First record of Melanorivulus decoratus (Costa, 1989) (Cyprinodontiformes: Rivulidae) in the lower-middle Sao Francisco River, Brazil

Abstract

The non-annual rivulid Melanorivulus decoratus is an endemic fish from the Sao Francisco River basin that was previously recorded only in small tributaries along river's middle stretch in northwestern Bahia. The present record marks the first appearance of M. decoratus in the lower-middle Sao Francisco, approximately 450 km northeast of the closest previously documented site, providing an updated map of the species' distribution. Currently classified as Least Concern (LC), this new occurrence record expands knowledge of the species' range and conservation status.

Alternate abstract:

O rivulídeo não-anual Melanorivulus decoratus é um peixe endêmico da bacia do Rio São Francisco que foi registrado anteriormente apenas em pequenos tributários no trecho médio do Rio, no noroeste da Bahia. Este é o primeiro registro de M. decoratus no submédio São Francisco, cerca de 450 km a nordeste do ponto mais próximo documentado anteriormente, gerando um mapa atualizado da distribuição da espécie. Classifcada atualmente como Pouco Preocupante (LC), este registro de ocorrência da espécie contribui para o conhecimento da sua distribuição e seu estado de conservação.

Headnote

Abstract: The non-annual rivulid Melanorivulus decoratus is an endemic fish from the Sao Francisco River basin that was previously recorded only in small tributaries along river's middle stretch in northwestern Bahia. The present record marks the first appearance of M. decoratus in the lower-middle Sao Francisco, approximately 450 km northeast of the closest previously documented site, providing an updated map of the species' distribution. Currently classified as Least Concern (LC), this new occurrence record expands knowledge of the species' range and conservation status.

Keywords: Brazilian semiarid; conservation; new record; non-annual rivulids; Rivulinae.

Resumo: O rivulídeo não-anual Melanorivulus decoratus é um peixe endêmico da bacia do Rio São Francisco que foi registrado anteriormente apenas em pequenos tributários no trecho médio do Rio, no noroeste da Bahia. Este é o primeiro registro de M. decoratus no submédio São Francisco, cerca de 450 km a nordeste do ponto mais próximo documentado anteriormente, gerando um mapa atualizado da distribuição da espécie. Classifcada atualmente como Pouco Preocupante (LC), este registro de ocorrência da espécie contribui para o conhecimento da sua distribuição e seu estado de conservação.

Palavras-chave: semiárido brasileiro; conservação; novo registro; rivulídeos não-anuais; Rivulinae.

The genus Melanorivulus Costa, 2006 (Rivulidae) comprises 64 valid species distributed across Central and northeastern South America, inhabiting diverse watersheds in Brazil (Costa, 2011; Ywamoto et al., 2020; Fricke et al., 2025). Species of Melanorivulus are typically found in muddy sediment areas with clear, shallow, and warm waters in ponds, canals, or small streams (Costa & Brasil, 2008). Within this genus, Melanorivulus decoratus (Costa, 1989), an endemic species of the Sao Francisco River basin (Barbosa et al., 2017), is distinguishable from its congeners by the following morphological traits in adult males: "14 scale rows around the caudal peduncle, a dark horizontal stripe extending from the middle of the operculum to the base of the caudal fin, and a dark stripe along the lower margin of the caudal fin" (Costa, 1989, p. 524). To date, M. decoratus had been documented only in the tributaries of the middle Sao Francisco River (Bahia state), according to Specieslink database and Lima & Caires(2011) (Figure 1).

Two specimens of M. decoratus (10 and 14 mm Standard Length) were collected on January 9, 2018 (Figure 2A), from the marginal pool of the lower-middle Sao Francisco River, coordinates 8°32'46.4"S 39°27'24.2"W, near Fazenda Toco Preto, in the municipality of Cabrobo, Pernambuco.

The site is located approximately 400 meters downstream of the Riacho Paredao mouth and 50 meters upstream of the water intake for the Sao Francisco River Integration Project (PISF) - North Axis (Figure 2A). Aquatic macrophytes such as Pontederia crassipes Mart., Nymphoides spp. and Azolla caroliniana Willd were present in the habitat. The specimens were captured using a seine net (10 m long, 1.5 m high and 5 mm mesh). Immediately following capture, the fish were photographed alive to document their coloration (Figure 2B). The specimens were then euthanized using clove oil (Eugenol - Brasil, 2018), fixed in a 5% formaldehyde solution for approximately three to five days, and subsequently preserved in 70% ethanol. The taxonomic confirmation was double-checked after consulting the original description manuscript (Costa, 1989) and its very own author, Dr. Wilson Costa. The specimens were deposited in the ichthyological collection of the Museu de Fauna da Caatinga (catalog number MFCI 006818), housed at the Universidade Federal do Vale do Sao Francisco - UNIVASF. Collection was conducted under Authorization for Capture, Collection and Transport of Biological Material No. 94/2014 and subsequent renewals. Registration code of SISGEN

(Sistema Nationalde Gestdo do. Patrimonio Genetico edo Conhecimento TraditionalAssotiado): A7000F1. The lower-middle stretch of the Sao Francisco River, spanning 686 km, is the second largest in the river's course and is partially obstructed by the Sobradinho and LuizGonzaga/Itaparicahydropower plants (Sato & Godinho, 1999). Despite these modifications, the lower-middle stretch supports diverse microhabitats, including rapids and lentic environments, with sandy and rocky substrates and areas of marginal riparian vegetation, which can be either preserved or anthropogenically impacted -deforested for agriculture and livestock (Godinho & Godinho, 2003).

Additionally, in the Brazilian Priority Areas for Conservation (Silva et al., 2003 - updated by the Ministerio do Meio Ambiente, Ordinance no. 463 of December 18) the Sao Francisco River area near the PISF intake is classified as a region of "High/Very High" biological importance. The SAD/E system (Sistema de Avaliagao de Risco de Extingdo da Biodiversidade) from Instituto Chico

Mendes de Conservagao da Biodiversidade {ICMBio) currently lists M. decoratus as Least Concern - LC (Brasil, 2025), and updated occurrence records are crucial for understanding the species' distribution and dispersal patterns, which are fundamental for future studies on population dynamics. The present records support the hypothesis that M. decoratus might present a wider distribution than previous thought. Therefore, we suggest additional collection efforts to elucidate the distribution of the species in the middle and lower-middle course of the Sao Francisco River.

Notably, species within the Rivulidae family constitute approximately 40% of potentially threatened Brazilian freshwater fishes (Castro & Polaz, 2020). The detection of M. decoratus in the lower-middle Sao Francisco stretches ensures the ecological significance of the area and highlights the need for conservation action plans for this endemic species, identified as one of the rivulids facing imminent threats.

Acknowledgements

The authors are grateful for the financial support provided by Secretaria Nacional de Seguranca Hidrica (SNSH) of Ministerio da Integracao e do Desenvolvimento Regional (MIDR), Universidade Federal do Vale do Sao Francisco (UNIVASF) and Centro de Conservacao e Manejo de Fauna da Caatinga (CEMAFAUNA). We also acknowledge MSc. Karlla Rios (CEMAFAUNA) for providing collaboration, guidance, and logistical support and Dr. Wilson J. E. M. da Costa (Universidade Federal do Rio de Janeiro) and DaltonT B. Nielsen (Universidade de Taubate) for guiding us to species identification.

Data availability

The entire dataset supporting the results of this study has been published in the article itself.

References

References

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Received: 08 November 2024 Accepted: 09 April 2025

Associate Editor: Fernando Mayer Pelicice.