Abstract
ABSTRACT Pacus of the genus Myloplus represent a formidable taxonomic challenge, and particularly so for the case of M. asterias and M. rubripinnis, two widespread and common species that harbor considerable morphological diversity. Here we apply DNA barcoding and multiple species discovery methods to find candidate species in this complex group. We report on one well-supported lineage that is also morphologically and ecologically distinct. This lineage represents a new species that can be distinguished from congeners by the presence of dark chromatophores on lateral-line scales, which gives the appearance of a black lateral line. It can be further diagnosed by having 25-29 branched dorsal-fin rays (vs. 18-24), 89-114 perforated scales from the supracleithrum to the end of hypural plate (vs. 56-89), and 98-120 total lateral line scales (vs. 59-97). The new species is widely distributed in the Amazon basin, but seems to have a preference for black- and clearwater habitats. This ecological preference and black lateral line color pattern bears a striking similarity to the recently described silver dollar Metynnis melanogrammus.
Highlights
Myloplus Gill, 1896 is currently represented by 15 valid species widely distributed in South America, being found in the Amazon, Orinoco, La Plata and São Francisco drainages, as well as the coastal rivers of the Guiana Shield in Brazil, Guyana, French Guiana, Suriname, and Venezuela (Jégu, 2003; Andrade et al, 2016a,b; Andrade et al, 2018, 2019; Nico et al, 2018)
Unlike Metynnis melanogrammus, the new species described presents variation in the degree of intensity of melanin concentration along the lateral line (Fig. 4), with specimens from the Branco and Tapajós rivers presenting a lateral line with very few melanophores (Figs. 4A–D)
Another feature shared between Myloplus nigrolineatus and Metynnis melanogrammus is that both species are widely distributed in black- and clearwater environments of the Amazon basin
Summary
Myloplus Gill, 1896 is currently represented by 15 valid species widely distributed in South America, being found in the Amazon, Orinoco, La Plata and São Francisco drainages, as well as the coastal rivers of the Guiana Shield in Brazil, Guyana, French Guiana, Suriname, and Venezuela (Jégu, 2003; Andrade et al, 2016a,b; Andrade et al, 2018, 2019; Nico et al, 2018). The species richness of the genus has been underestimated for decades, as attested by the recent descriptions of four species—M. lucienae Andrade, Ota, Bastos, Jégu, 2016, M. taphorni Andrade, López-Fernández, Liverpool, 2019, M. tumukumak Andrade, Jégu, Gama, 2018, and M. zorroi Andrade, Jégu, Giarrizzo, 2016— increasing the species diversity by 25% since the revalidation of the genus (Jégu et al, 2004). Jégu et al (2004) redescribed M. asterias (Müller, Troschel, 1844), the type species of the genus, and Myloplus rubripinnis (Müller, Troschel, 1844). Both M. asterias and M. rubripinnis were described from Guyana and are morphologically very similar, but diagnosable from one another by the presence of 25–28 dorsal-fin rays and circular-shaped anterior fontanel in M. asterias
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