Potamotrygon rex, De Carvalho, Marcelo R., 2016

De Carvalho, Marcelo R., 2016, Potamotrygon rex, a new species of Neotropical freshwater stingray (Chondrichthyes: Potamotrygonidae) from the middle and upper rio Tocantins, Brazil, closely allied to Potamotrygon henlei (Castelnau, 1855), Zootaxa 4150 (5), pp. 537-565 : 538-552

Potamotrygon rex , sp. nov.

( Figs. 1–13 , 15 , 16 a, Tables 1–2 )

Holotype. MZUSP 120371 , adult female, 1110 mm TL, 750 mm DL, 690 mm DW, rio Tocantins near the mouth of the right bank tributary rio Manoel Alves (type locality) , Tocantins state, Brazil, 11°18'55"S, 048°27'28"W, 10.vii.2003, col. NEAMB team (ex UNT 7139 ).

Paratypes. (19 specimens). MZUSP 120354 , rio Tocantins next to rio Santa Tereza , Tocantins state, Brazil, 11˚47’27”S, 048˚37’2”W, 8.ix.2003, col. NEAMB team (ex UNT 2198 ) ; MZUSP 120355 , rio Tocantins at rio Manoel Alves , Tocantins state, Brazil, 11˚18’55”S, 048˚27’28”W, 13.vi.2003, col. NEAMB team (ex UNT 2199 ) ; MZUSP 120356 , Rio Areias at confluence with rio Tocantins, 10°52'05''S, 048°21'39"W , Tocantins state, Brazil, 8.vi.2003, col. NEAMB team (ex UNT 2200 ) ; MZUSP 120358 , same data as MZUSP 120355, 12 .ix.2002, col . NEAMB team (ex UNT 7097); MZUSP 120360 , rio Maranhão at Traçadal farm , Tocantins state, Brazil, 12˚31’21”S, 048˚13’41”W, 03.iii.2000, col. NEAMB team (ex UNT 7116 ) ; MZUSP 120362 , rio Paranã at Traçadal Farm , Tocantins state, Brazil, 12˚30’35”S, 048˚12’57”W, 09.viii.2000, col. NEAMB team (ex UNT 7117 ) ; MZUSP 120363 , rio Tocantins at confluence with rio Santa Tereza , Tocantins state, Brazil, 11°47'26"S, 048°27'28"W, 14.x.2003, col. NEAMB team (ex UNT 7118 ) ; MZUSP 120364 , same data as MZUSP 120355, 20 .xii.2001, col . NEAMB team (ex UNT 7123); MZUSP 120365 , rio Tocantins at confluence with rio Santa Tereza , Tocantins state, Brazil, 11°47'26"S, 048°37'01"W, col. NEAMB ream (ex UNT 7125 ) ; MZUSP 120366 , rio Tocantins at Traçadal farm , Tocantins state, Brazil, 12˚28’1”S, 048˚14’47”W, 25.viii.1998, col. NEAMB team (ex UNT 7128 ) ; MZUSP 120367 , rio Maranhão at Traçadal farm , Tocantins state, Brazil, 12˚31’21”S, 048˚13’41”W, 27.v.1998, col. NEAMB team (ex UNT 7131 ) ; MZUSP 120368 , rio Paranã at Traçadal Farm , Tocantins state, Brazil, 12˚30’35”S, 048˚12’57”W, 03.vi.2000, col. NEAMB team (ex UNT 7133 ) ; MZUSP 120369 , rio Crixás , Brejinho de Nazaré , Tocantins state, Brazil, 11°08'15"S, 048°45'06"W, 22.vii.2000, col. NEAMB team (ex UNT 7136 ) ; MZUSP 120370 , rio São Valério at confluence with rio Tocantins , Tocantins state, Brazil, 11˚21’51”S, 048˚28’16”W, 06.x.2003, col. NEAMB team (ex UNT 7138 ) ; MZUSP 120372 , same data as MZUSP 120355, 08 .viii.2002, col . NEAMB team (ex UNT 7142); MZUSP 120373 , same data as MZUSP 120355, 14 .ix.2001, col . NEAMB team (ex UNT 7147); UNT 7111 , rio Tocantins next to rio Santa Tereza , Tocantins state, Brazil, 21.viii.2001, col. NEAMB team ; UNT 7144 , rio Tocantins at confluence with rio Sono , Tocantins state, Brazil, 8˚58’28”S, 048˚10’46”W, 27.vii.2000, col. NEAMB team ; UNT 7146 , rio Manoel Alves near confluence with rio Tocantins , Tocantins state, Brazil, 11˚19’19”S, 048˚26’57”W, 11.xi.1999, col. NEAMB team. Non-type specimens listed in Comparative Material below.

Diagnosis. A large, bulky species of Potamotrygon endemic to the middle and upper rio Tocantins, diagnosed by the following unique characters: blackish to blackish-brown dorsal color with intense yellow to orange spots (generally smaller than eye-diameter), usually forming distinct concentric clusters on dorsal disc and tail (clusters larger on central disc, smaller towards disc margins and on dorsal tail) separated by slender reticulate pattern of dark background color, usually with larger ocellate spots at center of clusters on posterior and outer disc, and with spots sometimes coalescing to form vermiculate shapes; ventral color dark gray to dark brown, with creamy white spots (smaller than eye-diameter) on central disc, pelvic fins and more numerous and smaller on ventral snout region, with a creamy white vertical streak over gill slits and anterior pelvic fins in most specimens, and sometimes with a creamy white snout and/or nasoral region. Other characters in combination that further diagnose P. re x, sp. nov., include: pelvic fins with broadly circular apices (also present in P. he n l e i and P. leopoldi , but these species lack bright orange spots forming concentric patterns on dorsal disc and usually have a lighter ventral coloration); absence of distinct labial grooves (strong grooves at least in P. constellata , P. orbignyi , P. hu m e ro s a, and P. marinae ); an irregular double row of slender, sharp, not too numerous, and moderately erect to raked thorns on dorsal tail ( P. leopoldi with three to four rows of closely packed, slender and evenly distributed thorns); presence of two angular cartilages, first angular anteriorly concave and well calcified, posterior angular less calcified, more slender, and not anteriorly concave (single angular present in P. t i g r i n a, P. schroederi , P. constellata , P. magdalenae , P. histrix , P. schuhmacheri , P. orbignyi , P. humerosa , P. marinae , and P. wallacei ; anterior angular wide and posterior angular absent or very small, abutting hyomandibula, in P. signata , P. pantanensis and P. amandae ; anterior and posterior angulars present in addition to a small lateral element in P. limai and P. scobina ).

Description. Large stingray, with thick, fleshy disc in larger specimens. Disc more or less round to slightly oval, disc length about 1.0–1.1 times DW; greatest disc width just anterior to its mid-length; posterior disc not tapering significantly. Knob-like protuberance on anterior snout small, more apparent in smaller specimens. Anterior disc margin broadly convex; males with slightly more oval snout. Head broad, clearly protruding above disc. Eyes moderately-sized, bulging only slightly; eye diameter about one-fourth interorbital distance. Preorbital distance 22.5–29.6% DW, greater than interorbital distance. Preorbital distance greater than preoral distance. Spiracles large, rhomboidal, greater than eyes, their length 5.5–7.8% DW. Spiracles extending anterolaterally to about mid-length of eyes. Interspiracular distance slightly greater than interorbital distance.

Nasal curtain trapezoidal, broader posteriorly, without median indentation, and covering most of mouth; posterior margin of nasal curtain weakly fringed in some specimens. Nostrils slender and elongate, almost reaching mouth corners. Mouth relatively wide, its width greater than internarial distance; mouth straight, not arched. Posterior margin of mouth with small median notch in specimens with slightly exposed lower tooth band. No shallow labial grooves present at lower jaw corners. Integument surrounding mouth ventrally and laterally highly crenulated, with many small grooves. Floor of mouth with five buccal papillae. Preoral distance 19.7–25.1% DW.

Upper tooth band slightly undulated. Teeth set in quincunx, in 21–44/20–47 total rows (n = 14); largest female (holotype) with 44/47 (with 25/18 exposed tooth rows with mouth closed, just slightly covered dorsally by nasal curtain); largest male with 31/33 tooth rows (with some 14/10 exposed tooth rows with mouth closed). Teeth low, rather flattened in females and with slightly higher, more elevated crowns in larger males, but generally without very sharp, pointed cusps; teeth hexagonal to rhomboidal, usually broader than long. Teeth in midlateral rows slightly wider in upper tooth band. Branchial basket relatively wide, its width just smaller than distance between fifth gill slits. Gill slits sinuous; fifth gill slits about half of width of first gill slits.

Pelvic fins broad and markedly rounded at apices, mostly covered by disc and only slightly protruding beyond disc posteriorly; anterior margin of pelvic fins broadly convex. Claspers in adult males slender, relatively elongate. Tail only moderately long, 58.9–84.3% DW. Tail strong, muscular, its width at base about two-thirds of interorbital space; distance between pectoral axils similar to interspiracular distance. Tail more slender, compressed laterally, just anterior to caudal sting origin. Lateral tail ridges wide and fleshy closer to tail base, tapering posteriorly, present from tail base to close to sting origin. Tail in cross-section slightly dorsoventrally flattened. Tail with welldeveloped dorsal finfold posterior to caudal stings, supported internally by cartilaginous elements; ventral finfold more elongate, not as tall as dorsal finfold. Caudal sting wider at base, highly tapering towards tip; sting length slightly smaller than interorbital distance. Usually one, sometimes two caudal stings present, with lateral serrations more developed closer to sting extremity.

Coloration. Dorsal surface variable in color, but with a unique overall pattern ( Figs. 1–9 ). Dorsal disc blackish (especially in fresh and live specimens) to blackish-brown (sometimes dark brown in preserved specimens) in larger juveniles and adults (olivaceous brown in some neonates and small juveniles), with numerous small (usually smaller than eye-diameter) orange to yellow spots forming concentric clusters on disc and tail, especially in larger specimens; spots generally larger toward center of clusters; clusters of spots separated by blackish-brown background color, faintly resembling a reticulate pattern; concentric clusters of spots may present a larger, ocelluslike central spot especially on posterior and outer disc margins; spots may coalesce into vermiculate, tear-drop and hour-glass shapes. Smaller specimens usually without clearly demarked clusters of spots, with numerous spots more evenly distributed on dorsal surface, but larger, ocellus-like blotches usually present closer to disc margins, equally spaced. Spots in smaller specimens (especially juveniles) bright orange. Some larger specimens with fewer and smaller spots dorsally on disc and tail. Orbital and interorbital region and preorbital snout with numerous, smaller orangish spots. Pelvic fins with more evenly distributed, larger spots. Small spots generally less numerous at disc margins, more numerous on central disc and on head region. Tail blackish, especially on its sides, with rounder, more regular, isolated and slightly larger orange to yellowish spots on lateral tail; dorsal tail with evenly separated clusters of small spots. Small spots present posterior to caudal stings. Tail finfolds black.

Ventral surface also somewhat variable ( Fig. 8 ). Ventral disc dark brown to grayish-brown, slightly lighter than dorsal surface, with numerous creamy white round and usually irregularly-shaped spots; spots usually smaller on anterior ventral snout region. Creamy white stripes on gill slits usually present. Anterior margins of pelvic fins with smaller and more numerous whitish spots, sometimes forming a distinct streak. The gill slit and anterior pelvic whitish streaks form a very distinct figure loosely resembling an "X" in some specimens. Nasoral region creamy white to yellowish around mouth, sometimes with irregularly distributed grayish brown blotches; nostrils darker inside. Some specimens with larger whitish area on anterior ventral snout, extending posteriorly to level of gill slits on central disc. Ventral tail also dark, similar to disc, but generally with isolated white spots closer to its base.

Dermal denticles and thorns. Dorsal disc somewhat prickly with many closely packed, small denticles ( Figs. 9–11 ), many close to 1 mm in diameter. Denticles star-shaped, with 5–9 coronal ridges radiating from central prominence; ridges frequently interrupted or with depressions close to central prominence. Central prominence usually very worn, rounded or blunt. Basal plate broad. Enlarged thorns present from posterior to pelvic girdle, at about level of pelvic fin mid-length, to almost origin of caudal sting. Thorns usually in two irregular rows; a single row usually present anteriorly, but thorns forming two parallel rows on central tail in most specimens. Juvenile specimens with less developed and fewer tail thorns. Thorns posteriorly pointed, with sharp, erect to raked apices in larger specimens, and with slightly rounded or broad bases. Tubercles with enlarged bases lacking in all specimens examined. Larger specimens with moderately developed and closely packed lateral spines on tail.

Ventral lateral-line canals. Suborbital loop (sol), formed by two branches of infraorbital canal (ioc), curved medially, extending toward anterior disc margin close to anterior segment of hyomandibular canal (hyoc) ( Fig. 12 ). Medial component of infraorbital canal highly undulated, reaching anteriorly to mid-distance between nostrils and snout margin; posterior portion of infraorbital canal extends to mid-distance between mouth and first gill slit, forming somewhat broad and subrectangular infraorbital loop (iol). Lateral portion of infraorbital canal (ioc) slightly undulated. Orbitonasal component of supraorbital canal more or less parallel to medial portion of infraorbital canal, adjacent to nostrils, and also highly undulated, but less so than medial component of infraorbital canal; orbitonasal component forms a very slender, triangular prenasal loop (pnl). Mandibular canal (man) slighty undulated, reaching anteriorly almost to mouth opening. Anterior jugular loop slender, digitiform; posterior jugular loop (pjl) more broad, anteriorly indented, formed medially by slightly concave jugular canal (jug). Jugular canal inflects abruptly laterally anterior to first gill slit. Subpleural component of hyomandibular canal (hyoc) expands laterally posterior to gill slits, somewhat undulated lateral to first three gill slits. Anterior subpleural tubules (ast) short, numerous, and closely packed. Posterior subpleural tubule (pst) extends posteriorly to level of cloaca, past anterior pelvic-fin margin. Subpleural loop (spl) slender, subtriangular.

Skeletal features. Neurocranium longer than wide, its greatest width at postorbital processes ( Fig. 13 ). Nasal capsules broadly rounded, without strong anterior median indentation; internasal septum very slender. Nasal apertures (no) broadly oval, much wider than long. Preorbital processes (prp) slender, triangular, and posterolaterally directed. Postorbital processes (pop) flattened, rectangular and broad, anterolaterally directed. Supraorbital process (sp) broadly triangular, just anterior to postorbital process. Precerebral (pcf) and frontoparietal (fpf) fontenellae about three-fourths of neurocranial length, key-hole shaped. Precerebral fontanelle circular, just over one-third of neurocranial length; frontoparietal fontanelle slender and tapering posteriorly, constricted at midlength. Otic capsule short and wider than orbital region. Parietal fossa (paf) shallow. Antorbital cartilage wider at articulation with posterolateral surface of nasal capsule, tapering distally, and very slender; antorbitals laterally compressed, extending posteriorly to level of preorbital processes. Prespiracular cartilages (psc) slender and uncalcified.

Meckel’s cartilages (mc) much stouter than palatoquadrates, but both highly calcified. Meckel’s cartilages with well-developed dorsally projecting lateral processes; palatoquadrates slender and with straight anterior margin. Lower tooth band greater than upper tooth band. Posterior margin of Meckel's cartilage with slender but marked protuberance directed posterolaterally. Hyomandibulae (hyo) anteroposteriorly compressed proximally at articulation with ventrolateral otic capsule, but relatively stout throughout its length. Hyomandibulae extend laterally to almost beyond level of propterygia, where they slightly curve medially. Two angular cartilages present. Anterior angular (ac) robust, fully calcified, with very concave anterior surface, and about one-fourth hyomandibula length. Anterior angular positioned at distal tip of hyomandibula. Posterior angular less calcified (faintly visible in radiographs), more slender than anterior angular, but about as wide or sometimes even slightly wider, and not anteriorly concave; posterior angular pointed at lateral extremities in some specimens, wider at center, and sometimes anteriorly convex. Angulars perpendicular to slightly oblique to neurocranium.

Cervicothoracic synarcual slender and elongated, slightly more than three-fourths neurocranial length; lateral stays triangular. Three vertebrae incorporated into cervicothoracic synarcual. Thoracolumbar synarcual slender and relatively short. Individual vertebral centra occur posteriorly to about one-half length of caudal sting. Slender, weakly calcified notochordal extension (cartilaginous rod) extending posterior to individual centra to tail tip. Mono- to diplospondyly transition occurs from 6–8 vertebral centra posterior to rear margin of pelvic girdle.

Propterygium (pro) wide at base, tapering anteriorly, much stouter than meso- and metapterygium. Anterior propterygial segment (apr) slender, lateral to and about as long as nasal capsule, articulating with five radial elements and to an enlarged element anteriorly. Mesopterygium (mes) anteriorly elongated, slender, more or less straight laterally. Metapterygium (met) slightly more broadly arched than propterygium, very slender, with four posterior segments decreasing in size caudally. Individual pectoral radial elements sometimes fused at base at posterior propterygium and at anterior metapterygium. Pectoral radials slender close to pectoral basals, slightly widening and shortening toward middisc, and slender again distally, bifurcating at about 7 th to 11th segment from basal elements; a total of up to 21 radial segments present at middisc from basals to outer disc margin.

Median prepelvic process (pp) of pelvic girdle very slender from base to tip, extending anteriorly to level of about 9th or 10th vertebral centrum. Pelvic girdle with concave anterolateral margins. Lateral prepelvic processes triangular. Iliac processes (ip) extending caudally beyond ischial processes (isp), distally broad. Ischial process wide, triangular. At least three obturator foramina (of) present in a single, oblique row. Posterior margin of puboischiadic bar highly concave. Basipterygium stout, just longer than first enlarged pelvic radial; first radial articulates to iliac projection of pelvic girdle, posteriormost three radials articulate with first basal segment of clasper. Proximal pelvic radial segment much longer than distal radial segments; a total of 6–7 segments present in each radial, bifurcating at fourth segment from basipterygium. Claspers with two basal segments ( Figs. 10 b, 13d).

Geographic distribution. Potamotrygon rex is endemic to the middle and upper rio Tocantins, having been collected from its main course and lower reaches of left and right bank affluents, and upland in the rios Paranã and Maranhão; it has also been collected from lagoons marginal to the Tocantins main course ( Fig. 14 ).

Common name. The proposed common Portuguese name is "raia preta do Tocantins "; in English, the adopted common name is Great freshwater stingray.

Etymology. The specific name rex is Latin for "king", in reference to its large, bulky size and striking color pattern—king of the rio Tocantins Potamotrygon species. A noun in apposition.

Biological notes. Examined stomachs of P. re x contained fishes, insects and insect larvae, mollusks, and crustaceans ( Santos, 2000; E. Marques, pers. comm.), with a predominance of insect larvae. As with other potamotrygonid species, P. re x may show ontogenetic dietary shifts. Sexual maturity for males occurs at about 580 mm TL, 375 mm DL, 356 mm DW (MZUSP 120369, claspers almost fully calcified), even though MZUSP 120355 (630 mm TL, 380 mm DL and DW) has proportionally smaller claspers that are even less calcified than in MZUSP 120369. Very large specimens (greater than 100 cm in TL) can easily weigh 20 kg. There is great disparity in the largest sizes achieved by males and females ( Fig. 15 ).

Figures

FIGURE 1. Dorsal (A) and ventral (B) views of holotype of Potamotrygon rex, sp. nov. (adult female, MZUSP 120371, 1110 mm TL, 750 mm DL, 690 mm DW).

FIGURE 2. Dorsal (A) and ventral (B) views of of Potamotrygon rex, sp. nov. (adult male, paratype, MZUSP 120373, 825 mm TL, 526 mm DL, 475 mm DW).

FIGURE 3. Dorsal (A) and ventral (B) views of Potamotrygon rex, sp. nov. (paratype, UNT 7146, 815 mm TL, 535 mm DL, 480 mm DW).

FIGURE 4. Dorsal (A) and ventral (B) views of Potamotrygon rex, sp. nov. (paratype, UNT 7144, 825 mm TL, 520 mm DL, 465 mm DW).

FIGURE 5. Dorsal (A) and ventral (B) views of juvenile specimen of Potamotrygon rex, sp. nov. (MZUSP 120352, 445 mm TL, 290 mm DL, 275 mm DW).

FIGURE 6. Morphological details of Potamotrygon rex, sp. nov. A) Dorsal view of snout region (paratype, MZUSP 120373); B) Lateral view of eye and spiracle (paratype, MZUSP 120373); C) Close-up of nasoral region (holotype, MZUSP 120371); D) Mouth of adult female showing external tooth rows (holotype, MZUSP 120371); E) Mouth of adult male showing external tooth rows (paratype, MZUSP 120373).

FIGURE 7. Dorsal view of posterolateral disc region in Potamotrygon rex, sp. nov., showing variation in dorsal color pattern. A) UNT 7137; B) UNT 7119; C) MZUSP 120365; D) MZUSP 120362; E) UNT 7111; F) UNT 7129; G) MZUSP 120373; H) MZUSP 120371 (holotype); I) MZUSP 120370.

FIGURE 8. Ventral disc in Potamotrygon rex, sp. nov., showing variation in ventral color pattern. A) MZUSP 120369; B) MZUSP 120368; C) MZUSP 120370; D) MZUSP 120367; E) UNT 7129; F) UNT 7143.

FIGURE 9. Tail region in Potamotrygon rex, sp. nov., showing morphology and distribution of dorsal tail thorns. A, B) Holotype (MZUSP 120371); C) Paratype (MZUSP 120373).

FIGURE 10. Radiographs of tails of Potamotrygon rex, sp. nov., showing arangement of thorns and denticles. A) MZUSP 120354; B) MZUSP 120356. Abbreviations: at, accessory terminal cartilage; ax, axial cartilage; b 1, basal (intermediate) segment 1; b 2, basal (intermediate) segment 2; bp, basipterygium; cr, cartilaginous rod; dm, dorsal marginal cartilage; dt 2, dorsal terminal 2 cartilage; vm, ventral marginal cartilage; vt, ventral terminal cartilage.

FIGURE 11. SEM photos of dorsal denticles near anterior middisc in Potamotrygon rex, sp. nov. A) Lateral view; B) and C) Dorsal views (all MZUSP 120372, adult female, 1050 mm TL, 660 mm DL, 620 mm DW).

FIGURE 12. Ventral lateral-line canals of Potamotrygon rex, sp. nov. (MZUSP 120352, 232 mm DW). Abbreviations: ast, anterior subpleural tubules; hyoc, hyomandibular canal; ioc, infraorbital canal; iol, infraorbital loop; jug, jugular canal; man, mandibular canal; pjl, posterior jugular loop; pnl, prenasal loop; pst, posterior subpleural tubule; sol, suborbital loop; spl, subpleural loop. The prenasal component of the nasal canal and the subrostral component of the supraorbital canal (both on the anterior snout) are not depicted for clarity.

FIGURE 13. Skeletal features of Potamotrygon rex, sp. nov., from radiographs, in dorsoventral view. A) Head region of MZUSP 120354; B) Head region of MZUSP 120353; C) Close-up of specimen in A; D) Base of tail region, showing distribution and development of dermal denticles and thorns (paratype, MZUSP 120369). Abbreviations: aac, anterior angular cartilage; ao, antorbital cartilage; apr, anterior segment of propterygium; fpf, frontoparietal fontanelle; hyo, hyomandibula; Mc, Meckel’s cartilage; no, nasal opening (nasal aperture); pac, posterior angular cartilage; pop, postorbital process; pq, palatoquadrate; pro, propterygium; prp, preorbital process; psc, prespiracular cartilage; sp, supraorbital process.

FIGURE 14. Map showing distribution of Potamotrygon rex, sp. nov., in the middle and upper rio Tocantins. Yellow star: holotype; red circles: other specimens examined (more than one specimen may be represented per circle).

FIGURE 15. Largest male (paratype, MZUSP 120373) on top of largest female (holotype, MZUSP 120371), demonstrating great size difference in Potamotrygon rex, sp. nov.

FIGURE 16. Comparative morphology of teeth and upper (top) and lower (bottom) tooth plates among (A, B) Potamotrygon rex, sp. nov. (MZUSP 120357, 340 mm TL, 210 mm DL, 201 mm DW, juvenile male), (C, D) P. h e nl e i (MZUSP 14768, 309 mm TL, 182 mm DL, 178 mm DW, juvenile female), and (E, F) P. leopoldi (MZUSP 35986, 346 mm TL, 221 mm DL, 211 mm DW, juvenile female). Note that specimen of P. re x is greater in size but has smaller teeth and lower tooth band in comparison to specimen of P. henlei.