Sergio Bogan, Yamila Paula Cardoso, Juan José Rosso, Juan Martín Díaz de Astarloa

New species of the highly diversified Hoplias malabaricus group (Characiformes, Erythrinidae) from La Plata Basin: Another brick in the wall

Introduction

New species of the highly diversified hoplias malabaricus group (characiformes, erythrinidae) from la plata basin: another brick in the wall. Describes a new Hoplias malabaricus fish species (Characiformes, Erythrinidae) from the La Plata Basin (Argentina, Brazil). Uncovers cryptic diversity via taxonomic & molecular analysis.

Abstract

With 13 species, the genus Hoplias contains more than 75% of the diversity of the family Erythrinidae. However, cytogenetic and molecular studies strongly suggest the existence of an outstanding cryptic diversity in this genus, with more than 20 likely new species only for the H. malabaricus group. Here, we described a new species for this group, including specimens from a wide geographic coverage within La Plata Basin: Iguazu River and Urugua-í Reservoir in Argentina and upper Parana River in Brazil. Traditional taxonomic and molecular analyses were performed. The new species is diagnosed by the following combination of characters: 15–17 predorsal scales, 38–41 lateral-line scales, 10–11 premaxillary teeth; 8–10 plate-like denticulate rakers in ceratobranchial, 39–41 vertebrae, pectoral-fin length (16.24–19.58 % SL), orbital diameter (13.15–20.06 % HL), the last vertical scale row in the caudal fin forming a straight line and the last portion of the ascending process of the premaxilla with a marked ridge. A key for the species of the H. malabaricus group is provided. This work contributes to the inventory of biological diversity of the study region but also aims to accurately identify species belonging to a group with complex taxonomy.

Review

This manuscript presents the description of a new species belonging to the highly diversified *Hoplias malabaricus* group, addressing a significant challenge in the taxonomy of the genus *Hoplias*. The authors aptly highlight the extensive cryptic diversity within *Hoplias*, where molecular and cytogenetic evidence far exceeds currently recognized morphological species. By utilizing both traditional taxonomic and molecular analyses across a broad geographic range within the La Plata Basin, the study makes a crucial contribution to disentangling the complex systematics of this charismatic characiform group, effectively adding another essential piece to the ongoing puzzle of its diversity. A particular strength of this work, as evidenced by the abstract, is the robust and detailed diagnosis provided for the new species. The authors meticulously list a comprehensive set of distinguishing characters, including specific meristic counts (predorsal, lateral-line scales, premaxillary teeth, vertebrae), precise morphometric measurements (pectoral-fin length, orbital diameter), and unique anatomical features (plate-like denticulate rakers, caudal fin scale row, premaxilla ridge). The promise of a diagnostic key for the *H. malabaricus* group further enhances the value of this publication, offering an indispensable resource for researchers grappling with the identification of species within this taxonomically difficult assemblage. The combined approach of morphological and molecular data is commendably suited for resolving species boundaries in groups known for cryptic diversity, lending strong support to the new species description. Overall, this work appears to be a well-executed and important contribution to Neotropical ichthyology. The findings are not only crucial for improving the inventory of biological diversity in the study region but also serve as an excellent example of how to approach the identification and description of species in taxonomically complex groups. The evocative title, "Another brick in the wall," aptly captures the incremental yet vital nature of fundamental taxonomic research. It is anticipated that the full paper will provide extensive data to support these findings, further solidifying its significance for future systematic and ecological studies of Erythrinidae and freshwater ecosystems in South America.

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