Potamotrygon tigrina

De, Marcelo R., Sabaj, Mark H. & Lovejoy, Nathan R., 2011, Potamotrygon tigrina, a new species of freshwater stingray from the upper Amazon basin, closely related to Potamotrygon schroederi Fernandez-Yépez, 1958 (Chondrichthyes: Potamotrygonidae), Zootaxa 2827, pp. 1-30 : 26-28

Comparisons of Potamotrygon tigrina with congeners

Potamotrygon tigrina is distinguished from all species of Potamotrygon , including P. schroederi , by its unique dorsal color pattern. Many other features (proportions of disc and tail, numbers of pectoral and pelvic fin rays, numbers of vertebrae, dentition, dermal covering, lateral line canals, etc.) further distinguish P. t i g r i n a from congeners, but we restrict our comparisons here mostly to coloration. Potamotrygon tigrina cannot be confused with P. brachyura , P. marinae , P. orbignyi , and P. h u m e ro s a, species with a dorsal disc color composed of a dark brown to blackish reticulate pattern over a light brown or grayish background; in P. orbignyi and P. h u m e ro s a, but especially in P. orbignyi , the reticulations may be mixed with yellowish spots and blotches of varying size ( Silva, 2010). Potamotrygon brachyura , restricted to the Paraná-Paraguay and Uruguay basins, has a markedly circular disc with a proportionally much shorter tail (only slightly greater than one-half DW).

The following species of Potamotrygon have a dorsal disc color composed of numerous spots, smaller irregular markings, and/or ocelli of variable size, but which do not form vermiculate patterns as in P. t i g r i n a: P. m o t o ro, P. s c o b i n a, P. signata , P. yepezi Castex & Castello, 1970 , P. boesemani Rosa, Carvalho & Wanderley, 2008 , P. h e n - lei (Castelnau, 1855), and P. leopoldi Castex & Castello, 1970 ; the latter two species also have a striking black to grayish-black dorsal background color. Potamotrygon magdalenae (Valenciennes, 1865) , a very small species from the Magdalena and Atrato basins in Colombia, also has numerous small spots on dorsal surface, but these may form vermiculate markings that are, however, much finer than in P. t i g r i n a (some smaller specimens of P. signata may also have finer vermiculations); P. magdalenae also has a lighter brown to olive background color and lacks wide alternating vertical stripes on tail (as do all species mentioned above).

Other species of Potamotrygon may have developed vermiculate patterns, such as P. falkneri and, to a much lesser degree, P. histrix , but never resembling the specific design of P. t ig r in a. Potamotrygon falkneri (including its junior synonym P. castexi Castello & Yagolkowski, 1969 ; Silva & Carvalho, 2011), a widely distributed species in the Paraná-Paraguay system but which also occurs in the Amazon basin (in the upper Rio Madeira and upper Río Marañon; Silva & Carvalho, 2011), may have significant vermiculate markings, but when present these are distinct from P. t ig r in a in being much finer, more tightly arranged. Potamotrygon falkneri is greatly variable in dorsal color, ranging from relatively small (smaller or equal to eye diameter) orange to tan spots to a fine vermiculate pattern, over a brownish background color (great color variation is present throughout its range). But this species, in contrast to P. tigrina , has a very particular ventral disc coloration (with brownish markings on posterior and lateral disc, as well as pelvic fins and tail), dorsal and lateral tail coloration usually also with small vermiculate markings (never with wide bars of alternating dark and light color), and two angular cartilages on each side (for further distinctions between P. falkneri and P. tigrina , see Remarks above under this species; also Silva & Carvalho, 2011). Potamotrygon histrix is a relatively small species from the Paraná-Paraguay basin with a dark gray to greenishblack dorsal and ventral coloration (ventral color pattern over most of disc and tail), with an incomplete reticulate or vermiculate pattern dorsally, and disc and tail region covered in sharp, well developed denticles (also present on tail sides). When vermiculate markings are present in P. histrix , these never form the conspicuous, relatively wide and highly convoluted vermiculate patterns present in P. tigrina .

Species of Potamotrygon with a single developed angular cartilage associated to each hyomandibula, as in P. tigrina , include P. schroederi , P. histrix , P. orbignyi , P. h u m e ro s a, and P. m a r i na e. Potamotrygon signata has a laterally elongate anterior angular (reaching at least one-half of hyomandibula length; Carvalho et al., 2004) and a smaller, somewhat rounded posterior angular that is closely adjacent to the hyomandibula (and easily missed in dissections; note that in P. ti gr i na and P. schroederi a very minute, uncalcified and concealed posterior angular element may be present in some specimens). The angular of P. h i s t r i x is anteriorly strongly concave in most specimens examined, and relatively wider than in the holotype of P. tigrina . In P. orbignyi and P. h u m e ro s a, and to a lesser extent P. marinae , the angular is also wider than in P. tigrina (but not as great as the anterior angular in P. signata ); the angular of P. t i g r i n a is more similar in size to that of P. schroederi , but this may be slightly variable, as in the paratype the angulars are proportionally slightly greater. With the exception of P. schroederi , however, the above species have markedly reticulate dorsal background patterns (less so in P. histrix , but see above) and cannot be confused with P. tigrina ( Silva, 2010) . The reticulate dorsal pattern in P. marinae , described from French Guiana ( Deynat, 2006), delimits large, irregular, lighter blotches formed from the association of smaller spots (much more clearly visible in live and freshly captured material). Ventral coloration in P. m a r i n a e, however, is composed of intense, dark brown blotches over mid, lateral and posterior disc, and most of pelvic fins; the ventral tail also presents a dark midline with lighter patterns laterally (in P. t i g r i n a, ventral coloration is a uniform creamy white, with a creamy white ventral tail that may have darker lateral blotches on sides). Potamotrygon marinae is further distinguished from P. t ig r in a in proportions of disc, pelvic fins, and tail, as well as in dermal covering, according to material we have examined (further information on this species is provided in Silva, 2010).

Potamotrygon tigrina and P. schroederi share features that appear to be derived within Potamotrygonidae . In particular, the strongly banded, alternating dark and light colored distal tail region is very similar and clearly divergent from the tail coloration in any other potamotrygonid species. Also, some of the anterior ventral lateral line canals of P. schroederi and P. t i g r i n a are differently arranged from those of other species of Potamotrygon (and Plesiotrygon iwamae as well, which has ventral lateral line canals similar to Potamotrygon ). These include the very slender, compressed posterior jugular loop (formed between the jugular and infraorbital canals; in other Potamotrygon species this loop is more broad), and the highly acute anterior angle formed by three different components: the suborbital and rostral loops of the infraorbital canal, and the orbitonasal component of the supraorbital canal (Figure 20); in other species of Potamotrygon the anterior angle in these canals is less acute (based on work in progress comparing the lateral line canals of all potamotrygonid species). These features, however, need further comparisons on more specimens of P. tigrina , as only the holotype was examined. Potamotrygon tigrina and P. schroederi also share specific angular cartilage morphology and angular cartilage proportions ( Figures 5 B, 17B), as well as general morphometrics and meristics, and arrangement of spines on dorsal tail region. The specific distal tail color, (single) angular cartilage morphology, patterns of the anterior ventral lateral line canals, and particular dorsal tailspine arrangement, are considered here derived features within potamotrygonids hypothesized to be indicative of their close relationship (to be tested in a morphological species level phylogeny currently in preparation by the senior author). Both P. schroederi and P. t i g r i n a also share a similar outer disc margin coloration composed of numerous small spots that may coalesce and form more elaborate patterns, but this feature is more widespread among species of Potamotrygon .

Potamotrygon tigrina and P. schroederi are easily distinguished from each other by their highly conspicuous dorsal color patterns. The background color is similar in both species, but in P. schroederi , due to the particular design and arrangement of the yellowish rosettes, brain-shaped figures and other lighter markings, the background color appears far more evenly reticulate or mesh-like than in P. tigrina (but note that the head and interorbital region of P. schroederi may present finer vermiculate markings). In P. tigrina , the dark brown to blackish dorsal background color is highly interwoven with the bright yellow to orange vermiculate markings, not forming a more evenly defined reticulate pattern, whereas in P. schroederi the darker background color is not as tortuously intricate with the rosettes and brain-shaped figures on mid, lateral and posterior disc regions.

Figures

FIGURE 5. Details of the skeleton and dermal covering in Potamotrygon tigrina, n. sp., from holotype (MUSM 39978, 495 mm TL, 271 mm DL, and 258 mm DW). A) Splanchnocranium and scapular region (most of gill arches removed). B) Close-up of angular cartilage. C) Base of tail region showing arrangement of dorsal spines and denticles.