Plesiotrygon nana, De Carvalho & Ragno, 2011

De Carvalho, Marcelo R. & Ragno, Maíra P., 2011, An Unusual, Dwarf New Species Of Neotropical Freshwater Stingray, Plesiotrygon Nana Sp. Nov., From The Upper And Mid Amazon Basin: The Second Species Of Plesiotrygon (Chondrichthyes: Potamotrygonidae), Papéis Avulsos de Zoologia 51 (7), pp. 101-138 : 101-138

Plesiotrygon nana sp. nov.

( Figures 1-15 , 27 ; Tables 1-2 )

Holotype

MUSM 20328 , adult male (1024 mm TL, 243 mm DL, 247 mm DW), Río Pachitea , tributary of Río Ucayali, up-river from town of Puerto Inca , Puerto Inca Province, Huánuco Department, Peru, 09°25’S, 74°55’E, 15 August 2002, coll. Edgardo Castro ( Figures 1 , 2 , 8 , 9 , 11 , 13-15 ).

Paratypes

MUSM 40243 , preadult male (671 mm TL, 174 mm DL, 170 mm DW), Río Amazonas, Aucayo Caserio , near Tamshiyacu , Sargento Lores District , Maynas Province, Loreto Department, Peru, 03°59’13.21”S, 73°10’02.80”W, altitude 89 m, 15 November 2010, coll. Homero Sanchez ; MZUSP 108777 , juvenile female (463 mm TL, 81 mm DL, 72 mm DW), Río Itaya , tributary of Río Nanay (itself an affluent of Río Amazonas), near Iquitos, Departamento Loreto, Peru, 18 October 2009, coll. F. Marques ( PU 09-45 ) ( Figures 3-7 , 11 , 12 , 27 ).

Non type specimen

MZUSP 57642 , juvenile or preadult female, 912 mm TL, 124 mm DL, 118 mm DW, from Rio Solimões , just down-river from the confluence with Rio Purus ( Brazil), 03°36’22.9”S, 61°20’14.0”W, to 03°36’26.0”S, 61°19’52.3”W, 1996 Calhamazon Project, 28 Jul 1996, coll. A. Zanata et al., collected 50 m from shore over predominantly clay bottom (AMZ-96-081) .

Diagnosis

A species of Plesiotrygon diagnosed by the following unique characters: (1) dorsal color pattern composed of a dark gray to dark brown background color, with tan to yellow highly curved, slender and convoluted stripes or small spots forming rosettelike pattern over dorsal disc, or with creamy white to yellowish irregularly shaped, scattered spots and ocelli, smaller than interorbital distance (pale gray or brown dorsal background color in P. iwamae , composed of faint, incomplete markings outlined by small spots, or small irregular blotches and creamy, faint spots); (2) disc circular in preadults and adults, about as wide as long (markedly oval in P. iwamae in all sizes); (3) broadly rounded anterior disc (anterior disc sharply oval in P. iwamae ); (4) spiracles only faintly rhomboidal, very small, ranging from 2.8 to 3.5% DW in adult and preadult specimens (spiracle strongly rhomboidal, with mean spiracle length 6.8% DW in P. iwamae ); (5) snout very short, mean preorbital length 21.2% DW, mean prenasal length 15.2% DW, and mean preoral length 18.7% DW (snout proportionally much more elongate in P. iwamae , with mean preoral length 27.4% DW, mean prenasal length 19.5% DW, and mean preoral length 25% DW); (6) mouth and nostrils very slender especially in adult and preadult specimens, with mean mouth and internarial width 6.3% DW (mouth and internarial distance much greater in P. iwamae , with mean mouth width 10.9% DW, and mean internarial distance 8.7% DW); (7) denticles on dorsal tail relatively small, scattered, not forming row of greatly enlarged spines ( P. iwamae usually with a single irregular row of enlarged spines on dorsal tail region); (8) adult and preadult specimens with few (20-21/19) tooth rows (adult specimens of P. iwamae have numerous tooth rows, ranging from about 40-60/42-64); (9) caudal vertebrae ranging from 86-88, with a modal count of 86 (93-98, with a modal count of 94 caudal vertebrae in P. iwamae ); (10) total pectoral radials in adult and preadult specimens 90-91 (in P. iwamae , total pectoral radials varied from 77-84); (11) overall size very small (probably not surpassing 250 mm DL or DW), males sexually maturing probably between 180 and 220 mm DL, and 175 and 225 mm DW ( P. iwamae attains great sizes, upwards of 650 mm DL or DW, reaching sexual maturity only at about 420 mm DL or DW); (12) distal coloration of tail, as of caudal stings, usually a dark purplish brown, remaining this color to extremity of whip (in P. iwamae , tail as of caudal stings creamy white ventrally and light gray dorsally, with creamy white distal whip); (13) ventral lateral-line canals with the following unique characters: hyomandibular canal very narrow, with external and internal components close together; infraorbital and supraorbital canals not undulated; supraorbital and infraorbital loops small and narrow, without wavy contours; supraorbital loop of anterior infraorbital canal very short, not extending posteriorly to level of mouth; jugular and posterior infraorbital canals short, not extending posteriorly to close to gill slits; subpleural loop very narrow posteriorly; anterior and posterior subpleural tubules of hyomandibular canal absent (for comparison with ventral lateral-line canals in P. iwamae and other anatomical differences between both species, as well as further details concerning features listed here, see Discussion below).

External morphology

Disc very circular, about as long as wide, and widest at more or less midlength, near level of scapulocoracoid (for description below, refer to Figures 1-10 ). Disc length 98.4% DW in holotype, ranging between 98.4 and 112.5% DW (mean 104.4% DW) ( Table 1 ). Disc very low and flat, tallest at head region. Snout with broadly rounded anterior margin, and with a minute but clearly visible, rostral knob-like projection protruding from anterior disc. Rostral knob somewhat fleshy, well developed in all specimens. Snout length considerably small. Preorbital snout length less than one-fourth disc width, slightly less than twice interorbital distance; preorbital snout length from 20.0 to 22.7% DW (mean 21.2% DW). Prenasal (12.1 to 18.1% DW, mean 15.2% DW) and preoral (16.2 to 22.2% DW, mean 18.7% DW) snout lengths also relatively short, shorter than preorbital snout length. Eyes very small, slightly rounded, protruding very little from top of head and disc in live specimens (see Figure 27 ); eyes smaller than spiracles in diameter. Spiracles closely adjacent to eyes, ovalrounded, not very rhomboidal, and relatively small. Spiracles without elevated spiracular rims or central knob posteriorly. Interspiracular distance slightly greater than interorbital distance.

Mouth very small and narrow, with opening somewhat straight across. Mouth width less about one-third distance between first gill slits, and more or less equal to internarial distance. Two small labial folds present at outer jaw corners posterior to tubular narial fold, extending anterolaterally away from jaw corners. Nostrils anteroposteriorly elongated, slit-like, close in length to internarial distance. Nasal curtain very narrow, straight and not widening posteriorly, with highly fringed and medially notched posterior margin ( Figure 8 ). Rounded, tubular narial fold present lateral to posterior corners of nasal curtain. Teeth set in quincunx, not visible externally with mouth closed in holotype. Tooth rows 20/ 19 in holotype, 21/ 19 in male paratype, and 13/ 16 in much smaller female paratype; teeth small, rhomboidal, with small cusps in holotype (an adult male). Five buccal papillae present inside mouth. Branchial basket about twice as wide as long, its length about one-tenth DW. Distance between first gill slits slightly greater than distance between fifth gill slits. Gill openings slightly obliquely positioned, very small; fifth gill slit smallest.

Pelvic fins with broadly rounded outer margins, much wider than long (48.6 to 57.6% DW, mean 54.0% DW), and with undulating posterior margins. Pelvic fins protruding significantly from posterior disc region, and triangular in dorsoventral view, broadest at more or less posterior apices. Anterior margins of pelvic fins with marked angle at more or less midlength of anterior surface, more prominent in larger specimens; outer portions of pelvic fins fleshy. Clasper relatively slender, projecting well beyond posterior margin of pelvic fin ( Figure 9 ). Clasper dorsoventrally flattened, with rounded posterior tip. Clasper groove deflects inward toward midline at clasper glans region; dorsal pseudosiphon relatively small, positioned at a slight angle. Hypopyle at beginning of clasper glans, extending posteriorly in a very straight line. Ventral pseudosiphon situated on external margin of clasper tip, about as elongated as hypopyle.

Tail at base about as wide as interorbital distance (tail width 11.1 to 15.3% DW, mean 12.9% DW). Tail strongly tapering from base, terminating far posteriorly as an elongated, filiform whip. In holotype (adult male), disc length and width only about one-fourth of total length; in female paratype, total length almost six times as long as disc length or width; in male paratype distal tail slightly shorter). Tail whip decreasing in diameter posteriorly, terminating as a very small point. Tail dorsoventrally flattened in crosssection throughout, with ventral, medially positioned groove present from tail base, extending very posteriorly beyond caudal stings. Relatively broad ventral tail fold originating within groove at more or less level of caudal sting origin, tallest at more or less midlength of caudal stings, and extending posteriorly for more than twice length of caudal stings. Tail without lateral or dorsal tail folds. Cloaca to distal tail length great (311.2 to 540.3% DW, mean 402.8% DW). Caudal stings positioned far posteriorly on dorsal tail (distance from tail base to their origin greater than one-half of disc width). Caudal stings clearly greater than interorbital distance or tail width at base. Caudal stings very slender, their width in male paratype about one-twentieth their length, but in female paratype caudal sting width about one-tenth of length. Caudal stings with acute distal apex and with posteriorly directed and sharp lateral serrations.

Coloration

Dorsal color of disc somewhat variable in material studied ( Figures 1-7 , 10 ). In live specimens, colors more intense but not significantly different. Dorsal disc with light to dark brown or blackish-brown background color, slightly lighter close to disc margin in preserved material. Background color forming a dark reticulate pattern surrounding tan to yellowish irregular patterns of rosettes, spots and/or incomplete ocelli of alternating sizes. In holotype (largest male specimen), rosettes very conspicuous, formed from very slender and convoluted individual lines and small spots of lighter color. Largest rosettes about equal to interorbital distance in diameter, smaller rosettes about half this size. Larger rosettes present on midlateral disc, reducing in size toward outer disc. Outer disc with interrupted yellow lines, and outermost disc region with numerous very small tan to yellowish spots. Rosettes closely adjacent, and may mesh together forming elaborate patterns over mid and outer disc regions. Rosettes over central disc obscured by intense covering of dermal denticles. Dorsal aspect of pelvic fins with more individual spots, these relatively more isolated, occasionally forming incomplete ocelli. Claspers with creamy white background and irregular, diffuse brown and grayish blotches dorsally, more concentrated on region of hypopyle, clasper groove and dorsal clasper glans; claspers creamy white ventrally. Color of base of tail also obscured by denticles, but with a speckled darker and lighter color, with fewer lighter spots than outer disc. Lateral aspect of tail, anterior to caudal stings, with alternating creamy white and grayish or brown stripes; grayish stripes with diffuse whitish areas within; white stripes with slightly darker areas interspersed. Dorsal aspect of tail also with alternating pattern but much more concealed by denticles, not as sharp. Ventral surface of holotype a uniform creamy white, with slight patches of dusky gray at posterior disc and posterior pelvic fin margins. Ventral tail creamy white until about midlength of ventral tail fold, with darker bands alternating with lighter bands present until about just greater than one-half of tail length. Posterior more or less two-thirds of tail uniformly dark purplish brown; distal tail extremity evenly dark on all sides.

Preadult male paratype with slightly distinct col- or pattern, with more background color present, and with spots and ocellated markings relatively smaller than in holotype. Spots and ocelli more scattered, much smaller than interorbital distance, and irregularly shaped. Clearly defined rosettes, as in holotype, absent. Spots and dorsal markings tan to yellow, reducing in diameter closer to disc outer margin. Laterally elongated and irregularly shaped spots present on disc. Pelvic fins with similar dorsal pattern as disc, but with slightly more regularly spaced creamy white spots. Claspers also with dorsal pigmentation most concentrated on dorsal clasper glans region, similar to holotype. Base of tail more marked with creamy white bands on lateral aspect, and with more whitish markings compared to holotype. Alternating bands of darker gray and creamy white present distally on tail posterior to caudal stings. Distal filiform tail creamy white ventrally, not dark as in holotype and female paratype. On ventral disc and base of tail, male paratype creamy white throughout.

Female paratype with color pattern more or less similar to male paratype, composed of numerous creamy white to yellowish irregularly shaped spots and ocelli ( Figures 5-7 ). Spots very reduced in size, especially at outer disc. Spots and irregular ocelli greater at middisc region. Incomplete ocelli present mostly over central disc area. Laterally elongated and irregularly shaped spots present on disc. On anterior disc region, fine vermiculate pattern present. Base of tail with darker midline region and dorsolaterally positioned lighter ocelli. Lateral tail with alternating bands of lighter and darker from base of tail, where bands are less defined, to about one-half of tail length; tail uniformly very dark purplish-brown at posterior half to tail extremity. Solimões specimen with similar color pattern, but with more reticulate background, and larger, more rectangular lighter markings on central dorsal disc region; spots diminish in size closer to outer disc margin ( Figure 10 ). Caudal whip also dark purplish-brown in this specimen.

Dermal denticles

Dermal covering in holotype and preadult male paratype with low and relatively wide, intensely packed dermal denticles on anterior disc and snout, middisc and tail-base regions, with slightly larger, more acute spines on posterolateral disc, and sharp, taller spines on dorsal and lateral aspects of tail from tail base posteriorly to caudal stings ( Figures 11 , 13 , 14C ). Larger dorsal disc denticles with stellate bases, usually with five to ten basal ridges radiating from denticle base. Denticles thoroughly covering disc, even on outer disc margins. Denticles larger, with wider bases, and more closely packed on middisc especially over disc midline. Outer disc with slightly enlarged spines, more developed on anterior and posterior outer disc regions; these larger, outer disc denticles evenly spaced apart, relatively straight, not organized in rows, and surrounded by smaller denticles. These smaller denticles are also very erect, without radiating basal ridges. Base of tail region with numerous enlarged spines not forming regular, discrete rows. Enlarged spines much smaller than in P. iwamae , Paratrygon and species of Potamotrygon . Spines sharp, with enlarged, wide bases and short, radiating basal ridges; central crown well developed with acute, posteriorly directed, tall central spine. Spines greatest on anterior dorsal tail base region. Smaller spines present in between larger spines, evenly spaced apart, on base of tail, dorsal tail extending posteriorly to caudal stings, and lateral tail regions. Denticles on lateral tail region also well developed, extending posteriorly well beyond level of caudal stings. Distal tail whip with very minute denticles (‘prickles’) until more or less midtail area in both male specimens. Dorsal aspect of pelvic fins also with evenly scattered denticles, but smaller and less packed than on disc. Small female paratype with dorsal disc denticles less developed, not as sharp, but denticles numerous, and without developed dorsolateral disc spines or enlarged spines on dorsal and lateral tail regions. Solimões specimen with slightly more denticles and with small spines on dorsal disc and tail base (from photograph).

Ventral lateral-line canals

The principal canals of the ventral lateral-line system resemble those present in Plesiotrygon iwamae ( Figure 12 ) and in species of Potamotrygon , but are very distinct from Paratrygon or Heliotrygon (e.g. Carvalho & Lovejoy, 2011). In general, ventral lateral-line system narrow, occupying small portion of ventral disc between outer disc and branchial slits, but anterolateral disc and snout canals proportionally less narrow. Prenasal canal extends vertically from the nasal curtain to the anterior snout tip, running more or less parallel to its homologous component on the oth- er side. Prenasal is crossed anteriorly by infraorbital canal and is continuous posteriorly with supraorbital canal, where it forms a small, markedly triangular figure on nasal curtain. Infraorbital and supraorbital canals relatively straight, not undulated. Infraorbital canal with very straight, vertical external margin from supraorbital loop to infraorbital loop; supraorbital and infraorbital loops small, relatively narrow and uniform in width, without wavy contours. Supraorbital loop of anterior infraorbital canal very short, not extending posteriorly to level of mouth. Jugular canal somewhat narrow and also not undulated. Jugular and infraorbital canals not very elongate anteroposteriorly, not extending posteriorly to close to first pair of gill slits. Mandibular canal not observed. Hyomandibular canals forming very narrow loop posteriorly. External hyomandibular canal very straight, not curved or inflected, running very close to external infraorbital canal anteriorly and branchial slits at midlength. Internal hyomandibular canal slightly curved medially away from gill slits. Hyomandibular canal widest at more or less its posterior one-fifth where it bulges laterally, anterior to subpleural loop. Subpleural loop very narrow, somewhat acute posteriorly. Anterior and posterior subpleural tubules of hyomandibular canal absent.

Skeletal morphology

Neurocranium elongate, longer than twice greatest width, and widest at postorbital processes and nasal capsules. Neurocanium very slender at orbital region and central cranial floor, strongly tapering posteriorly from nasal capsules to postorbital processes. Nasal capsules relatively large, oval, broadly rounded anteriorly, and slightly inclined toward midline; internasal septum very slender ( Figures 13-15 ). Orbital region very concave. Preorbital processes broadly triangular and posterolaterally oriented. Postorbital processes very elongate, anterolaterally directed, reaching level of angular cartilages anteriorly. Precerebral and frontoparietal fontenellae long, but not as long as usually present in species of Potamotrygon , and about two-thirds of neurocranial length. Supraorbital process relatively wide and broadly triangular, situated just anterior to postorbital process. Neurocranium widens significantly at its posterior third where it articulates with hyomandibulae. Neurocranium very elongate posterior to postorbital processes; its length posterior to postorbital processe about 40% of neurocranial total length. Antorbital cartilage slightly laterally compressed, triangular, very slender and elongate, widest anteriorly, and extending posteriorly to level of palatoquadrates, anterior to angular cartilage.

Meckel’s cartilage very stout, with strong anteromedial deflection toward midline. Dorsally projecting lateral process of Meckel’s cartilage low and broadly triangular, not slender and elongate. Palatoquadrates very slender, shorter than Meckel’s cartilage, and also somewhat inclined toward midline ( Figures 14A, B ). Hyomandibulae relatively short, slender and more or less straight, widest at midlength; hyomandibula only slightly curved anteriorly toward midline, and faintly concave distally to accommodate hyomandibular- Meckelian tendon. Angular cartilage well developed, very stout, at least as stout as hyomandibula, and relatively short, about one-third length of hyomandibula ( Figures 14B, C , 15B ). Angular cartilage more or less straight, without concave anterior or posterior margins, slightly thicker closer to Meckel’cartilage. Hypobranchials slender, inclined toward midline, reaching anteriorly to level of postorbital processes of neurocranium. Basihyal element(s) not calcified, not apparent in radiographs. Basibranchial anteriorly triangular, extending forward to level of hyomandibular facet of neurocranium. Pseudohyoid arch more slen- der than subsequent branchial arches; all branchial arches short, not elongated laterally. Gill rays extending to propterygium.

Cervicothoracic synarcual elongate, its greatest width just under greatest width of neurocranium. Thoracolumbar synarcual very slender, not strongly calcified. Individual vertebral centra occurring posterior to level of caudal sting origin, last discernible centrum at more or less caudal sting extremity. Distal to caudal stings, an uncalcified notochordal extension (cartilaginous rod) present, continuing caudally toward whip extremity. Transition from mono- to diplospondyly occurs at fourth to fifth centra posterior to pelvic girdle.

Propterygium widest posteriorly, more stout than meso- and metapterygium ( Figures 13 , 14A , 15A ). Propterygium anteriorly also relatively wide, much more so than in species of Potamotrygon , extending anteriorly to level of posterior nasal capsules; anterior segment of propterygium smaller than width of nasal aperture, extending forward to almost nasal capsule anterior margin. Metapterygium more slender and slightly more broadly arched than propterygium, with two smaller, more slender posterior segments. Mesopterygium elongate and more slender anteriorly, slightly convex externally, and highly concave internally where it articulates with lateral aspect of scapulocoracoid. Articular surface with scapulocoracoid extensive, more so than in species of Potamotrygon . Mesopterygium extends posteriorly only slightly. Pectoral radial elements sometimes fused at base between mesopterygium and anterior metapterygium. Pectoral radials slender close to pectoral basals, slightly wider and shorter at middisc, and slender again distally; some 16 total lateral pectoral radial segments present (from pectoral basals to outer disc); pectoral basals bifurcating at distal segments 9 and 10. Scapulocoracoid, in ventral view, very elongate anteroposteriorly from where it articulates anteriorly with propterygium to articular area with metapterygium, proportionally much longer than in any potamotrygonid. Coracoid bar with straight posterior margin but highly concave and relatively narrow anterior border. Articular surface for fifth ceratobranchial on anterior surface of scapulocaoracoid markedly protruding.

Pelvic girdle with concave anterior margins lateral to prepelvic process, more concave than in P. iwamae . Lateral prepelvic processes rather low, not very acute. Iliac processes extending caudally beyond triangular ischial processes; both structures triangular and relatively slender. Puboischiadic bar anteroposteriorly elongate at sides, more so than in P. iwamae and species of Potamotrygon . Posterior margin of puboischiadic bar very concave, highly oval, extending posteriorly to a significant degree ( Figure 13 ). Three to four obturator foramina present. Basipterygium relatively wide, tapering posteriorly, about equal in length to one-half of puboischiadic bar width. First enlarged pelvic radial element articulating with lateral projection of iliac region, and about twice thickness of following radial segments. Pelvic radials subdivided laterally into three or four segments; segment contacting basipterygium much longer than others. Pelvic fin widest at sixth pelvic radial. Posteriormost radials articulating with basipterygium splayed. Clasper not dissected for skeleton, but in radiographs two basal segments discernible.

Remarks

The variation in dorsal color pattern observed among the three type-specimens of Plesiotrygon nana , although seemingly significant, is not enough to consider them separate species. Nor is the distinction in color pattern between the Rio Solimões specimen ( Figure 10 ) and those from Peru very great. All specimens represent different size-classes and stages of sexual maturity (and all come from different localities, although the paratypes were collected not too far from each other). The paratypes are more similar in color, with more isolated irregular spots and incomplete ocelli present on dorsal disc. In the preadult male, the light yellow dorsal markings are relatively more spaced-apart, which, in relation to the smaller female, represents a pattern that can be achieved with growth. The holotype, though, is somewhat distinct in dorsal color. The irregular rosettes formed by very slender lines and small spots on disc are not present in the other specimens. The preadult male (174 mm DL, 170 mm DW) is significantly smaller than the holotype (243 mm DL, 247 mm DW), but it is difficult to envision its color changing to resemble the holotype if it were to have kept growing, although this cannot be discarded based on our small sample size. Of the four specimens depicted in one aquarium source ( Ross & Schäfer, 2000: 140, 144), one has similarities in color with the male paratype (p. 144, figure on lower right), and another resembles the holotype to some degree (p. 140, figure on top). Clearly, P. nana should be expected to be somewhat variable in dorsal color.

The creamy white distal tail extremity in the preadult male paratype ( MUSM 40243; Figure 3 ) is, however, more difficult to account for. The caudal filiform whip in this specimen resembles that of the holotype from tail base to more or less level of caudal stings. The mid region of the whip in the holotype, however, has a predominantly dark grayish color, even where it is alternately banded with light- er stripes anterior to its midlength. But in MUSM 40243 the mid caudal whip region beyond the caudal stings is predominantly light colored ventrally, and remains so to caudal extremity. In contrast, in the holotype, the smaller female paratype and the Solimões specimen, the caudal whip becomes a uniform dark purplish-brown anterior to its midlength and remains this color posteriorly to caudal extremity. All aquarium specimens depicted in the literature (e.g. Ross & Schäfer, 2000) also have a very dark caudal whip for about two-thirds of tail length (hence one of their popular names, “black-tailed” antenna ray). Plesiotrygon iwamae , on the other hand, has a much lighter distal tail from level of caudal stings posteriorly (for about two-thirds tail length), and even when darker, the tail of P. iwamae is much lighter than the tail of P. nana . At present we can only conclude that this feature may be variable, even if minimally so (note that the holotype of P. iwamae has a grayish white distal tail whip, slightly darker than other specimens from Rio Solimões and from Peru). The preadult male paratype ( MUSM 40243) clearly shares many diagnostic features with the holotype and female paratype of P. nana , and cannot be identified with P. iwamae .

There is also variation in disc shape among the four specimens of Plesiotrygon nana . The small female paratype has a markedly oval disc, but the Solimões specimen has a rounder disc, whereas both larger males have clearly rounded discs (especially the holotype). These variations are ontogenetic, and not sexually dimorphic, as larger females depicted in the aquarium literature also have more circular discs compared to the small female paratype ( Ross & Schäfer, 2000).

Plesiotrygon nana is one of the smallest potamotrygonids known; it appears to be even smaller than Potamotrygon magdalenae and a new species of Potamotrygon from the Rio Negro basin. The holotype is a fully mature male (243 mm DL, 247 mm DW) that is probably close to the largest size reached by males of this species; females probably reach slightly larger sizes as is common in the family (in some species females are significantly larger; Araújo, 1998; Rosa et al., 2010). Males become sexually mature probably around 180 mm DL or DW but perhaps even smaller, as judged from the preadult male paratype that has well developed claspers, but which are still not fully rigid (it is probable that sexual maturity for this specimen would not have depended on additional growth). Remarkably, the smaller female, at 81 mm DL and 72 mm DW, was a free-living individual (there is no sign of an umbilical scar, just a small, dark-pigmented area), and at this size represents the smallest free living potamotrygonid recorded (the Rio Solimões specimen depicted in Figure 10 , which is slightly larger than the female paratype at 124 mm DL and 118 mm DW, was also a free-living specimen collected alone). Plesiotrygon is the only batoid genus (and probably chondrichthyan genus) in which one species reaches great sizes while its sister-group is so remarkably small in comparison; P. nana is almost three times smaller than P. iwamae , an interesting phenomenon in a family notorious for the great sizes (over 120 cm DW in some cases), attained by many of its members [e.g. Paratrygon aiereba (Müller & Henle, 1841) , Heliotrygon spp. , Potamotrygon brachyura ].

Geographic distribution

As far as is known, Plesiotrygon nana occurs in the upper Río Amazonas basin of Peru, both in small- er tributaries and in the main Río Amazonas channel, and in the lower course of Rio Solimões in Brazil, just down-river from the mouth of Rio Purus ( MZUSP 57642). The small female paratype is from Río Itaya, a small tributary of Río Nanay (near Iquitos). The male paratype was collected in Río Amazonas slightly farther south, near Tamshiyacu, and the holotype was found in Río Pachitea, a triburay of Río Ucayali near Puerto Inca ( Figure 16 ). This species may be expected to occur in Río Napo and other regions of the upper Río Amazonas inhabited by P. iwamae . Plesiotrygon nana is not restricted to the main Amazonas channel as previously thought (accounts from the aquarium fish trade conflict).

Etymology

The specific epithet nana is in reference to its dwarf size (from the Latin nanus). Gender feminine.

Common name

Dwarf antenna ray (as in aquarium literature; also known as “black-tailed” antenna ray).

Figures

FIGURE 1: Holotype of Plesiotrygon nana sp. nov., in dorsal (A) and ventral (B) views (MUSM 20328, adult male, 1024 mm TL, 243 mm DL, 247 mm DW, from Río Pachitea, tributary of Río Ucayali, near Puerto Inca, Peru).

FIGURE 2: Morphological details of holotype of Plesiotrygon nana sp. nov. (MUSM 20328, adult male, 1024 mm TL, 243 mm DL, 247 mm DW, from Río Pachitea, tributary of Río Ucayali, near Puerto Inca, Peru). A) Dorsal view of anterior disc and head region. B) Nasoral region. C) Dorsal view of base of tail and pelvic fins. D) Ventral view of pelvic fins and clasper. E) Dorsal view of tail region at caudal stings (removed). F) Lateral view of tail at region of caudal stings (removed), showing ventral tail-fold. Scale bar in A, C-F = 2 cm; B = 1 cm.

FIGURE 3: Paratype of Plesiotrygon nana sp. nov., in dorsal (A) and ventral (B) views (MUSM 40243, preadult male, 671 mm TL, 174 mm DL, 170 mm DW, Río Amazonas, near Tamshiyacu, Peru).

FIGURE 4: Morphological details of paratype of Plesiotrygon nana sp. nov. (MUSM 40243, preadult male, 671 mm TL, 174 mm DL, 170 mm DW, Río Amazonas, near Tamshiyacu, Peru). A) Dorsal view of anterior disc and head region. B) Nasoral region. C) Dorsal view of base of tail and pelvic fins. D) Dorsal view of claspers and pelvic fins. E) Ventral view of pelvic fins and claspers.F) Dorsal view of base of tail and tail region at caudal stings. G) Lateral view of tail at caudal sting region, showing ventral tail-fold. Scale bar in A, C, E, F, G = 2 cm; B, D = 1 cm.

FIGURE 5: Paratype of Plesiotrygon nana sp. nov., in dorsal (A) and ventral (B) views, shortly after capture (MZUSP 108777, juvenile female, 463 mm TL, 81 mm DL, 72 mm DW, Río Itaya, tributary of Río Nanay, near Iquitos, Peru). Image courtesy of F. Marques.

FIGURE 6: Paratype of Plesiotrygon nana sp. nov., in dorsal (A) and ventral (B) views (MZUSP 108777, juvenile female, 463 mm TL, 81 mm DL, 72 mm DW, Río Itaya, tributary of Río Nanay, near Iquitos, Peru).

FIGURE 7: Dorsal (A) and ventral (B) discs of paratype of Plesiotrygon nana sp. nov. (MZUSP 108777, juvenile female, 463 mm TL, 81 mm DL, 72 mm DW, Río Itaya, tributary of Río Nanay, near Iquitos, Peru).

FIGURE 8: Nasal curtain and mouth (closed) of holotype of Plesiotrygon nana sp. nov., showing triangular mark on nasal curtain formed by junction of suborbital and prenasal canals of ventral lateral-line system, tubular narial grooves, and labial folds (MUSM 20328, 243 mm DL, 247 mm DW). Distance between nostrils = 1.5 cm.

FIGURE 9: Dorsal (A, B) and external (C) aspect of clasper of holotype of Plesiotrygon nana sp. nov. (MUSM 20328, adult male, 243 mm DL, 247 mm DW). Abbreviations: CG, clasper groove; DPS, dorsal pseudosiphon; VPS, ventral pseudosiphon (mostly concealed in dorsal view). Scale bar = 1 cm.

FIGURE 10: Small, free-living specimen of Plesiotrygon nana sp. nov. MZUSP 57642, juvenile or preadult female 912 mm TL, 124 mm DL, 118 mm DW. Characters that allow it to be identified as P. nana include its dorsal color pattern, dark purplish-brown caudal whip, proportionally smaller spiracles and nasoral region, and ventral lateral-line canals. Photo courtesy M. Toledo-Piza.

FIGURE 11: Dermal denticles and larger spines in holotype (left column) and male paratype (right column) of Plesiotrygon nana sp. nov. A, B) Enlarged spine from dorsal tail base. C, D) Scale bars: A, B = 1 mm; C, D = 0.5 mm; E, F = 0.1 mm.

FIGURE 12: Ventral lateral-line system in species of Plesiotrygon. A) P. nana sp. nov. (based on female paratype, MZUSP 108777, 81 mm DL, 72 mm DW). B) P. iwamae (MZUSP 59899, 325 mm DL, 310 mm DW). Abbreviations: AST, anterior subpleural tubules; HYC, hyomandibular canal; IOC, infraorbital canal; IOL, infraorbital loop; JUG, jugular canal; PNS, prenasal canal; PST, posterior subpleural tubule; SOC, supraorbital canal; SOL, suborbital loop; SPL, subpleural loop. Scale bar = 3 cm.

FIGURE 13: Radiograph of skeleton of holotype of Plesiotrygon nana sp. nov. (MUSM 20328, adult male, 1024 mm TL, 243 mm DL, 247 mm DW). Distal tail filament not shown.

FIGURE 14: Details of the skeleton and dermal covering in holotype of Plesiotrygon nana sp. nov., from radiograph (MUSM 20328, adult male, 1024 mm TL, 243 mm DL, 247 mm DW). A) Splanchnocranium and scapular region. B) Angular cartilage magnified. C) Base of tail region showing arrangement of dorsal spines and denticles. For an anatomical guide, refer to Figure 15. Scale bar = 1 cm.

FIGURE 15: Skeletal aspects of holotype of Plesiotrygon nana sp. nov. (MUSM 20328, adult male, 1024 mm TL, 243 mm DL, 247 mm DW). A) Articulation between pectoral basal elements and shoulder girdle, showing pectoral radials and lateral aspect of scapulocoracoid. B) Detail of propterygium, neurocranium, and mandibular and hyoid arches in ventral view. Abbreviations: AC, angular cartilage; AOC, antorbital cartilage; HYO, hyomandibula; MC, Meckel’s cartilage; MES, mesopterygium; MET, metapterygium; NA, nasal aperture; POP, postorbital process; PQ, palatoquadrate; PR, pectoral radials; PRO, propterygium; SCP, scapulocoracoid.

FIGURE 16: Distribution of specimens of Plesiotrygon examined in this study (see text for details). Square with star: holotype of P. nana (Río Pachitea, tributary of Río Ucayali, near Puerto Inca, Peru). Open square: paratypes of P. nana (representing two separate, but proximal, localities). Closed square: specimen collected and photographed from Rio Solimões in 1996. Closed circle with star: holotype of P. iwamae (Rio Solimões, above Tefé). Open circles: paratypes of P. iwamae. Closed circles: non-type specimens of P. iwamae. Base map by M.J. Weitzman.

FIGURE 27: Anterior disc and head region depicting relative proportions of eyes and spiracles in (A) subadult male paratype of Plesiotrygon nana sp. nov. (MUSM 40243, 174 mm DL, 170 mm DW), and (B) juvenile male Plesiotrygon cf. iwamae (MUSM 39977, 221 mm DL, 211 mm DW). Scale bar in A = 1 cm; B = 2 cm.