cm

Fig. 21 Acroteriobatus stehmanni sp. nov., ZMH 25553, adult male holotype, 597 mm TL, head in ventral view. Scale bar: 5 cm

Fig. 22 Acroteriobatus stehmanni sp. nov., ZMH 25553, adult male holotype, 597 mm TL, close-up of mouth-nasal region. Scale bar: 2 cm

Fig. 23 Acroteriobatus stehmanni sp. nov., ZMH 25553, adult male holotype, 597 mm TL; first dorsal (a), second dorsal (b), and caudal (c) fins in lateral views. Scale bar: 2 cm

Fig. 24 Acroteriobatus stehmanni sp. nov., enlarged dermal denticles of a–e adult male holotype, ZMH 25553, 597 mm TL, and f–i smallest juvenile paratype, ZMH 25560, male, 201.4 mm TL. a, b preorbital denticles in a overview and b close-up view; c postorbital den- ticle; d spiracular denticles; e midline denticles; f preorbital denticles; g postorbital denticles; h spiracular denticle; i midline denticles. Scale bars: a 2 mm and b–i 1 mm

Fig. 25 Acroteriobatus stehmanni sp. nov., row of enlarged dermal denticles along midline from nape to first dorsal-fin origin of a adult male holotype, ZMH 25553, 597 mm TL, and b smallest juvenile paratype, ZMH 25560, male, 201.4 mm TL. Scale bars: a 5 cm and b 2 cm

Fig. 26 Acroteriobatus stehmanni sp. nov., ZMH 25553, adult male holotype, 597 mm TL, radiograph of cranium and snout in dorsal view

Fig. 27 Acroteriobatus stehmanni sp. nov., ZMH 25560, juvenile male paratype, 201.4 mm TL, head in ventral view. Scale bar: 2 cm

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