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Cytogenetic studies in the subfamily Potamotrygoninae have provided valuable insights into the understanding of the evolution and diversification of its species. In the present study, the chromosomal features of seven nominal potamotrygonin species are provided: Plesiotrygon iwamae (2n=74, FN=120), Potamotrygon amazona (2n=66, FN=107), P. constellata (2n=66, FN=110), P. leopoldi (2n=64, FN=102), P. motoro (2n=66, FN=106) from four different localities, and P. orbignyi (2n=66, FN=106), P. scobina (2n=66, FN=104), from Central Amazon. Additionally, we found a new karyomorph in P. wallacei. We considered the localization of Nucleolus Organizer Regions (NORs), as well as the pattern of constitutive heterochromatin, as species-specific characters. We found an XX/XY sex chromosome system in P. orbignyi, and we suggest that P. scobina and P. amazona also possess the same sex chromosome system. Overall, the chromosomal evolution in this group appears to have progressed towards a reduction in diploid number, with a concomitant increase in the number of bi-armed and nucleolar chromosomes. © 2019, Brazilian Journal of Genetics. All rights reserved.

Durophagous predators consume hard-shelled prey such as bivalves, gastropods, and large crustaceans, typically by crushing the mineralized exoskeleton. This is costly from the point of view of the bite forces involved, handling times, and the stresses inflicted on the predator's skeleton. It is not uncommon for durophagous taxa to display an ontogenetic shift from softer to harder prey items, implying that it is relatively difficult for smaller animals to consume shelled prey. Batoid fishes (rays, skates, sawfishes, and guitarfishes) have independently evolved durophagy multiple times, despite the challenges associated with crushing prey harder than their own cartilaginous skeleton. Potamotrygon leopoldi is a durophagous freshwater ray endemic to the Xingu River in Brazil, with a jaw morphology superficially similar to its distant durophagous marine relatives, eagle rays (e.g., Aetomylaeus, Aetobatus). We used second moment of area as a proxy for the ability to resist bending and analyzed the arrangement of the mineralized skeleton of the jaw of P. leopoldi over ontogeny using data from computed tomography (CT) scans. The jaws of P. leopoldi do not resist bending nearly as well as other durophagous elasmobranchs, and the jaws are stiffest nearest the joints rather than beneath the dentition. While second moment has similar material distribution over ontogeny, mineralization of the jaws under the teeth increases with age. Neonate rays have low jaw stiffness and poor mineralization, suggesting that P. leopoldi may not feed on hard-shelled prey early in life. These differences in the shape, stiffness and mineralization of the jaws of P. leopoldi compared to its durophagous relatives show there are several solutions to the problem of crushing shelled prey with a compliant skeleton.

In the Xingu river basin, Brazil, occurs two genera of Potamotrygonidae family: Potamotrygon and Paratrygon. In this region, the taxa have significant economic importance for the ornamental fishing industry, being intensively captured, especially the species Potamotrygon leopoldi, which is endemic to this basin. In the attempt to propose a species-specific DNA marker for the species from Xingu, as well as ensuring a robust and reliable molecular identification, the present work analysed mitochondrial gene portions Cytochrome Oxidase C – subunit I (COI) and Cytochrome B (Cytb) of five species: P. leopoldi; Potamotrygon orbignyi; Potamotrygon motoro; Potamotrygon scobina and Paratrygon aiereba. We found haplotype sharing, with a total absence of reciprocal monophyly in the majority of taxa. Individuals morphologically identified as a species showed mitochondrial DNA from another, suggesting the first record of hybridization amongst freshwater stingrays of Xingu. Also, we detected a deep divergence among Paratrygon aiereba haplotypes, indicative of speciation, suggesting the possibility of a new species for the Xingu river. Therefore, although the is still confusing and controversial taxonomy of freshwater stingrays, and evidencing hybridization processes that may have shaped the evolutionary history of this Family, the genes COI and Cytb can successfully help in the their species identification.

BackgroundChondrichthyans represent a monophyletic group of crown group gnathostomes and are central to our understanding of vertebrate evolution. Like all vertebrates, cartilaginous fishes evolved concretions of material within their inner ears to aid with equilibrium and balance detection. Up to now, these materials have been identified as calcium carbonate-bearing otoconia, which are small bio-crystals consisting of an inorganic mineral and a protein, or otoconial masses (aggregations of otoconia bound by an organic matrix), being significantly different in morphology compared to the singular, polycrystalline otolith structures of bony fishes, which are solidified bio-crystals forming stony masses. Reinvestigation of the morphological and chemical properties of these chondrichthyan otoconia revises our understanding of otolith composition and has implications on the evolution of these characters in both the gnathostome crown group, and cartilaginous fishes in particular.ResultsDissections ofAmblyraja radiata,Potamotrygon leopoldi, andScyliorhinus canicularevealed three pairs of singular polycrystalline otolith structures with a well-defined morphology within their inner ears, as observed in bony fishes. IR spectroscopy identified the material to be composed of carbonate/collagen-bearing apatite in all taxa. These findings contradict previous hypotheses suggesting these otoconial structures were composed of calcium carbonate in chondrichthyans. A phylogenetic mapping using 37 chondrichthyan taxa further showed that the acquisition of phosphatic otolith structures might be widespread within cartilaginous fishes.ConclusionsDifferences in the size and shape of otoliths between taxa indicate a taxonomic signal within elasmobranchs. Otoliths made of carbonate/collagen-bearing apatite are reported for the first time in chondrichthyans. The intrinsic pathways to form singular, polycrystalline otoliths may represent the plesiomorphic condition for vertebrates but needs further testing. Likewise, the phosphatic composition of otoliths in early vertebrates such as cyclostomes and elasmobranchs is probably closely related to the lack of bony tissue in these groups, supporting a close relationship between skeletal tissue mineralization patterns and chemical otolith composition, underlined by physiological constraints.

Potamotrygon boesemani, new species, is described from the Corantijn river drainage in Surinam. The species has a diagnostic dorsal color pattern formed by deep orange to red ocellated spots of irregular form, encircled by relatively broad black rings. Potamotrygon boesemani is distinguished from other ocellated congeners (P. motoro, P. henlei and P. leopoldi) by the more intensely colored ocelli, which are usually yellow in the latter species. From P. motoro it is also distinguished by the darker dorsal background coloration, by the broader black contour of the dorsal ocelli, and by the irregular form of the ocelli as compared to the more rounded shape in the latter species. From P. henlei and P. leopoldi, it is distinguished by the lack of ocelli on tail. From the tentatively identified specimen of P. ocellata, which also has dark orange ocelli, the irregular contour of the ocelli in the new species is also distinctive. The teeth are relatively smaller and in greater number than in P. motoro and P. ocellata, with up to 45 rows in the upper jaw. Potamotrygon boesemani, espécie nova, é descrita da bacia do rio Corantijn, no Suriname. A espécie apresenta um padrão de coloração dorsal formado por ocelos com coloração alaranjada intensa a vermelha, de formato irregular, margeados por anéis pretos e relativamente grossos. Potamotrygon boesemani distingue-se de outras espécies congêneres oceladas (P. motoro, P. henlei e P. leopoldi) pela coloração mais intensa dos ocelos, que são normalmente amarelados nestas últimas espécies. De P. motoro, distingue-se também pela coloração de fundo mais escura no dorso, pelo contorno preto mais largo nos ocelos dorsais, e pela forma irregular dos ocelos comparada à forma arredondada em P. motoro. De P. henlei e P. leopoldi, distingue-se pela falta de ocelos na cauda. Da espécie tentativamente identificada como P. ocellata, que também possui ocelos laranja escuro, o contorno irregular dos ocelos também é distintivo. Os dentes são relativamente menores e em maior número que em P. motoro. e P. ocellata, com até 45 fileiras na mandíbula superior.