Heliotrygon rosai, De, Marcelo R. & Lovejoy, Nathan R., 2011

De, Marcelo R. & Lovejoy, Nathan R., 2011, Morphology and phylogenetic relationships of a remarkable new genus and two new species of Neotropical freshwater stingrays from the Amazon basin (Chondrichthyes: Potamotrygonidae), Zootaxa 2776, pp. 13-48 : 29-36

Heliotrygon rosai , n. sp.

Figures 11–16 ; Tables 3–4

Holotype. MZUSP 104996, adult male, Baía de Marajó, 0°55’34’’S, 48°17’25’’W, Colares, Pará, Brazil, coll. P. C.-Almeida. ( Figures 11 , 16 ).

Paratypes. (4 specimens). AMNH 251885, preadult male, Rio Nanay, Rio Amazonas basin, Iquitos, Loreto, Peru (aquarium trade specimen) ( Figures 12 , 13 ); MZUSP 108200, juvenile female, Rio Nanay, Rio Amazonas basin, Iquitos, Loreto, Peru, coll. F. Marques (PU 09-06), 17.x.2009 ( Figure 14); MZUSP 108201, juvenile female, same locality and collection date as MZUSP 108200, coll. F. Marques (PU 09-05) ( Figure 14); MZUSP 108202, juvenile male, same locality as MZUSP 108200, coll. F. Marques (PU 09-19), 18.x.2009 ( Figure 14).

Other material. (2 specimens). MZUSP 63604, juvenile male, Rio Purus, 1.1 km downriver from São Tomé, 03°48’10”S, 061°25’17”W, Amazonas state, Brazil, coll. S. Jewett & collaborators (ACFSLJ 93010), 25.x.1993 ( Figure 15 ); MZUSP 108295, adult male, Baía de Marajó, Colares, Pará, Brazil, 4.xi.2003, coll. F. Marques & P. C.-Almeida (PA03-49) (very large specimen, between 700 and 800 mm DW, heavily dissected, missing lateral disc, snout and tail regions).

Diagnosis. A species of Heliotrygon diagnosed by its unique dorsal color pattern, composed of numerous white to creamy-white vermiculate markings over a light brown, tan or gray background color. Heliotrygon rosai is tentatively further separated from H. gomesi by presenting a slightly more slender tail width at base (5.5% DW in holotype of H. rosai vs. 4.5% DW in holotype of H. gomesi ), slightly greater preorbital snout length (31% DW in holotype of H. rosai vs. 33.2% DW in holotype of H. gomesi ), and smaller pectoral axil to pelvic inner length (4.6% DW in holotype of H. rosai , ranging from 1.3 to 4.9% DW in all specimens, with a mean of 3.7% DW vs. 2.1% DW in holotype of H. gomesi , and ranging from 1.0 to 3.3% DW in all specimens, with a mean of 1.9% DW).

Description. Measurements are presented as raw data in mm and percentages of DW (Table 3); meristic characters are provided in Table 4 . As with H. gomesi , the description is based on all specimens, with significant features of the holotype separately mentioned. For the description below, refer to Figures 11–16 .

External morphology. Disc flat, its greatest height more or less equal to interorbital space (less than one-tenth of disc width). Disc extremely circular, widest at midlength near, or just anterior to, scapulocoracoid. Disc length varying from 101.9% to 108% DW (x = 104.9% DW; in holotype disc length 104.7% DW). Snout broadly rounded, elongated, and convex. Snout length about four times interorbital distance. Preorbital snout length ranging from 31 to 34.2 % DW (x = 33.1% DW; in holotype, preorbital snout length 31% DW). Prenasal snout length ranging between 26.5 and 29.4% DW (x = 28.3% DW; in holotype, prenasal snout length 26.5% DW). Preoral snout varying from 29.6% to 34.2% DW (x =32.3% DW; in holotype, preoral snout length 29.6% DW). Snout with strongly circular anterior margin, and with minute rostral knob protruding from anterior disc (more evident in juveniles). Posterior disc region slightly more oval than anterior disc region.

Eyes very small, smaller than spiracles, and not protruding significantly above disc. Spiracles closely adjacent to eyes, rhomboidal. Spiracles about as wide as long, without elevated spiracular rims or central knob posteriorly. Mouth opening relatively straight across, its width close to one-half distance between first gill slits, and about equal to internarial distance. Posterior profile of lower jaw presenting rugose grooves; labial folds absent. Outer corners of mouth also with marked grooves. Nostrils about equal or slightly smaller than eye-diameter; nostrils circular. Nasal curtain medially notched posteriorly, with posterior margin greatly fringed. Lateral margins of nasal curtain reaching to nostril midlength. Teeth externally visible with mouth closed; tooth row counts 28/ 28 in holotype. Adult males with sharp, pointed cusps; cusps in adult females smaller, less pointed and more evenly triangular. Shallow integumental rugae present ventrally posterior to mouth and anterior to branchial basket region. Branchial basket relatively short anteroposteriorly, much shorter than wide (branchial basket length ranges from 8.2 to 11.5% DW, whereas distance between first gill openings varies between 17.3 to 20.9% DW). Distance between first gill slits slightly greater than distance between fifth gill slits. Gill openings with slightly curved gill flap; fifth gill slit smallest, slightly more obliquely positioned compared to first gill slit.

Pelvic fins completely hidden in dorsal view, relatively short anteroposteriorly (much wider than long), and with undulating posterior margins. Pelvic fins broadest at more or less their midlength, with broadly oval apices. Tail very slender at base, its width more or less one-half of interorbital distance (tail width ranging between 4.9% and 5.8% DW, x = 5.3%; in holotype, tail width 5.6% DW). Tail tapering to an elongated whip, usually greater than disc length but frequently broken, even in juveniles. Adults with much shorter tail, usually extending posteriorly a distance similar to width of pelvics. Tail with very slender, ridge-like lateral tail folds originating at base of tail and extending to more or less level of caudal sting; dorsal tail fold lacking in most specimens but present in holotype; ventral tail fold present in holotype as low ridge, originating posterior to level of caudal sting and extending caudally as a very low keel; ventral keel absent in some other specimens. Caudal sting significantly reduced, smaller or more or less equal to spiracle length. Caudal sting single in specimens in which they were not lost, without sharp, lateral, posteriorly oriented serrations; serrations extremely reduced in size and blunt. Caudal sting very slender, usually less than 1 mm in width, and positioned just posteriorly to posterior margin of disc.

Clasper dorsoventrally depressed ( Figure 24 ), relatively short (length 12.1% DW) and stout, wide at base and narrowing only slightly distally, with rounded to more or less straight extremity. Clasper groove beginning proximally, well before level of posterior margin of pelvics. Anterior half of clasper groove running obliquely from inner margin to outer margin of clasper. Posterior half of clasper groove curving inward at level of dorsal pseudosiphon, reaching midline caudal to posterior border of dorsal pseudosiphon, and extending to clasper tip. Dorsal pseudosiphon small in relation to clasper length, near inner clasper edge. Dorsal pseudosiphon elliptical, and obliquely oriented in relation to midline. Ventral pseudosiphon well developed, located at lateral distal edge of clasper.

Coloration. Dorsal coloration somewhat variable among specimens, more so than in H. gomesi . Dorsal coloration composed of numerous beige to whitish vermicular markings forming complex, reticular patterns over a brown, tan or gray background color. Vermiculations may delimit circular areas of only background color, usually smaller than interorbital distance, on dorsal disc region (as in holotype; Figure 11 ). These brownish blotches distributed in a regular fashion over disc, more or less evenly spaced apart, and diminishing in size near outer disc margins. Specimen AMNH 104996 (preadult male paratype) lacks brownish blotches on dorsal disc, with dorsal color composed solely of more regularly distributed vermiculate markings ( Figure 12 ). Variation in dorsal color pattern not a function of gender ( Figure 14). Conspicuous dorsal coloration pattern also over pelvic fins and base of tail region. Ventral coloration mostly a uniform, creamy-white, but with darker blotches on posterior disc margins, ventral tail region and pelvic fins (both posteriorly and laterally) in larger specimens. Darker posterior disc margins reach anteriorly to posterior third of disc length in holotype. Pelvic fin ventral color varied among specimens as some present faint markings whereas others present stronger patterns. Ventral tail coloration darker posteriorly.

TABLE 3. Measurements of specimens of Heliotrygon rosai , n. sp. A: MZUSP 104996 (holotype), adult male. B: AMNH 251885 (paratype), preadult male. C: MZUSP 108200 (paratype), juvenile female. D: MZUSP 108201 (paratype), juvenile female. E: MZUSP 108202 (paratype), juvenile male. F: MZUSP 63604, juvenile male. x: mean. SD: standard deviation.

Remarks. For comments on identifying both H. rosai and H. gomesi based on proportional measurements, see Remarks for the latter species, above. Information on sexual maturity for H. rosai females, based on preserved material is not as clear as in H. gomesi , as the larger specimens are male. The holotype is clearly sexually mature, but the large paratype (AMNH 281885) has claspers that are still flexible, easily bent dorsally, not as stout and firm as in the holotype. Sexual maturity for males therefore occurs between 483 and 577 mm DW (and 500 and 604 mm DL).

In terms of the size that these stingrays may reach, we examined and dissected a specimen (MZUSP 108295) that had both sides of the disc removed, along with the tail and snout region. This specimen is clearly much larger than the adult male holotype and also much thicker at the scapulocoracoid region. The measurements obtained from this specimen are as follows: interorbital width: 59 mm; interspiracular width: 67 mm; eye length: 9 mm; spiracle length: 27 mm; mouth width: 65 mm; distance between first gill slits: 121 mm; distance between fifth gill slits: 110 mm; branchial basket length: 61 mm; pelvic fin anterior width: 92 mm; and tail width at base: 28 mm. From the relative proportions deduced from some of these measurements, in particular interspiracular and interorbital distances and mouth width (which is not easily distorted even in a mutilated specimen), and compared to other specimens in hand, we may infer that this specimen may have been around 700 to 750 mm in DW, and slightly greater in DL. This is no surprise if we examine the very large sizes reached in the closely related Paratrygon aiereba (specimens greater than 100 cm in DW have been reported).

The holotype of H. rosai had ingested a catfish (Siluriformes, possibly a doradid) about the size of the metapterygium (visible in radiographs). The catfish was ingested whole, head first, and is relatively intact in the abdominal cavity.

Geographic distribution. Heliotrygon rosai is known from the upper, mid and lower Rio Amazonas basin, and probably also enters, we believe, the lower reaches of its major tributaries ( Figure 17 ). As with H. gomesi , its distribution is similar to that of Plesiotrygon iwamae ( Rosa et al., 1987; Carvalho et al., 2003).

Etymology. This new species is named after Ricardo S. Rosa, whose excellent revision of potamotrygonids ( Rosa, 1985a) represents a landmark in our understanding of the taxonomy and diversity of this family.

Proposed common name. Rosa’s round ray (also referred to as "coly" ray in the aquarium trade).

Figures

FIGURE 11. Holotype of Heliotrygon rosai, n. sp., in dorsal (A) and ventral (B) views (MZUSP 104996, adult male, Baía de Marajó, Pará state, Brazil; 984 mm TL, 604 mm DL, 577 mm DW).

FIGURE 12. Paratype of Heliotrygon rosai, n. sp., in dorsal (A) and ventral (B) views (AMNH 251885, preadult male, Rio Nanay, Rio Amazonas basin, Iquitos, Peru; 599 mm TL, 500 mm DL, 483 mm DW).

FIGURE 13. Illustration of paratype of Heliotrygon rosai, n. sp., in dorsal view (based on AMNH 251885; see Figure 12).

FIGURE 15. Illustration of Heliotrygon rosai, n. sp., in dorsal (A) and ventral (B) views (MZUSP 63604, juvenile male, Rio Purus, Amazonas state, Brazil, 680 mm TL, 247 mm DL, 231 mm DW). Note that dorsal color pattern has faded significantly in this specimen.

FIGURE 16. Morphological details of holotype of Heliotrygon rosai, n. sp. A) Eyes and spiracles. B) Nasoral region. C) Dorsal tail region (claspers barely noticeable; specimen is an adult). D) Ventral view of pelvic fins and base of tail.

FIGURE 17. Geographic distribution of Heliotrygon rosai, n. sp. Star depicts approximate locality of holotype.

FIGURE 24. Clasper morphology of Heliotrygon rosai, n. sp. A) External morphology (holotype, MZUSP 104996). B) Clasper skeleton in dorsal view (MZUSP 108295). C) Clasper skeleton in ventral view (MZUSP 108295). Anterior facing top of page. Abbreviations: AT, accessory terminal cartilage; AX, axial cartilage; BE, beta cartilage; B 1, first basal segment; B 2, second basal segment; CG, clasper groove; DM, dorsal marginal cartilage; DPS, dorsal pseudosiphon; DT 2, dorsal terminal 2 cartilage; VM, ventral marginal cartilage; VPS, ventral pseudosiphon; VT, ventral terminal cartilage.

Tables

TABLE 4. Meristc data for specimens of Heliotrygon rosai, n. sp. A: MZUSP 104996 (holotype), adult male. B: AMNH 251885 (paratype), preadult male. C: MZUSP 108200 (paratype), juvenile female. D: MZUSP 108201 (paratype), juvenile female. E: MZUSP 108202 (paratype), juvenile male. F: MZUSP 63064, juvenile male.

CHARACTER	A	B	C	D	E	F	Range	Mode
precaudal vertebrae	32	29	31	32	28	31	28–32	32
caudal vertebrae	71	71	50	56	65	64	50–71	71
total vertebrae	103	100	81	88	93	95	81–103	-
diplospondylous vertebrae	70	66	-	51	62	60	51–70	-
upper tooth rows (radiograph)	26	-	-	-	-	21	21–26	-
lower tooth rows (radiograph)	28	-	-	-	-	21	21–28	-
upper tooth rows (specimen)	28	19	11	18	18	20	11–28	18
lower tooth rows (specimen)	28	19	10	18	18	21	10–28	18
propterygial radials	50	47	46	42	46	49	42–50	46
mesopterygial radials	26	25	26	25	26	23	25–26	26
metapterygial radials	37	37	36	36	33	38	33–38	37
total pectoral radials	113	-	108	103	105	111	103–113	-
pelvic radials	21	21	18	18	17	22	17–22	21