Heliotrygon gomesi De & Lovejoy, 2011, n. sp.

Heliotrygon gomesi, n. sp.

Figures 1–9; Tables 1–2

Holotype. MZUSP 104988, adult female, Rio Jamari, 04°18’15”S, 070°04’19”W, Benjamin Constant, Amazonas, Brazil, 05.ix.2006, coll. M. C. Júnior & M. V. Domingues (TA 06–17). (Figures 1, 8, 9 B).

Paratypes. (6 specimens). AMNH 251884, adult female, Rio Nanay, Rio Amazonas, Iquitos, Loreto, Peru (aquarium trade); ANSP 178014, adult male, Rio Nanay, Rio Amazonas basin, upstream from Pampa Chica, village 4.5 km west of Iquitos, Maynas, Loreto, Peru, coll. Linder Isuiza & ornamental fishermen, August 1999 to August 2001; ANSP 178015, adult (?) female, same data as ANSP 178014; ANSP 178016, adult male, same data as ANSP 178014; ANSP 178017, preadult (?) female, same data as ANSP 178014; MZUSP 108203, preadult female, Rio Nanay, Rio Amazonas basin, Iquitos, Loreto, Peru, coll. F. Marques (PU 09-33), 19.x.2009.

Other material. (6 specimens). AMNH 58402, preadult male, aquarium trade specimen reportedly from Ecuador (CJF, 24.xi.1988); AMNH (uncat.), preadult male (BES 04-00), Iquitos, Loreto, Peru (aquarium trade, probably from Rio Nanay, Rio Amazonas basin); AMNH (uncat.), preadult male, Iquitos, Loreto, Peru (aquarium trade, probably from Rio Nanay, Rio Amazonas basin); CU 78485, preadult (?) female, Iquitos, Loreto, Peru (aquarium trade, probably from Rio Nanay, Rio Amazonas basin); CU 78486, preadult (?) female, Iquitos, Loreto, Peru (aquarium trade, probably from Rio Nanay, Rio Amazonas basin); MZUSP 108294, aborted late-term pup from holotype, female, 125 mm DW, 135 mm DL, same data as holotype (specimen is cleared and stained).

Diagnosis. A species of Heliotrygon diagnosed by its unique dorsal color pattern, composed of a uniform gray to light tan or brown color, without strong vermiculations, rosettes or other conspicuous patterns dorsally. Heliotrygon gomesi is tentatively further separated from H. rosai (described below) by presenting a slightly more slender tail width at base (holotype tail width 4.5% DW vs. 5.6% DW in holotype of H. rosai), slightly greater preorbital snout length (holotype preorbital snout length 33.2% DW vs. 31.0% DW in holotype of H. rosai), and proportionally smaller pelvic inner length (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 vs. 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).

Description. Measurements, as both raw data in mm and transformed into %DW, are presented in Table 1; meristic data are given in Table 2. The description below is based on all specimens, but salient features of the holotype are separately mentioned. For the description below, refer to Figures 1–9.

External morphology. Disc very flat, its greatest height more or less equal to, or slightly greater than, interorbital space. Disc very circular, widest more or less at its midlength. Disc slightly longer than wide in all specimens; disc length varying from 102.2 to 108.0% DW (x = 104.7% DW; disc proportionally longest in holotype). Snout very broad, elongated, convex. Preorbital snout length about four times interorbital distance, ranging from 32.6 to 35.7% DW (x = 34.2% DW; 33.2% DW in holotype). Prenasal (26.7 to 31.2% DW, x = 29% DW, holotype 28.4% DW) and preoral (29.5 to 35.7% DW, x = 32.5% DW, holotype 32.2% DW) snout lengths also proportionally elongate. Snout with strongly circular anterior margin, and with minute rostral knob protruding from anterior disc (more prominent in smaller specimens). Posterior disc region slightly more oval than anterior disc region.

TABLE 1. Measurements of specimens of Heliotrygon gomesi, n. sp. A: MZUSP 104988 (holotype), adult female. B: AMNH 251884 (paratype), adult female. C: MZUSP 108203 (paratype), juvenile female. D: ANSP 178014 (paratype) adult male. E: ANSP 178015 (paratype), adult (?) female. F: ANSP 178016 (paratype), adult male. G: ANSP 178017 (paratype), juvenile female. H: AMNH 58402, preadult male. I: AMNH (uncat.), preadult male. x: mean.: standard deviation.

TABLE 1 (cont.). Measurements of specimens of Heliotrygon gomesi, n. sp. J: CU 78485, preadult female. K: CU 78486, preadult female. L: AMNH uncat.), preadult male. x: mean. SD: standard deviation.

PARAMETER J K L RANGE x SD Eyes minute, smaller than spiracles, and not greatly protruding above disc. Spiracles closely adjacent to eyes, rhomboidal in shape; spiracles about as wide as long, without elevated spiracular rims or central knob posteriorly. Mouth opening relatively straight across, its width about one-half distance between first gill slits, and more or less equal to internarial distance. Posterior profile of lower jaw marked with rugose grooves, but labial folds absent; outer corners of mouth also with marked grooves. Nostrils slightly smaller than eye-diameter, relatively circular. Nasal curtain medially notched at posterior margin; posterior margin highly fringed. Lateral margins of nasal curtain reaching to midlength of nostrils. Teeth externally visible with mouth closed; tooth row counts from radiographs 21‒29/21‒32; 29/28 exposed rows in holotype. Adult males with sharp, pointed cusps; adult females with smaller, less pointed and more broadly triangular cusps. Shallow integumental rugae posterior to mouth and anterior to gill-basket region. Branchial basket relatively short anteroposteriorly, much shorter than wide (branchial basket about half as long as wide; in holotype distance between first gill slits is 20.1% DW, and branchial basket length is 10.7% DW). Distance between first gill slits slightly greater than distance between fifth gill slits. Gill openings with slightly undulated gill flap; fifth gill slit smallest, slightly more obliquely positioned.

Pelvic fins completely concealed in dorsal view. Pelvic fins 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 varying from 3.9 to 7.8% DW, x = 5.4% DW; holotype tail width 4.5% 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 but ventral tail fold present, ridge-like, originating posterior to level of caudal sting (more concealed in juveniles) and extending caudally a short distance.

Ventral tail-fold anteriorly wider, extending posteriorly as a very shallow ridge. Caudal sting very reduced, smaller or more or less equal to spiracle length. Caudal sting single in specimens, without sharp, lateral, posteriorly oriented serrations; serrations extremely reduced (observable with magnification) and blunt. Caudal sting very slen- der, usually less than 1 mm in width, and positioned just posteriorly to posterior margin of disc.

Coloration. Dorsal color varying from a uniform gray to light tan or brown background, generally without conspicuous spots, vermiculations, or other distinctive markings. Holotype with darker, irregularly shaped spots, smaller than eye diameter, over dorsal disc region; spots vary in size and distribution, but do not form conspicuous vermiculations, rosettes or other patterns; holotype dorsal coloration may be indicative of older, larger individuals. Ventral coloration uniform creamy white over most of ventral disc and pelvic fins. Some specimens with small, diffuse brownish or grayish patterns over posterior disc margins, base of tail region, and outer and posterior pelvic fins; other specimens without any markings on posterior disc and pelvic fins. Ventral surface of holotype with very slender part of posterior disc and pelvics with darker markings. Ventral tail coloration with brownish marks usually posterior to caudal sting; ventral tail region of holotype with a central whitish stripe. Distal tail region darker in comparison to base of tail.

Remarks. Identification to species level of larger specimens of Heliotrygon is easily accomplished by examining dorsal coloration. However, the proportional measurements in the diagnosis employed to separate H. gomesi from H. rosai, n. sp. (see also description below), are in need of further verification in additional material of both species. The length of the pelvic fin internal margin appears to be a slightly more consistent proportional character, allowing for the separation of both species. The values provided above were extracted from all specimens and not only from the holotypes of both species, which are clearly distinct (2.1% DW in holotype of H. gomesi, and 4.6% DW in holotype of H. rosai). Taking all specimens into account, the differences in the mean concerning pelvic fin internal length is as great as the differences between the holotypes of both species (mean of 1.9% DW in H. gomesi, and 3.7% DW in H. rosai; Table 1).

The comparison of preorbital length proportions (and prenasal and preoral lengths) is slightly more difficult. The ranges of both species overlap for all specimens (32.6 to 35.7% DW in H. gomesi, and 31.0 to 34.2% DW in H. rosai), but directly comparing specimens of similar size, as with both holotypes (33.2% DW in holotype of H. gomesi and 31.0% DW in holotype of H. rosai), reveals slight distinctions. For example, preorbital snout length is relatively similar between MZUSP 108203 (H. gomesi) and MZUSP 108202 (H. rosai), even though the former is smaller in disc length (290 mm vs. 315 mm, respectively) and disc width (269 mm vs. 309 mm, respectively); preorbital snout length is 35.7% DW in MZUSP 108203 (H. gomesi) and 33.0% DW in MZUSP 108202 (H. rosai). Placing both individuals with their eyes aligned on same level, with ventral surfaces against each other, clearly shows that preorbital snout length is very similar even though the specimen of H. rosai is greater in disc length and width. In contrast, carrying out this exercise with a smaller specimen of H. rosai (MZUSP 108201) and the slightly larger MZUSP 108203 (H. gomesi) provided a different result, in which the preorbital snout length of the former was clearly smaller. Whether the snout length is truly diagnostic needs to be corroborated with the examination of more individuals from more localities. This is also true of tail width at base, which separates the holotypes quite easily, and certain specimens of similar proportions, but which proved more complex for all specimens examined.

According to our material, male specimens of H. gomesi become sexually mature between 345 mm DW (AMNH 58402, 361 mm DL, a specimen with claspers protruding beyond posterior margin of pelvic fins, but claspers very slender and uncalcified), and 400 mm DW (ANSP 178014, 420 mm DL, a specimen with stout claspers that protrude well beyond posterior pelvic margin). Size of maturity for females, based on preserved material, is more difficult to discern. The holotype aborted a very late-term pup when captured, but sexual maturity must occur well before 624 mm in DW. In potamotrygonids, and elasmobranchs in general, females grow to proportionally much greater sizes than males (Araújo, 1998; Rosa et al., 2010), but sexual maturation in H. gomesi probably occurs for females at about 440 mm in DW.

Geographic distribution. Heliotrygon gomesi is distributed in the upper Rio Amazonas basin according to our material (Figure 10), but enters, we suspect, the lower reaches of most major tributaries and the lower Rio Amazonas basin as well. Its distribution is very similar to that of Plesiotrygon iwamae (Rosa et al., 1987; Carvalho et al., 2003).

Etymology. This new species honors Ulisses L. Gomes, a pioneer in the study of elasmobranch morphology and systematics in Brazil, and an esteemed colleague and collaborator of the first author.

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