Influências biogeográficas na forma dos otólitos lagenares de populações isoladas de Psalidodon bifasciatus em sistemas hidrográficos do sul do Brasil

Abstract

Neotropical ichthyofauna exhibits high diversity and pronounced spatial structuring, particularly in river basins with a history of natural and anthropogenic isolation. Psalidodon bifasciatus, an abundant characid in the Iguaçu River, represents a suitable model to investigate intraspecific differentiation because it combines broad regional distribution, phenotypic plasticity, and occurrence in reaches that are intensely fragmented by rapids, waterfalls, and dams. This study aimed to characterize and quantify morphological variation in the right lagenar otoliths (asteriscus) of P. bifasciatus and to evaluate patterns of intra- and interpopulation differentiation across subregions of the Iguaçu system and adjacent basins. A total of 90 individuals were sampled in seven hydrographic systems in southern Brazil: upper Iguaçu (IA, n = 13), lower Iguaçu (IB, n = 14), middle Iguaçu (IM, n = 15), Ivaí River (IV, n = 12), coastal rivers draining to the South Atlantic (LI, n = 14), Piquiri River (PI, n = 9), and Jordão River (RJ, n = 13). Right asterisci, standardized for orientation and scale, were photographed under a stereomicroscope; otolith outlines were extracted, normalized, and described by coefficients obtained via a non-decimated discrete wavelet transform (DWT), with allometric effects removed by regression on fish total length. Shape variation was explored using Principal Component Analysis (PCA); differences among populations were tested by Multivariate Analysis of Variance (MANOVA) and pairwise contrasts; group separation was evaluated by Linear Discriminant Analysis (LDA). The first two principal components explained 45.7% of the total variation, and the global MANOVA indicated significant differences among regions (Pillai = 4.049; F ≈ 2.917; df = 222/312; p < 0.001). The greatest divergence was concentrated in the anterior sector of the outline (≈160–240°), involving the pseudoexcisura, antirostrum, and pseudoantirostrum. The integrated DWT–PCA–MANOVA–LDA approach revealed three consistent morphological units of asteriscus shape in P. bifasciatus: (G1) upper Paraná River + middle/lower Iguaçu + Jordão River (IV, PI, IM, IB, RJ; n = 63), (G2) upper Iguaçu River (IA; n = 13), and (G3) South Atlantic coastal rivers (LI; n = 14). The upper Iguaçu group was distinct from all other populations, whereas LI differed from several members of G1 (except PI). These patterns are consistent with historical isolation (waterfalls, geomorphological compartmentalization), reinforced by recent fragmentation (reservoirs), and possibly modulated by local environmental convergence. We conclude that asteriscus shape records geographic structuring in P. bifasciatus and supports the hypothesis of allopatric divergence among populations in southern Brazil. From an applied perspective, the identified morphological units support treating the species as a set of evolutionarily significant units for conservation planning, with emphasis on maintaining/restoring connectivity and managing dams. Integrative studies combining otolith morphology, genomic data, and cytogenetics are recommended to test population boundaries and refine taxonomic diagnoses.