Sunday, April 19, 2020

[Ichthyology • 2020] Myloplus nigrolineatus • Integrative Taxonomy Reveals A New Species of Pacu (Characiformes: Serrasalmidae: Myloplus) from the Brazilian Amazon

 Myloplus nigrolineatus 
Ota, Machado, Andrade, Collins, Farias & Hrbek, 2020

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.

Keywords: COI gene; Cryptic species; Myloplus asterias; Myloplus rubripinnis; Neotropical

FIGURE 1: Myloplus nigrolineatus, INPA 54773, holotype, 221.9 mm SL, male, lago Tracajá, tributary of rio Juma, ca. 60 km south of Manaus, Amazonas State, Brazil.
FIGURE 2: Osteology of the head of Myloplus nigrolineatus, INPA 53281, 180.3 mm SL, Manaus, rio Negro nearby Manaus: A. lateral view; B. dorsal view.
 afo, anterior fontanel; epi, epiotic; eth, lateral ethmoid; fro, frontal; int, interopercle; io, infraorbitals; max, maxillary; mth, mesethmoid; nas, nasal; neu, neural; ope, opercle; oph, orbitosphenoid; par, parietal; pfo, posterior fontanel; pmx, premaxillary; pre, preopercle; psh, parasphenoid; ptr, pterotic; pts, pterosphenoid; qu, quadrate; so, supraorbital; sub, subopercle; sup, supraoccipital; vom, vomer.

FIGURE 3: Myloplus nigrolineatus, paratypes, INPA 23312, lago Mabi, São Gabriel da Cachoeira, Amazonas State, Brazil. A. 199.1 mm SL, male; B. 182.0 mm SL, female.

FIGURE 4: Freshly collected Myloplus nigrolineatus with different color patterns.
A. INPA 23312, 182.0 mm SL, female, lago Mabi, São Gabriel da Cachoeira, Amazonas; B. INPA 53281, 170.7-180.3 mm SL, female, rio Negro nearby Manaus, Amazonas;
C. INPA 23446, 160.0 mm SL, male, tissue voucher CTGA 15401, Tapajós Itaituba, Pará; D. INPA 46279, 172.3 mm SL, male, tissue voucher CTGA 12201 (GenBank accession MG752197), rio Branco, boundary between Caracaraí and Rorainópolis, Roraima;
E. INPA 58772, 189.3 mm SL, male, rio Purus, Tapauá, Amazonas; F. INPA 52505, 186.4 mm SL, male, tissue voucher CTGA 12329 (MG752209), rio Negro nearby Barcelos, Amazonas.

Myloplus nigrolineatus, new species

Myloplus asterias Machado et al., 2018: 8 (Brazil, rio Nhamundá, fig. 3d).
Myloplus sp.  Zeinad, Prado, 2012: 148 (Brazil, rio Itapará, tributary of rio Branco, photo).
Myloplus aff. asterias Morales et al., 2019: 6, 9 (Brazil, Tucuxi lake, rio Purus, Tab. 2, fig. 2, 3 and 4).

Diagnosis. Myloplus nigrolineatus can be easily diagnosed from all congeners by highly concentrated dark chromatophores on the lateral-line scales, resulting in a markedly, dark pigmentation along the lateral line. Additionally, the new species can be distinguished from M. arnoldi, M. lobatus, M. lucienae, M. planquettei, M. rhomboidalis, M. schomburgkii, and M. zorroi by having 25-29 branched dorsal-fin rays (vs. 18-24). It can be distinguished from M. ternetzi by the presence of a symphyseal pair of conical teeth posterior to the main row of dentary (vs. absence); and from M. asterias, M. levis, M. rubripinnis, M. taphorni, M. tiete, M. torquatus, and M. tumukumak by having 89-114 lateral line scales from supracleithrum to end of hypural plate (vs. 56-89) and 98-120 total perforated lateral line scales (vs. 59-97).

FIGURE 5: Map of northern South America, showing the distribution of Myloplus nigrolineatus. Black star represent the type locality, black circles the paratypes, yellow the non-types localities, and the gray represents the locality of exemplars used exclusively in the molecular analyses (without voucher in museums).

Geographical distribution. Myloplus nigrolineatus is widespread in the Amazon basin, occurring in the Negro, Branco, Nhamundá, Purus, Madeira, Tapajós, Tocantins, and Uatumã rivers (Fig. 5). In whitewater rivers with high sediment loads such as the Madeira, Branco, and Purus, Myloplus nigrolineatus was only captured in tributaries with black- or clearwater (see Ecological notes).

Ecological notes. The species inhabits slow-flowing habitats such as backwaters and lakes, feeding mainly on aquatic and terrestrial plants (Goulding, 1980). Of the collection localities, the Aripuanã, rio Água Boa do Univini (rio Branco), Nhamundá, Tapajós, Tocantins, and Uatumã rivers are clearwater rivers chemically characterized by a low concentration of suspended sediments, dissolved minerals and humic compounds (Crampton, 2011), while the rio Negro basin, rio Jutaí, Pauapixuna lake (rio Juruá), Tapauá lake (rio Purus), Serpa lake (rio Amazonas), and rio Maués-Açu are extremely acidic oligotrophic blackwaters (Sioli, 1984; Goulding et al., 1988). The Tracajá lake (holotype locality) is also a blackwater system that has its headwaters in the Purus-Madeira interfluvium and like other blackwater rivers, is of low sediment load and low pH. These blackwater environments are inhospitable to certain species of fish (Wallace, 1889; Roberts, 1972; Goulding et al., 1988; Araújo-Lima, Goulding, 1997; Saint-Paul et al., 2000; Crampton, 2011; Lima, Ribeiro, 2011).

Etymology. The epithet nigrolineatus, from the Latin nigro meaning black, and lineatus meaning lined, an allusion to the black, pigmented lateral line. An adjective.

Rafaela Priscila Ota, Valéria Nogueira Machado, Marcelo C. Andrade, Rupert A. Collins, Izeni Pires Farias and Tomas Hrbek. 2020.  Integrative Taxonomy Reveals A New Species of Pacu (Characiformes: Serrasalmidae: Myloplus) from the Brazilian Amazon. Neotropical Ichthyology. 18(1). DOI: 10.1590/1982-0224-20190112 

RESUMO: Pacus do gênero Myloplus representam um desafio taxonômico formidável, e particularmente o caso de M. asterias e M. rubripinnis, duas espécies amplamente distribuídas e comuns que abrigam uma considerável diversidade morfológica. Aplicamos aqui a tecnologia do DNA barcoding e múltiplos métodos de descoberta de espécies para encontrar possíveis espécies novas nesse grupo complexo. Registramos uma linhagem bem suportada que também é distinta morfológica e ecologicamente. Essa linhagem representa uma nova espécie que pode ser distinguida das demais congêneres por apresentar cromatóforos escuros nas escamas da linha lateral que conferem uma aparência de linha lateral preta. Ela pode ser adicionalmente diagnosticada por ter 25-29 raios ramificados na nadadeira dorsal (vs. 18-24), 89-114 escamas perfuradas do supracleitro até o final da placa hipural (vs. 56-89) e 98-120 escamas totais na linha lateral (vs. 59-97). A nova espécie é amplamente distribuída na bacia Amazônica, mas aparentemente possui preferência por habitats de água preta e clara. A preferência ecológica e o padrão de colorido escuro da linha lateral consistem em semelhanças impressionantes com o silver dólar recém descrito Metynnis melanogrammus.
Palavras-chave: Espécie críptica; Gene COI; Myloplus asterias; Myloplus rubripinnis; Neotropical