A new species of Hoplias malabaricus species complex is herein described from the Munim River Basin, Maranhão state, Northeastern of Brazil. Hoplias cazumba differs from its congeners by the following combination of characters: 19–22 scales around caudal peduncle, 40–42 lateral-line scales, 39–41 vertebrae, 11–13 dorsal-fin rays and, always 15 caudal-fin rays.

Uma nova espécie do complexo de espécies Hoplias malabaricus é descrita aqui, proveniente da Bacia do Rio Munim, estado do Maranhão, Nordeste do Brasil. Hoplias cazumba difere de seus congêneres pela seguinte combinação de caracteres: 19–22 escamas ao redor do pedúnculo caudal, 40–42 escamas na linha lateral, 39–41 vértebras, 11–13 raios na nadadeira dorsal e sempre 15 raios na nadadeira caudal.

Biodiversity, freshwater fish, Maranhão, Neotropical, Trahira

Biodiversidade, Maranhão, Neotropical, Peixe de água doce, Traíra

The family Erythrinidae is a group of Characiformes widely distributed in Neotropics with 17 valid living species belonging to three genera, Erythrinus Scopoli, 1777, Hoplerythrinus Gill, 1896 and Hoplias Gill, 1903 (Oyakawa 2003; Fricke et al. 2023), and one extinct species, Paleohoplias assisbrasiliensis Bocquentin, Negri, 2003 from State of Acre, Brazil (Toledo-Piza et al. 2024). Hoplias, the most species-rich genus of the family, comprises 13 valid species (Toledo-Piza et al. 2024) allocated into three different groups (Oyakawa 1990; Oyakawa and Mattox 2009): the H. lacerdae group with five species, the monotypic H. aimara group and H. malabaricus group with currently seven species.

The H. lacerdae group is defined by the absence of teeth on the basihyal and basibranchials bones and the medial margins of contralateral dentaries lying almost parallel to each other, forming a U shape margin in ventral view (Oyakawa and Mattox 2009). In the H. malabaricus group, tooth plates are present on the basihyal and basibranchials bones and the medial margins of contralateral dentaries converge towards the symphysis, forming a V shape in ventral view of head (Oyakawa and Mattox 2009). The H. aimara group is defined by the absence of an accessory ectopterygoid and the presence of a dark spot on the opercular membrane (Mattox et al. 2006; Oyakawa and Mattox 2009).

Currently, H. malabaricus group comprises the following species: H. malabaricus (Bloch, 1794), Hoplias microlepis (Günther, 1864); Hoplias teres (Valenciennes, 1847); Hoplias mbigua Azpelicueta, Benítez, Aichino & Mendez, 2015; Hoplias misionera Rosso, Mabragaña, González-Castro, Delpiani, Avigliano, Schenone & Díaz de Astarloa, 2016; Hoplias argentinensis Rosso, González-Castro, Bogan, Cardoso, Mabragaña, Delpiani & Díaz de Astarloa, 2018 and Hoplias auri Guimarães, Rosso, González-Castro, Souza, Díaz de Astarloa & Rodrigues, 2021.

The existence of cryptic lineages in the H. malabaricus species complex has been known to the ichthyologists since the 1970s. After several morphological, molecular and chromosomal studies, the diversity of H. malabaricus species group remains unresolved, mainly due to its conservative morphological characters (Bertollo et al. 2000; Marques et al. 2013; Rosa et al. 2014; Guimarães et al. 2022a). Nevertheless, four new species belonging to the H. malabaricus species group were described in the last decade (Azpelicueta et al. 2015; Rosso et al. 2016; Rosso et al. 2018; Guimarães et al. 2022b), shedding light on the methods and characters that are informative for discriminating taxa within this group.

Some studies carried out in recent years with samples from the Maranhão Hydrological Unit (MHU) indicated the existence of several lineages and possible new species of Hoplias for the region (Piorski 2010; Souza 2014; Pires et al. 2019). However, none of these studies found diagnostic characters strong enough to differentiate species in these samples.

In the last years, there has been a focused effort to understand the process responsible for the distribution and diversification of ichthyofauna of MHU (Abreu et al. 2019, 2020a, b; Koerber et al. 2022). This effort resulted in at least a dozen new species described for the region and a dramatic increase from 83 to 287 species known from the region (Koerber et al. 2022, 2023). Herein we contribute to this effort, describing a new species belonging to the Hoplias malabaricus complex, one of the best-known, but most difficult to decipher, species complexes. The new species inhabits the Munim River, in Maranhão State, Northeastern, Brazil.

Traditional measurements and counts were made on the left side of the body following Fink and Weitzman (1974) and Mattox et al. (2006). Meristic data were taken under a stereomicroscope and the morphometric data point-to-point with a digital caliper to 0.1 mm. Measurements are presented as percentages of standard length (SL) unless those referring to subunits of head, which are presented as percentages of head length (HL). Vertebral counts were taken from radiographs and included the anterior four vertebrae of the Weberian apparatus. Counts of the holotype are indicated with asterisks.

Information about congeners was based on examined material (see comparative material section) and literature (Azpelicueta et al. 2015; Rosso et al. 2016; Rosso et al. 2018; Guimarães et al. 2022b). Specimens analyzed here are deposited in Coleção de Peixes da Universidade Federal do Maranhão (CPUFMA), Museu de Ciência e Tecnologia, Pontifícia Universidade Católica do Rio Grande do Sul (MCP), Museu de Zoologia da Universidade de São Paulo (MZUSP), Núcleo de Pesquisa em Limnologia, Ictiologia e Aquicultura, Universidade Estadual de Maringá (NUP), Museum für Naturkunde, Leibniz-Institut für Evolutions-und Biodiversitätsforschung, Berlin, Germany (ZMB), Museum d’Histoire Naturelle de Neuchâtel, Neuchâtel, Switzerland (MHNN), Muséum National d’Histoire Naturelle, Systématique et Évolution, Laboratoire d’Ichthyologie Générale et Appliquée, Paris, France (MNHN), National Museum of Natural History, Smithsonian Institution, Washington DC, United States of America (USNM).

Herein, we described a new species belonging to the H. malabaricus species group, based on morphological and morphometric characters. This new species is endemic to the Munim Basin in Northeastern Brazil. Hoplias cazumba is distinguished from H. malabaricus by the lower number of dorsal-fin rays, number of caudal-fin rays, higher number of lateral-line scales and higher number of scales around caudal peduncle. A combination of several characters, especially number of pre-dorsal scale, number of lateral-line scales and number of vertebrae, can be used to distinguish H. cazumba from the congeners in the H. malabaricus species group.

The H. malabaricus group is one of the most intriguing Neotropical fish species complexes. Its complexity ranges from karyotypic overlaps in disjunct populations (Bertollo et al. 2000; Marques et al. 2013; Rosa et al. 2014) to molecular differences in sympatric populations with high morphological overlap (Cardoso et al. 2018; Pires et al. 2021; Guimarães et al. 2022a). Initially recognized as comprising H. malabaricus, new species within the group have only been identified since 2015, using both morphological and molecular characters (Azpelicueta et al. 2015; Rosso et al. 2016; Rosso et al. 2018; Guimarães et al. 2022b). Notably, three species with restricted distribution emerged during this period: H. mbigua from the Paraná and Paraguay rivers, H. argentinensis from the lower Río de La Plata basin, and H. auri from the Crepori River drainage in the Tapajós basin. Additionally, H. misionera with a restricted distribution to Río de La Plata was expanded to the lower Amazon basin (Guimarães et al. 2021).

Despite the crucial role of molecular tools in uncovering cryptic species, some recent taxa in the H. malabaricus group have been described solely based on anatomical characters, like H. mbigua (Azpelicueta et al. 2015). This suggests that the karyotypic and molecular variations observed within the group may be associated with morphological differences identifiable through refined analyses of anatomical characters. For instance, Piorski (2010) proposed the existence of new Hoplias species in the Maranhão Hydrological Unit (MHU) using geometric morphometry and mitochondrial DNA data. This pattern was also observed by Abreu-Souza (2014). However, those researchers were unable to describe diagnostic characters for the lineages identified. Cardoso et al. (2018), through analyses of Cytochrome c Oxidase I (COI) gene sequences, reported the occurrence of Hoplias malabaricus sensu stricto in the state of Maranhão, as well as a potentially new species in the Itapecuru River basin, with 3.2% of genetic distance – a finding later confirmed by Guimarães et al. (2022), with 2.8% of genetic distance.

In our examination of multiple Hoplias specimens from various hydrographic basins in Maranhão, we identified not only H. cazumba but also H. malabaricus and putative new species from Itapecuru and Mearim basins (these new species are being described in an ongoing study by another research group).

Despite the increasing number of studies involving specimens from Maranhão, it is important to note that only Piorski (2010) and Abreu-Souza (2014) included samples from the Munim River basin—and in very limited numbers (2 and 6 individuals, respectively). This may reflect the basin’s relatively small size and consequently lower research interest, or possibly the scarcity of available specimens in ichthyological collections. This limitation is also evident in the present study, which includes only a few specimens from two localities within the basin. Nonetheless, these were sufficient to demonstrate the presence of a distinct, previously unrecognized species.

More recently, Conde-Saldaña et al. (2025), based on an analysis of 2,519 ultraconserved element (UCE) loci, recovered the monophyly of Erythrinidae genera and confirmed that H. malabaricus represents a broadly distributed species. However, their dataset did not include any samples from the hydrographic basins of Maranhão. These authors also suggested that the diversification and dispersal of species within the H. malabaricus complex intensified over the last 10 million years—coinciding with the timing of recent biogeographic events that drove freshwater fish diversification in Maranhão’s basins (Abreu et al. 2019, 2020a, 2020b).

Abreu et al. (2020b) further demonstrated that populations of Schizodon dissimilis and Prochilodus lacustris underwent recent diversification in this region, around 0.6 million years ago. This may indicate that H. malabaricus populations in Maranhão also experienced recent diversification. Given the conservative morphological traits of this species, this could have hindered the recognition of new taxa within the genus in previous studies.

The Maranhão Hydrological Unit (MHU), located in the Eastern Amazon region, has been identified as a potential endemic area for freshwater fishes (Hubert and Renno 2006). Recent studies based on local scientific collections and the availability of new samples encouraged the discovery of new taxa in this region (Leão et al. 2019; Brito et al. 2019; Saraiva et al. 2021; Aguiar et al. 2022; Dutra et al. 2022; Lustosa-Costa et al. 2022; Oliveira et al. 2022). Nevertheless, the ichthyofauna of the MHU is likely still underestimated, influenced by its geomorphological history and historical connections with adjacent river basins (Abreu et al. 2019, 2020a, 2020b).

Recent studies have reaffirmed Piorski’s (2010) pioneering findings on Hoplias from the MHU, using the mitochondrial marker COI (Pires et al. 2020; Guimarães et al. 2022). However, a comprehensive analysis of the previously identified molecular groups and subgroups, aimed at clarifying their taxonomic status, remains lacking.

The authors thank Carlos Lucena from Museu de Ciência e Tecnologia da PUC-RS (MCP) and Carla Pavanelli from Universidade Estadual de Maringá (Nupélia) for the loan of specimens, Celia Bueno for the photos and x-ray of specimens deposited in Museum d’Histoire Naturelle de Neuchâtel (MHNN), Edda Assel and Peter Bartsch for the photos and x-ray of specimens deposited in Museum fuer Naturkunde Berlin (ZMB). We are grateful to Bete from Universidade Estadual do Maranhão (UEMA) for technical support with obtaining radiographs of Hoplias cazumba and to Mark Sabaj for the review of the previous manuscripts. JMSA thanks Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq for the Post-doctoral fellowship (150387/2023-7) and Fundação de Amparo à Pesquisa e ao Desenvolvimento Científico e Tecnológico do Maranhão – FAPEMA for granting to LT (4487/2023) and NMP (UNIVERSAL-01331/18).