400 | wileyonlinelibrary.com/journal/jzs J Zool Syst Evol Res. 2019;57:400–417. © 2019 Blackwell Verlag GmbH 1 | INTRODUCTION The morphology of an organism is an integral component of all as- pects of its biology and is typically determined by natural and sexual selection pressures, which may vary significantly within the spa- tial range of a species (Arnold, 1983). Natural selection and sexual selection may affect morphological traits in distinct ways, and the type and degree of morphological change may provide insights into the relative importance of different selective forces in the shaping of species and populations (Kirkpatrick & Ravigné, 2002; Schluter, 2001). Phenotypic plasticity, that is, the environment-driven variation in the phenotypes produced from a single genotype, is common in nature (De Witt & Scheiner, 2004), allowing organisms to adapt to local conditions (Pigliucci, 2001; Schlichting, 2004). Ichthyologists have long used phenotypic variation to differenti- ate species or populations of the same species (Murta, 2000; Tudela, 1999), given that the influence of environmental variables on fish morphology is well documented (Grünbaum, Cloutier, Mabee, & Le François, 2007; Peres-Neto & Magnan, 2004). In fish, morphological plasticity is an important strategy of adaptation to distinct environ- ments (Robinson & Parsons, 2002; Wimberger, 1991), including both biotic factors and abiotic ones, such as competition and predation, Received: 29 May 2018 | Revised: 9 November 2018 | Accepted: 16 November 2018 DOI: 10.1111/jzs.12261 ORIGINAL ARTICLE Incipient speciation, driven by distinct environmental conditions, in the marine catfishes of the genus Aspistor (Siluriformes, Ariidae), from the Atlantic coast of South America Alexandre Pires Marceniuk 1 | Tibério Cesar Tortola Burlamaqui 2 | Claudio Oliveira 1 | Jeferson Carneiro 3 | Bruno Eleres Soares 4 | João Bráullio de Luna Sales 2 Contributing authors: Tibério Cesar Tortola Burlamaqui (tburla@gmail.com), Claudio Oliveira (claudio.oliveira@unesp.br), Jeferson Carneiro (jeferson.carneiro@yahoo.com), Bruno Eleres Soares (soares.e.bruno@ gmail.com), João Bráullio de Luna Sales (braziliancephalopod@gmail.com) 1 Departamento de Morfologia, Instituto de Biociências, Universidade Estadual Paulista (UNESP), Botucatu, Brazil 2 Museu Paraense Emílio Goeldi (MPEG), Belém, Brazil 3 Universidade Federal do Pará (UFPA), Belém, Brazil 4 Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil Correspondence Alexandre Pires Marceniuk, Departamento de Morfologia, Instituto de Biociências, Universidade Estadual Paulista, (UNESP), Botucatu, Brazil. Email: a_marceniuk@hotmail.com Funding information Conselho Nacional de Desenvolvimento Científico e Tecnológico, Grant/Award Number: 118452/2017-7 and 151731/2010- 1; São Paulo State Research Foundation, Grant/Award Number: 2016/09204-6 Abstract The Atlantic coast of South America is characterized by significant variation in envi- ronmental attributes, such as the salinity and transparency of the water, substrate, temperatures, and oceanic currents, which contribute to the existence of heteroge- neous environments with distinct biotic and abiotic conditions. The present study shows that the Aspistor quadriscutis from the equatorial Amazon–Orinoco plume and Aspistor luniscutis from the northeast to southeast coast of Brazil represent different morphological lineages, with distinct life habits and behavior, belonging to a single genetic lineage. This condition is related to the distinct selective pressures (ecological conditions) found in the geographic regions in which the two species occur. Intermediate morphological patterns correspond to the ancestral traits, present in fossil species, which, together with the absence of differentiation in mitochondrial markers, characterize a process of incipient speciation. KEYWORDS Atlantic South America, incipient speciation, marine catfish, morphological lineages, phenotypic plasticity, selective pressure