Saturday, August 1, 2015

[Ichthyology • 2011] Heliotrygon gen. nov. • Morphology and Phylogenetic Relationships of A Remarkable New Genus and Two New Species of Neotropical Freshwater Stingrays (Chondrichthyes: Potamotrygonidae) from the Amazon Basin

Rosa’s Round Ray | Heliotrygon rosai
de Carvalho & Lovejoy, 2011

The morphology and phylogenetic relationships of a new genus and two new species of Neotropical freshwater stingrays, family Potamotrygonidae, are investigated and described in detail. The new genus, Heliotrygon, n. gen., and its two new species, Heliotrygon gomesi, n. sp. (type-species) and Heliotrygon rosai, n. sp., are compared to all genera and species of potamotrygonids, based on revisions in progress. Some of the derived features of Heliotrygon include its unique disc proportions (disc highly circular, convex anteriorly at snout region, its width and length very similar), extreme subdivision of suborbital canal (forming a complex honeycomb-like pattern anterolaterally on disc), stout and triangular pelvic girdle, extremely reduced caudal sting, basibranchial copula with very slender and acute anterior extension, and precerebral and frontoparietal fontanellae of about equal width, tapering very little posteriorly. Both new species can be distinguished by their unique color patterns: Heliotrygon gomesi is uniform gray to light tan or brownish dorsally, without distinct patterns, whereas Heliotrygon rosai is characterized by numerous white to creamy-white vermiculate markings over a light brown, tan or gray background color. Additional proportional characters that may further distinguish both species are also discussed. Morphological descriptions are provided for dermal denticles, ventral lateral-line canals, skeleton, and cranial, hyoid and mandibular muscles of Heliotrygon, which clearly corroborate it as the sister group of Paratrygon. Both genera share numerous derived features of the ventral lateral-line canals, neurocranium, scapulocoracoid, pectoral basals, clasper morphology, and specific patterns of the adductor mandibulae and spiracularis medialis muscles. Potamotrygon and Plesiotrygon are demonstrated to share derived characters of their ventral lateral-line canals, in addition to the presence of angular cartilages. Our morphological phylogeny is further corroborated by a molecular phylogenetic analysis of cytochrome b based on four sequences (637 base pairs in length), representing two distinct haplotypes for Heliotrygon gomesi. Parsimony analysis produced a single most parsimonious tree revealing Heliotrygon and Paratrygon as sister taxa (bootstrap proportion of 70%), which together are the sister group to a clade including Plesiotrygon and species of Potamotrygon. These unusual stingrays highlight that potamotrygonid diversity, both in terms of species composition and undetected morphological and molecular patterns, is still poorly known.

Key words: Heliotrygon rosai, Heliotrygon gomesi, taxonomy, systematics, Myliobatiformes, South America

Family Potamotrygonidae Garman, 1913

Genus Heliotrygon, n. gen.

Type-species. Heliotrygon gomesi, n. sp., by original designation.

Included species. Currently the new genus is composed of two species, Heliotrygon gomesi, n. sp., and Heliotrygon rosai, n. sp., both described below.

Etymology. From the Greek helios, meaning “sun”, in reference to its distinctively arranged pectoral disc radials that appear to “radiate” outward; and trygon, Greek for stingray. Gender feminine.

FIGURE 4. Paratype of Heliotrygon gomesi, n. sp., in dorsal (A) and ventral (B) views (ANSP 178017, preadult female, Rio Nanay, Rio Amazonas basin, 4.5 km west of Iquitos, Peru; 425 mm TL, 230 mm DL, and 224 mm DW).

Heliotrygon gomesi, n. sp.

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).

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).

Heliotrygon rosai, n. sp. 

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). 

Marcelo R. de Carvalho and Nathan R. Lovejoy. 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: 13–48. 

Pictures: Odd Stingless Stingrays Discovered in Amazon via @NatGeo