An exploratory survey of fish species inhabiting rapids from the Tocantins-Araguaia River Basin: perspectives on species diversity and conservation

Inventário exploratório das espécies de peixes que habitam as corredeiras da Bacia do Rio Tocantins-Araguaia: perspectivas sobre diversidade e conservação

Abstract

Rapid habitats are characterized by an environment of intense hydrologic dynamic, hosting freshwater biodiversity that has adapted to these conditions. Knowledge of the diversity of rapids-dwelling fish is still unsatisfactory in South America’s rivers. Our study surveyed fish species within rapids from the Tocantins-Araguaia River Basin (TARB). We assessed 27 stretches of rapids in different localities along the TARB, using various fishing gears along with free diving, preferably in deeper areas of rocky stretches. Our survey yield 1668 specimens, from 107 species, 79 genera, 28 families, and 10 orders. Among these, 11 species are categorized as being under some threat, according to IUCN and ICMbio. The species Rhynchodoras xingui and Mylesinus paucisquamatus, both listed as threatened at ICMbio, were not recorded during our expeditions in the surveyed area. Baryancistrus longipinnis was only recorded in the Pedral do Lourenço, suggesting a possible distribution retraction. This is the first species surveying focused exclusively on ichthyofauna associated with the rapids of the Tocantins-Araguaia River Basin (TARB). Despite significant alterations in the past four decades, particularly due to the construction of hydroelectric power plants, still house a rich biodiversity. Our results indicate that the remnants of rapids and bedrock habitats in the TARB provide suitable niches for the ichthyofauna, highlighting their importance as potential conservation units.

Keywords
Dams; Inventory; Loricariidae; Lotic environments; Threatened species

Resumo

As corredeiras são caracterizadas por ser um ambiente de intensa dinâmica hidrológica, abrigando uma biodiversidade que se adaptou a essas condições. O conhecimento da diversidade de peixes que habitam em corredeiras ainda é insatisfatório nos rios da América do Sul. Com o intuito de melhorar esse conhecimento, este estudo inventariou as espécies de peixes que ocorrem áreas de corredeiras da bacia do rio Tocantins-Araguaia (TARB). Avaliamos 27 pontos em diferentes localidades ao longo do TARB, utilizando várias artes de pesca e mergulho livre, que preferencialmente aconteceram em áreas mais profundas. Nosso estudo coletou 1668 espécimes, de 107 espécies, 79 gêneros, 28 famílias e de 10 ordens. Entre estas, 11 espécies são categorizadas estando sob algum nível de ameaça, de acordo com os critérios da IUCN e ICMbio. As espécies Rhynchodoras xingui e Mylesinus paucisquamatus, ambas listadas como ameaçadas pelo ICMBio, não foram registradas durante nossas expedições na área pesquisada. Baryancistrus longipinnis foi registrada apenas no Pedral do Lourenço, sugerindo uma possível retração da distribuição. Aqui apresentamos o primeiro levantamento de espécies focado exclusivamente na ictiofauna associada às corredeiras da Bacia do Rio Tocantins-Araguaia (TARB). Apesar das alterações significativas nas últimas quatro décadas nesta bacia, particularmente em trechos de corredeiras devido à construção de usinas hidrelétricas, as áreas remanescentes ainda abrigam uma rica diversidade de espécies. Nossos resultados indicam que os trechos remanescentes de corredeiras na TARB fornecem nichos estáveis para a ictiofauna, destacando sua importância como potenciais áreas a serem designadas unidades de conservação.

Palavras-chave
Ambientes lóticos; Barragens; Espécies ameaçadas; Levantamento de espécies; Loricariidae

Introduction

The Tocantins-Araguaia River Basin (TARB) is the fourth-largest drainage located entirely within Brazilian territory (Goulding 2003, Akama 2017). This drainage runs mostly throughout Cerrado and partially in the Amazon biomes, discharging directly into the Atlantic Ocean in the Marajó Bay (Barthem and Schwassmann 1994, Akama 2017). Due to these distinct phytophysiognomies, some authors exclude the TARB as part of the Amazon River basin. Geomorphological history indicates a relatively recent isolation of the TARB from the Amazon basin, which is also corroborated by the diversity of fish presented in the TARB (Rosseti and Valeriano 2007, Dagosta and Pinna 2017). Despite it currently flowing directly into the Atlantic Ocean without a clear connection with the Amazon River basin, the ancient geological processes that formed this watershed and its water biological community suggest earlier connection between the two.

The Tocantins-Araguaia drainage can be divided into ten main sections (Ribeiro et al. 1995), according to its geomorphology and biodiversity distribution, and can be defined into three major areas: the Lower Tocantins, located downstream from Imperatriz (state of Maranhão) and Itaguatins (state of Tocantins); the Middle Tocantins, which encompasses the area near Imperatriz (state of Maranhão), presenting a stretch of rapids, flooded plains and fluvial terraces; and the Upper Tocantins, upstream of Imperatriz and Itaguatins, where the river flows through a crystalline basement composed of Precambrian rocks and Paleozoic sediments from the Paraná Basin (Ribeiro et al. 1995, Dagosta and Pinna 2019, Jesus et al. 2020). The Araguaia River also has well-defined Upper, Middle, and Lower sections, flowing through an area dominated by Cerrado biome, contacting areas of Amazon forest on the Middle Araguaia portion (Aquino et al. 2008, Jarduli et al. 2014, Corrêa et al. 2022). The rivers originating in the Brazilian Central Plateau are particularly vulnerable to anthropic impacts, arising from the advance of agricultural activities (especially soybean cultivation and livestock raising). The construction of large-scale projects, such as hydroelectric power plants, also disrupts the natural environment and leads to unintended ecological consequences (Agostinho et al. 2007, Akama 2017, Choueri and Azevedo 2017, Perônico et al. 2020, Pereira et al. 2021). There are 73 Hydroelectric plants operating along the Tocantins-Araguaia River basin, with the Tucuruí and Serra da Mesa being the largest (Chamon et al. 2022). Although hydroelectric plants are Brazil’s main source of energy supply, their construction promotes significant environmental and social changes, including submersion of rapids and waterfalls, which severely impacts the associated ichthyofauna, including accelerating the invasion of alien species (Pelicice et al. 2017, Akama 2017, Doria et al. 2021, Chamon et al. 2022).

The fish diversity in the TARB corresponds to the overall diversity found in the Amazon basin, where Characidae species are predominant (Lucinda et al. 2007, Dagosta and Pinna 2019, Chamon et al. 2022). The most detailed study on the ichthyological diversity of the TARB was carried out by Chamon et al. (2022), recognizing 751 species, distributed in 314 genera, 51 families, and 16 orders, with Characiformes, Siluriformes, and Cichliformes being the most diverse orders, and Characidae, Loricariidae, and Cichlidae being the most well-represented. Notably, around one-third of the species (243) are endemic to the TARB, with the highest number of endemics found in Characidae, Loricariidae, Rivulidae, and Cichlidae (Reis et al. 2024). This diversity shares similarities with other Amazonian rivers from the Brazilian Shield, such as the Tapajós and Xingu rivers (Rosetti and Valeriano 2007, Akama 2017, Dagosta and Pinna 2017, 2019).

While the study by Chamon et al. (2022) compiled the most comprehensive list of fish species for the TARB to date, it lacks detailed specific characterization of strictly rheophilic species (river-dwelling) or species that occasionally inhabit this habitat. This knowledge gap is concerning because the environmental changes over the last four decades have significantly impacted the rapids areas along the TARB, leaving only a small portion of the initial bedrock background environment in this river basin (Ribeiro et al. 1995, Lucinda et al. 2007, Winemiller et al. 2016, Akama 2017). Therefore, these previously heterogeneous habitats with high dissolved oxygen levels have been transformed into hypoxic lentic environments (Fitzgerald et al. 2018, Hrbek et al. 2018).

The environmental characteristics of the rapids are structurally complex and heterogeneous containing species that are highly specialized for life in areas with high flow speed, elevated turbulence and high levels of diluted oxygen (Fitzgerald et al. 2018; Hrbek et al. 2018; Lujan and Conway 2015; Lujan et al. 2015). Geologically, they can be defined as stretches of river with visible rock formations, that create waterfalls or rapids with heterogeneous water flow (Hrbek et al. 2018). Considering the geomorphological processes that formed the South American continent, a large part of these environments are part on the Brazilian Shield, consisting of Precambrian rocks (~540 Ma) (Alkim 2015). The Brazilian Shield encompasses the extensive South American plateau that extends from the lowlands of the Amazon to the estuary of the La Plata River in the south. It is limited in the west by the plains of the Madeira and Paraguay rivers, reaching the coastal plains of the Atlantic Ocean in the east (Hawkins et al. 1993; Lundberg et al. 1998). Therefore, these rocky substrates featuring burrows, slabs, caves, cracks and crevices with tree trunks, provide ecological niches that allow various organisms to exploit resources. In general compared to other environments, rapids provide an abundance of food (algae and insect larvae), provide protection and have low predation pressure (Zuanon 1999).

Inventories of fish species in rapids remain relatively rare in South America, with much of the data coming from unpublished results. The Xingu River, the Upper Tocantins River, the São Francisco River, and more recently, the lower Vaupés River Basin are exceptions (Casatti and Castro 1998, Zuanon 1999, Lucinda et al. 2007, Zualang-Gómez et al. 2015, Fitzgerald et al. 2018, Gonçalves 2019, Keppler et al. 2022, Urbano-Bonilla et al. 2024). This lack of studies likely arises from the difficulty of accessing these fast-flowing habitats and collecting specimens in deep areas of high-flowing waters. This has an impact on the knowledge of this diversity and, consequently, the assessment of their conservation status (Lundberg et al. 2000, Vörösmarty et al. 2010, Collen et al. 2014). To address this, we conducted an exploratory survey of fish species inhabiting rocky habitats throughout the remnant rapid areas along the TARB, aiming to document the species and characterize the fish community that occurs in these environments.

Material and Methods

1. Study area

The TARB spans across the Brazilian Shield, flowing in the direction of the South-North drain, in an area of 767,000 km² (Ribeiro et al. 1995). The river basin is located between the 4º and 18º Southern parallels and the meridians 46º and 55º West. A significant portion of the TARB traverses the Cerrado biome, flowing to the Atlantic Ocean within the Amazonian ecosystem (Akama 2017). According to the Köppen’s climate classification, the region has dry and warm weather, typical of tropical savannah (Kottek et al. 2006). The highest precipitation rates occur between November and March (Kottek et al. 2006, Alvares et al. 2013).

We sampled 27 rapids over TARB, consisting of ten sampling sites that refer to the Middle Tocantins River sector (transitional zone), nine from the Araguaia River and nine from the Tocantins River (Figure 1).

Figure 1
Map of the rapid habitats surveyed in the TARB. The two biomes along the sampling area are presented: the green area refers to the Amazon forest and the yellow to the Cerrado. Triangles represent rapids from the Araguaia River, and the squares are related to rapids’ habitats close to the city of Marabá, where the Tocantins and Araguaia rivers join their water flow (Lower Tocantins – Downstream Tocantins). The circles correspond to rapids from the Middle Rio Tocantins (Itaguatins and Porto Franco areas - Upstream Tocantins). The red square refers to the Pedral do Lourenço sampling site, where there are plans to build a waterway that will destroy the rapids if this project is implemented. Numbers refers to each sampling site.

To enable the analysis of fish diversity, we divided the sampling sites into three main areas based on their geomorphology and fish biogeographical units and updating the physiographic units by Ribeiro et al. (1995): 1) Tocantins downstream (Transitional zone), where the waters of Araguaia and Tocantins rivers converge. This area is near Marabá city and presents stretches of rapids, flooded plains and fluvial terraces, with sediments dating from Mesozoic (~252 Ma) (Ribeiro et al. 1995, IBGE 2019, Jesus et al. 2020). 2) Tocantins upstream, the rapids include the main Tocantins’s channel river sector, flowing northward from Porto Franco to Itaguatins. 3) Araguaia River (Middle Araguaia), includes bedrock sediments dating from Paleozoic (~541 Ma), near São Geraldo do Araguaia (Ribeiro et al. 1995, IBGE 2019).Details about sampling sites are found in Table 1.

Table 1
Rapids sampled in the Tocantins-Araguaia River basin; surveyed sites coordinates are provided as well as details about sampling gears used.

Sampling site	Locality/municipality/state	Month of survey	Basin	Sampling method	Coordinates
1	Nova Marabá – Marabá-Pará	November 2019	Tocantins	casting nets, gill nets, diving, mesh nets, handmade casting net	5°18’42’’S, 49°04’24’’W
2	Ilha das Cabras, São Joao do Araguaia-Pará	November 2019	Tocantins	casting nets, gill nets, diving, mesh nets	5°17’37’’S, 48°57’32.7’’W
3	Ilha do Jaú, Bom Jesus do Tocantins-Pará	November 2019	Tocantins	casting nets, gill nets, diving, handmade casting net	5°20’11.9’’S, 48°51’15.5’’W
4	Mãe Maria, Rio Jacundá, Bom Jesus do Tocantins-Pará	November 2019	Tocantins	mesh nets e handmade casting net	5°19’48.7’’S, 48°49’53.9’’W
5	Região do Caju Amigo, Bom Jesus do Tocantins-Pará	November 2019	Tocantins	casting nets, gill nets, diving, mesh nets, handmade casting net	5°20’27’’S, 48°47’18.3’’W
6	Cachoeira do Mãe Maria. “Barca velha afundada”–São Joao do Araguaia-Pará	November 2019	Tocantins	diving	5°16’12.5’’S, 48°57’09.1’’W
7	Pedral do Lourenção, Nova Ipixuna-Pará	November 2019	Tocantins	casting nets, diving	4°59’31.9’’S, 49°20’09’’W
8	Praia próximo à grota do pedral do Lourenço, Nova Ipixuna-Pará	November 2019	Tocantins	diving and gill nets	4°59’33.5’’S, 49°19’30.8’’W
9	Pedral, Cachoeira do São Miguel, Ilha do Porta Aberta–São Geraldo do Araguaia-Pará	July 2022	Araguaia	diving, mesh nets	6º22’44’’S, 48º31’35’’W
10	Pedral, Corredeira, São Geraldo do Araguaia-Pará	July 2022	Araguaia	diving, mesh nets, harpoon	6º21’29’’S, 48º29’41’’W
11	Pedral, Cachoeira do São Miguel, Ilha do Porta Aberta, São Geraldo do Araguaia-Pará	July 2022	Araguaia	diving	6º22’44’’S, 48º31’35’’W
12	Pedral do Chiqueirão, Xambioá-Tocantins	July 2022	Araguaia	diving	6º21’37’’S, 48º29’56’’W
13	Remanso do Boto, Xambioá-Tocantins	July 2022	Araguaia	diving	6º22’48’’S, 48º23’16’’W
14	Pedral da Gaivota, São Geraldo do Araguaia-Pará	July 2022	Araguaia	diving	6º25’33’’S, 48º34’48”W
15	Pedral em frente a Ilha dos martírios, São Geraldo do Araguaia-Pará	July 2022	Araguaia	diving	6°13’23”S, 48°25’45”W
16	Travessão da Santa Isabel do Araguaia, São Geraldo do Araguaia-Pará	July 2022	Araguaia	diving	6°08’02.3”S, 48°19’47.6”W
17	Paredão das Três bocas, Boca feia, São Geraldo do Araguaia-Pará	July 2022	Araguaia	diving	6º8’53’’S, 48º22’56’’W
18	Gorgulhão do Sobradinho, São Geraldo do Araguaia–Pará	July 2022	Araguaia	diving	6º9’48”S, 48º25’29”W
19	Pedral em frente ao Porto da Balsa, Porto Franco-Maranhão	August 2022	Tocantins	diving, mesh nets	6º20’28’’S, 47º24’32’’W
20	Pedral da Ceval, próximo à Cachoeira seca, Porto Franco-Maranhão	August 2022	Tocantins	diving	6º26’11”S, 47º25’56”W
21	Perto da boca do rio Itaueiras, Porto Franco-Maranhão	August 2022	Tocantins	diving	6º23’7”S, 47º24’47”W
22	Pedral da Santa, Tocantinópolis-Tocantins	August 2022	Tocantins	diving	6º23’7’’S, 47º24’47’’W
23	Pedral, corredeira acima de Itaguatins–Itaguatins-Tocantins	August 2022	Tocantins	diving, casting nets	5º46’10’’S, 47º28’9’’W
24	Ilha das bacias, Itaguatins-Tocantins	August 2022	Tocantins	diving	5º46’7’’S, 47º28’26’’W
25	Pedral da Cachoeirinha, Governador Edison Lobão- Maranhão	August 2022	Tocantins	diving	5º46’5”S, 47º28’5”W
26	Pedral acima da cidade Itaguatins, Itaguatins–Tocantins	August 2022	Tocantins	diving, mesh nets, gill nets	5º45’59”S, 47º28’23”W
27	Pedral do rolador	August 2022	Tocantins	diving	5º46’39’’S, 47º28’3’’W

2. Fish sampling

Fish were collected in two expeditions which occurred in November 2019 and between the months of July and August 2022. Both field campaigns targeted various rapids areas across the Lower and Araguaia rivers (Figure 2). As this was an exploratory fish survey, we did not apply a standardized sampling effort.

Figure 2
Examples of rapid habitats sampled in the TARB. (A) Rocky stretches in the Pedral do Lourenço, in the municipality of Itupiranga – PA, near to the Marabá city, Tocantins River drainage, (B) View of rapids in front of Porto Franco city in Maranhão state, Tocantins River drainage, (C) Rapids habitats in the municipality of São Geraldo do Araguaia-PA, in the Serra dos Martírios/Andorinhas reserve, Araguaia River drainage, (D) Rapids in the front of Itaguatins city-TO, Tocantins River drainage, (E, F, and G) Rapids near the city of São Geraldo do Araguaia-PA (Corredeira do São Miguel), Araguaia River drainage (photographs A, B, E-G by Leandro Sousa, C by Renan Leão, and D by Felipe Araújo).

Sampling occurred between 08:00 AM to 01:00 PM, ensuring the greatest clarity of specimens in shallower areas (<3 m). At each site, we collected fish using a variety of methods: capture by free diving using a handmade casting net (2 mm mesh), gill nets (30 mm mesh), a harpoon for large specimens, and mesh nets for marginal areas and macrophyte banks. While multiple collection techniques were used, diving dominated the sampling effort. In areas with bedrock, we carefully overturned or removed loose rocks to capture specimens, positioning preferably opposite to the water flow.

For deeper sampling sites, exceeding 4 m of profundity with limited visibility, we used active sampling technique with scuba diving equipment. Divers used an LED flashlight to aid in locating specimens and capturing them.

All specimens were caught without any selection bias, except for endangered taxa. Which was collected a few representatives to obtain tissue samples for forthcoming genetic analysis. Upon capture, specimens were kept in containers for acclimatization in clean water. They were posteriorly euthanized using an overdose of eugenol (clove oil). Biological samples (fins or muscle) were obtained from most of the specimens, stored in 90% ethanol. Specimens were fixed in 10% formalin for 50 hours before being transferred to 70% ethanol for storage.

Following current legislation, the capture and transport of specimens were authorized under license 70940-1 SISBIO/IBAMA. After the identification at the lowest taxonomic level, the specimens were deposited in the fish collection of the Museu Paraense Emílio Goeldi. The tissues vouchers were stored in the Molecular Biology Laboratory of the Emílio Goeldi Museum to be integrated into the institution’s animal tissue collection. A complete list of voucher specimens from this study is provided in Appendix S1. The species are listed alphabetically by family, following a systematic organization based on Fricke et al. (2023).

To distinguish strictly rheophilic species from those with a less dependent association with rapids, we classified them according on their degree of rheophily (Table 2), such as: i) Strongly rheophilic (SR), species that occur in stretches of fast to moderate flow over a rocky substrate; ii) Moderately rheophilic (MR), species that can occupy both flowing and backwater habitats; iii) Opportunists (OP), species that explore areas of backwaters or with moderate flow, some of these species are migratory; and iv) Cryptobiotics (CB), species sheltered amid substrates in areas of strong to moderate water flow. This classification scheme follows Zuanon (1999), who characterized the degree of rheophily of fish inhabiting the bedrock environment of Xingu River according to visual sighting in diving sessions.

Table 2
Annotated list of the fish species collected in the rapid habitats of the TARB. N refers to number of specimens by species. Conservation status (CS) are presented according to International Union for Conservation of Nature (IUCN) and Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio) criteria: CR – Critically Endangered, DD – data deficient, EN – Endangered, LC- Least concern, NE – Not evaluated, NT – Near Threatened, VU – Vulnerable. The drainage where the species is discriminated (TO – Tocantins, AR – Araguaia). The degree of rheophily (DR) of fish species is indicated by MR (moderate rheophilic); SR (strictly rheophilic); OP (Opportunists); and CR (Cryptobiotic).

Order/family/species	N	CS		Basin		DR
MYLIOBATIFORMES		IUCN	ICMBIO	TO	AR
Potamotrygonidae
Paratrygon aiereba (Walbaum, 1792)	1	DD	CR		x	MR
Potamotrygon henlei (Castelnau, 1855)	2	LC	DD	x		MR
CHARACIFORMES
Anostomidae
Abramites hypselonotus (Günther, 1868)	1	LC	LC	x		MR
Hypomasticus cf. pachycheilus (Britski, 1976)	1			x		MR
Hypomasticus julii (Santos, Jégu & Lima, 1996)	1	LC	LC	x		SR
Hypomasticus sp.	2			x		MR
Leporellus vittatus (Valenciennes, 1850)	2	LC	LC	x		SR
Leporinus aff. taeniofasciatus Britski, 1997	6			x		MR
Leporinus bistriatus Britski, 1997	8	NT	NT	x		MR
Leporinus cf. unitaeniatus Garavello & Santos, 2009	1			x	x	MR
Leporinus fasciatus (Bloch, 1794)	3	LC	LC	x		MR
Leporinus maculatus Müller & Troschel, 1844	3	LC	LC	x		MR
Leporinus tigrinus Borodin, 1929	3	LC	LC	x		MR
Leporinus unitaeniatus Garavello & Santos, 2009	11	LC	LC	x	x	MR
Sartor tucuruiense Santos & Jégu, 1987	4	NE	EN	x		MR
Characidae
Astyanax sp.	2			x		OP
Exodon paradoxus Müller & Troschel, 1844	1	NE	LC		x	MR
Knodus savannensis Géry, 1961	2	LC	LC	x		OP
Moenkhausia gracilima Eigenmann, 1908	2	LC	LC	x		OP
Moenkhausia sp.	1			x		OP
Roeboides sp.	1			x		OP
Bryconidae
Brycon falcatus Müller & Troschel, 1844	3	NE	LC	x		OP
Brycon gouldingi Lima, 2004	1	EN	EN	x		OP
Brycon pesu Müller & Troschel, 1845	1	NE	LC	x		OP
Chilodontidae
Caenotropus labyrinthicus (Kner, 1858)	3	NE	LC	x		OP
Ctenoluciidae
Boulengerella cuvieri (Spix & Agassiz, 1829)	1	NE	LC	x		OP
Curimatidae
Curimata cf. inornata Vari, 1989	1			x		OP
Curimata inornata Vari, 1989	3	LC	LC	x		OP
Erythrinidae
Hoplias malabaricus (Bloch, 1794)	1	LC	LC	x		OP
Hemiodontidae
Bivibranchia velox (Eigenmann & Myers, 1927)	2	NE	LC	x		OP
Hemiodus argenteus Pellegrin, 1909	2	NE	LC	x		MR
Hemiodus sp.	1			x		MR
Prochilodontidae
Prochilodus nigricans Spix & Agassiz, 1829	3	NE	LC		x	MR
Serrasalmidae
Acnodon normani Gosline, 1951	9	LC	LC	x		OP
Myleus arnoldi Ahl, 1936	2	LC	NE	x		MR
Myleus setiger Müller & Troschel, 1844	1	NE	LC	x		MR
Tometes ancylorhynchus Andrade, Jégu & Giarrizzo, 2016	16	NE	NE	x		MR
Tometes sp.	4			x		MR
GYMNOTIFORMES
Apteronotidae
Apteronotus albifrons (Linnaeus, 1766)	4	LC	LC	x		MR
Apteronotus bonapartii (Castelnau, 1855)	2	LC	LC	x		MR
Parapteronotus hasemani (Ellis, 1913)	1	LC	LC	x		MR
Sternarchogiton nattereri (Steindachner, 1868)	11	LC	LC	x		MR
Sternarchorhynchus axelrodi de Santana & Vari, 2010	2	NE	NE	x		MR
Sternarchorhynchus cf. mesensis Campos-da-Paz, 2000	3			x		MR
Sternarchorhynchus goeldii de Santana & Vari, 2010	14	LC	NE	x		MR
Sternarchorhynchus sp.	3				x	MR
Sternopygidae
Archolaemus blax Korringa, 1970	13	LC	LC	x		MR
Archolaemus janeae Vari, de Santana & Wosiacki, 2012	1	LC	LC	x		MR
Eigenmannia macrops (Boulenger, 1897)	26	NE	LC	x		MR
Rhamphichthyidae
Gymnorhamphichthys cf. bogardusae Lundberg, 2005	6			x		MR
Rhamphichthys rostratus (Linnaeus, 1766)	2	LC	LC	x		MR
SILURIFORMES
Aspredinidae
Amaralia hypsiura (Kner, 1855)	1	LC	LC	x		CB
Bunocephalus coracoideus (Cope, 1874)	1	LC	LC		x	CB
Auchenapteridae
Centromochlus schultzi Rössel, 1962	5	LC	LC		x	OP
Cetopsidae
Cetopsis cf. candiru Spix & Agassiz, 1829	1			x		OP
Doradidae				x
Hassar wilderi Kindle, 1895	3	LC	LC	x		MR
Megalodoras uranoscopus (Eigenmann & Eigenmann, 1888)	5	NE	LC	x		MR
Rhinodoras boehlkei Glodek, Whitmire & Orcés V., 1976	22	LC	NE	x		MR
Rhinodoras armbrusteri Sabaj-Pérez, 2008	2	NE	NE	x		MR
Rhinodoras gallagheri Sabaj-Pérez, Taphorn & Castillo G., 2008	2	LC	NE	x		MR
Tenellus leporhinus (Eigenmann, 1912)	1	NE	NE	x		MR
Heptapteridae
Ceptopsorhamdia sp.	2			x		OP
Imparfinis sp.	1			x		OP
Leptorhamdia sp.	1			x		OP
Pimelodella cristata (Müller & Troschel, 1849)	15	LC	NE	x		OP
Rhamdia cf. quelen (Quoy & Gaimard, 1824)	1			x		OP
Loricariidae
Acanthicus hystrix Spix & Agassiz, 1829	41	NE	LC	x	x	MR
Ancistomus feldbergae (De Oliveira, Rapp Py-Daniel, Zuanon & Rocha, 2012)	10	LC	NE		x	SR
Ancistomus sp.	16			x		SR
Ancistomus spilomma (Cardoso & Lucinda, 2003)	58	LC	NE	x	x	SR
Ancistrus ranunculus Muller, Rapp Py-Daniel & Zuanon, 1994	9	NT	NT	x	x	SR
Ancistrus sp1.	10			x	x	SR
Ancistrus sp2.	45			x	x	SR
Ancistrus sp3.	14			x		SR
Ancistrus sp4.	26			x		SR
Aphanotorulus emarginatus (Valenciennes, 1840)	8	NE	NE	x		MR
Baryancistrus cf. niveatus (Castelnau, 1855)	3				x	SR
Baryancistrus longipinnis (Kindle, 1895)	24	NE	CR	x		SR
Baryancistrus niveatus (Castelnau, 1855)	38	NE	CR	x	x	SR
Baryancistrus sp.	7			x	x	SR
Farlowella cf. henriquei Miranda-Ribeiro, 1918	1			x		OP
Farlowella cf. oxyrryncha (Kner, 1853)	5			x		OP
Farlowella henriquei Miranda-Ribeiro, 1918	3	DD	DD	x	x	OP
Harttia cf. duriventris Rapp Py-Daniel & Oliveira, 2001	2				x	SR
Harttia duriventris Rapp Py-Daniel & Oliveira, 2001	37	LC	LC	x	x	SR
Harttia punctata Rapp Py-Daniel & Oliveira, 2001	19	LC	LC	x	x	SR
Harttia sp.	2				x	SR
Hypoptopoma gulare Cope, 1878	1	NE	LC	x		MR
Hypostomus gr. cochliodon	1			x		SR
Hypostomus faveolus Zawadzki, Birindelli & Lima, 2008	3	LC	LC	x		MR
Lamontichthys parakana Paixão & Toledo-Piza, 2009	19	NE	NE	x	x	SR
Lasiancistrus sp.	13			x		SR
Leporacanthicus galaxias Isbrücker & Nijssen, 1989	86	NE	LC	x	x	SR
Limatulichthys cf. griseus (Eigenmann, 1909)	1			x		MR
Limatulichthys griseus (Eigenmann, 1909)	5	LC	LC	x	x	MR
Panaque armbrusteri Lujan, Hidalgo & Stewart, 2010	3	LC	LC		x	MR
Parancistrus aurantiacus (Castelnau, 1855)	188	NE	NT	x	x	SR
Peckoltia oligospila (Günther, 1864)	6	LC	LC	x	x	SR
Peckoltia sabaji Armbruster, 2003	2	NE	NE	x		SR
Peckoltia sp1.	93			x		SR
Peckoltia sp2.	34			x		SR
Peckoltia vittata (Steindachner, 1881)	49	LC	LC	x	x	SR
Pseudacanthicus major Chamon & Costa e Silva, 2018	10	NE	NE	x	x	MR
Pseudacanthicus pitanga Chamon, 2015	3	NE	NE	x		MR
Rineloricaria cf. lanceolata (Günther, 1868)	1				x	MR
Rineloricaria lanceolata (Günther, 1868)	2	NE	LC	x		MR
Scobinancistrus pariolispos Isbrücker & Nijssen, 1989	3	VU	VU	x		SR
Scobinancistrus aff. pariolispos Isbrücker & Nijssen, 1989	1			x		SR
Spatuloricaria evansii (Boulenger, 1892)	20	LC	LC	x		MR
Spectracanthicus javae Chamon, Pereira, Mendonça & Akama, 2018	10	NE	NE	x	x	MR
Spectracanthicus tocantinensis Chamon & Rapp Py-Daniel, 2014	63	NE	NE	x		SR
Sturisoma rostratum (Spix & Agassiz, 1829)	3	LC	LC	x		MR
Pimelodidae
Aguarunichthys tocantinsensis Zuanon, Rapp Py-Daniel & Jégu, 1993	2	EN	EN	x		MR
Pimelodus tetramerus Ribeiro & Lucena, 2006	7	LC	LC	x		MR
Pimelodus quadratus Lucinda, Ribeiro & Lucena 2016	6	DD	NE	x		MR
Pseudopimelodidae
Microglanis sp.	4			x	x	CB
Pseudopimelodus bufonius (Valenciennes, 1840)	1	LC	LC	x		CB
Rhyacoglanis sp.	1			x		CB
Trichomycteridae
Pseudostegophilus nemurus (Günther, 1869)	1	NE	LC	x		CB
BATRACHOIDIFORMES
Batrachoididae
Potamobatrachus trispinosus Collette, 1995	1	DD	EN	x		CB
SYNBRANCHIFORMES
Synbranchidae
Synbranchus marmoratus Bloch, 1795	2	LC	LC	x	x	CB
Synbranchus gr. marmoratus	5			x		CB
CARANGIFORMES
Achiridae
Hypoclinemus mentalis (Günther, 1862)	1	LC	LC	x		CB
CICHLIFORMES
Cichlidae
Biotodoma cupido (Heckel, 1840)	1	NE	LC	x		OP
Cichla cf. pinima Kullander & Ferreira, 2006	2			x		OP
Cichla pinima Kullander & Ferreira, 2006	2	LC	LC	x		OP
Cichla piquiti Kullander & Ferreira, 2006	12	LC	LC	x		OP
Cichla sp.	1			x		OP
Cichlasoma araguaiense Kullander, 1983	1	NE	LC	x	x	MR
Crenicichla aff. cametana Steindachner, 1911	2			x		SR
Crenicichla cametana Steindachner, 1911	33	LC	LC	x	x	SR
Crenicichla cyclostoma Ploeg, 1986	2	CR	CR	x		SR
Crenicichla jegui Ploeg, 1986	12	EN	EN	x		SR
Crenicichla reticulata (Heckel, 1840)	1	NE	LC	x		OP
Crenicichla sp.	14			x		OP
Geophagus altifrons Heckel, 1840	1	LC	LC	x		MR
Geophagus sveni Lucinda, Lucena & Assis, 2010	3	LC	LC	x		MR
Geophagus neambi Lucinda, Lucena & Assis, 2010	47	LC	LC	x		MR
Heros severus Heckel, 1840	3	NE	LC	x		OP
Lugubria strigata (Günther, 1862)	1	LC	LC	x		OP
Retroculus lapidifer (Castelnau, 1855)	2	LC	LC	x	x	SR
Satanoperca jurupari (Heckel, 1840)	1			x	x	OP
Saxatilia cf. labrina (Spix & Agassiz, 1831)	1			x		OP
Teleocichla cinderella Kullander, 1988	15	EN	EN	x	x	SR
ACANTHURIFORMES
Sciaenidae
Pachyurus junki Soares & Casatti, 2000	1	LC	LC	x		MR
TETRAODONTIFORMES
Tetraodontidae
Sphoeroides tocantinensis (Amaral, Brito, Silva & Carvalho, 2013)	13			x		MR

To assess overall species diversity, a rarefaction curve of specimen abundance accumulation across sampling sites was created. The rarefaction and extrapolation were measured using the iNEXT package v. 3.0.1 (Hsieh et al. 2024). Figures were produced using the ggplot2 package v. 3.6.3 (Wickham 2016).

The conservation status of fish species was assessed according to both the Brazilian endangered fish list (ICMBio, 2022) and the International Union of Conservation of Nature categories: CR – Critically Endangered, DD – data deficient, EN – Endangered, LC- Least concern, NE – Not evaluated, NT – Near Threatened, VU – Vulnerable. Complete categorization of species endangerment status can be found at Table 2.

Results

In this study a total of 1668 specimens were surveyed in the lotic habitats from the TARB, representing 107 species, 79 genera, 28 families and 10 orders (Table 2). The Siluriformes order was the most speciose, with 44 species (41.1%), followed by Characiformes (27 species, 25.2%), Cichliformes (18 species, 16.8%), and Gymnotiformes (13 species, 10.2%) (Figure 3). Other orders represented 2% of the inventoried diversity, containing one or two species each.

Figure 3
Contribution total number of the different fish orders to the rapids habitats from the TARB.

Among the families, Loricariidae was the most representative (Figure 4), with 29 species, followed by Cichlidae, with 18 species, and Anostomidae with nine species (some illustrated in Figure 5). In some cases, identification was only possible at the genus level, such as Heptapteridae and Pseudopimelodidae, where the individuals were juveniles, or the species delimitation relied on inner morphologic characters. Among the whole ichthyofauna, the most abundant species were Parancistrus aurantiacus and Leporacanthicus galaxias, both from Loricariidae.

Figure 4
Number of species organized by family recorded in the rapids of the TARB.

Figure 5
Examples of fish species collected in the TARB rapids. (A) Aguarunichthys tocantinsensis, MPEG 039432, 340 mm SL; (B) Apteronotus albifrons, MPEG 039446, 174 mm SL; (C) Baryancistrus longipinnis, MPEG 039575, 93.2 mm SL; (D) Brycon falcatus, MPEG 039467, 255 mm SL; (E) Crenicichla jegui, MPEG 039205, 177.8 mm SL; (F) Parancistrus aurantiacus; MPEG 39404, 116.6 mm SL; (G) Leporinus maculatus, MPEG 039183, 77 mm SL; (H) Potamobatrachus trispinosus, MPEG 039525, 72.2 mm SL; (I) Sartor tucuruiensis, MPEG 039533, 86.7 mm SL; (J) Teleocichla cinderella, MPEG 039212, 79 mm SL (all photographs by Julia Nascimento).

Considering the degree of rheophily, the majority of species occurring in the TARB’s rapids were categorized as moderate rheophilic (66 species), followed by opportunists (35 species). Strictly rheophilic species count 24 species, while cryptobiotics totaled five species. A radar figure summarizes the diversity among the degrees of rheophily (Figure 6).

Figure 6
Degree of rheophily of ichthyofauna inhabiting the rapids of the TARB: absolute frequency by species status following Zuanon (1999) and our field diving observations: Strongly rheophilic (SR); Moderately rheophilic (MR); Opportunists (OP); Cryptobiotics (CB).

In relation to the diversity of fish in the TARB rapids, the rarefaction curve corroborates the accumulation of species with an increase in the number of individuals sampled and shows a tendency towards stability, thus indicating that the sampling carried out is able to reflect the richness of species on it (Figure 7).

Figure 7
Rarefaction and extrapolation curves for species richness along the TARB. Shaded areas indicate 95% confidence intervals.

Considering the conservation status of the inventoried rapids-dwelling species, about eleven are listed as endangered on the Brazil’s Red Species List (ICMBIO 2018, 2022). Being the species Aguarunichthys tocantinsensis (EN), Baryancistrus longipinnis (CR), Baryancistrus niveatus (CR), Brycon gouldingi (EN), Crenicichla cyclostoma (CR), Crenicichla jegui (EN), Potamobatrachus trispinosus (EN), Teleocichla cinderella (EN), Sartor tucuruiensis (EN) and Scobinancistrus pariolispos (VU). Non-endemic species for the TARB, Paratrygon aiereba (CR) was listed.

Discussion

We have presented the first survey of fish species occupying remnant rapids habitats from the TARB. The ichthyofauna which explore such environments of TARBS’s is similar to that of other river basins, with a predominance of species of Loricariidae, Cichlidae, Characidae, and Anostomidae (Casatti and Castro 1998, Lucinda et al. 2007, Fitzgerald et al. 2018, Gonçalves 2019). Among the Gymnotiformes, Apteronotidae showed a high diversity with seven species, a result that was expected, since the taxon has several species associated with this type of environment, particularly those with the genus Apteronotus (Peixoto et al. 2021). Noticeably, previous studies focused on TARB’s overall fish fauna, without characterizing the rapid-dwelling fish community (Lucinda et al. 2007, Chamon et al. 2022). Our survey bridges the knowledge gap from the lack of studies of fish diversity specifically for strictly rheophilic species and those with moderate or occasional rheophily in the rapids of the TARB.

In comparison to other studies in the TARB, the present survey sampled a lower number of characin species, which is the richest fish group in the basin (Chamon et al. 2022). This discrepancy is likely due to the sampling methods employed, as this study relied primarily on diving to capture specimens. The bedrock substrate of the rapids habitat made it challenging to use traditional fishing gears for specimen collection, such as gillnets and seine nets, which are more commonly used in collecting in river margins, sand beaches and backwaters. Some studies indicate that morphological variation of the dorsal fin size and shape among characiforms lineages can also impact their habitat occupation ability (Cassati and Castro 2006, Guisande et al. 2012). This was corroborated by our field observations in which few characin species (Characidae) were identified occupying areas of moderate water flow, being only seen near the river margins. Consequently, it corroborates the assumption that they are not able to thrive in strong water flows.

Our analysis of the rheophilic composition in the TARB’s rapids revealed a dominance of moderately rheophilic species, followed by opportunists. Considering the diversity within strictly rheophilic species, Loricariidae showed the highest number of species (27), likely due to their adaptations for exploring these environments such as the cup-shaped mouth morphology and flattering of the body (Reis et al. 2003, Lujan et al. 2012, Collins et al. 2018). It is followed by Cichlidae and Anostomidae. This pattern aligns with observations reported by Fitzgerald et al. (2018) in the rapids of Xingu River, where its fish assemblage was characterized by a prevalence of moderate to strictly rheophilic fishes, with important species abundance on Loricariidae, Cichlidae and Anostomidae. However, our data has a noteworthy discrepancy when compared to the findings of Cassati and Castro (1998). They documented a higher proportion of strictly rheophilic species in six families, such as Anostomidae, Characidae, Cichlidae, Loricariidae, Parodontidae, and Pimelodidae in the rapids of the Upper São Francisco River (Minas Gerais state). The Upper Tocantins rapid-dwelling ichthyofauna, Lucinda et al. (2007) also documented a high diversity of Characidae, followed by Loricariidae and Anostomidae. This contrasts with our findings. While Lucinda et al. (2007) survey encompassed both lentic and lotic habitats, they did not characterize the degree of rheophily in the species that occur in the Upper Tocantins. Consequently, the Upper Tocantins rheophilic fish community lacks a detailed survey of the degree of rheophily among the species that inhabit its bedrock environments. Unfortunately, these habitats are severely impacted by several dams along this river sector, potentially leading to the loss of many rheophilic species (Agostinho et al. 2007, Lucinda et al. 2007, Araújo et al. 2013, Akama 2017, Perônico et al. 2020, Pereira et al. 2021, Obeso et al. 2024).

In respect of the state of knowledge of the evolution and biogeography of the Neotropical freshwater fish there are models of continental radiation through allopatry among lineages rather than adaptive radiation, resulting in the remarkable biodiversity observed today (Bloom et al. 2013, Machado et al. 2018, Albert et al. 2020). Consequently, continuous surveys can lead to an increase in the knowledge of its ichthyofauna and could provide an accurate assessment of the conservation status for many taxa, specially rheophilic lineages, whose evolutionary history still needs to be better researched and understood. For comparison, species restricted to fast-flowing habitats outside the Neotropics include those from the Congo River in the African continent (Kisekelwa et al. 2020). Its diversity is composed predominately of Cypriniformes, with Cyprinidae being the most speciose (30 species); then Siluriformes with 26 species; followed by Characiformes including 14 species (Kisekelwa et al. 2020). Markedly we observed that the TARB has more Siluriformes species compared to Congo´s rapids. This might be explained as the catfish diversity from the two continents evolved and diversified in different ways. It is known that for Neotropical fish fauna, palaeohydrological changes and river captures facilitated lineage dispersal (Lundberg et al. 1998, Dagosta and Pina 2017, Dagosta and Pina 2019, Cassemiro et al. 2023). While for continental African fish fauna, especially catfishes, events of dispersal and colonization of new habitat through time appear to be prime drivers of diversification (Day et al. 2023).

Our results show that much of this diversity is closely associated with rapids habitats, and the loss of these habitats might result in local extirpation, especially species which are suffering population decline. For example, Baryancistrus longipinnis, a species whose distribution is restricted to the Tocantins River basin, has not been recorded in dammed areas after the construction of several dams (Lajeado, Serra da Mesa, Cana Brava, São Salvador, and Peixe Angical Power plants) (ICMBio 2018, Oliveira 2018, Chamon et al. 2022). In our survey on the remnant’s rapids throughout the TARB, B. longipinnis was only recorded in the Pedral do Lourenço (Tocantins downstream). This rocky stretch is under threat as there is a project to build a waterway to facilitate the transport of agricultural commodities (CONAB 2021). The proposed environmental alteration to deepen the river channel, which will be operated via implosion of rocky substrate, will directly impact on its biodiversity, boosting the possibility of disappearance of many strictly rheophilic taxa, including the endemic and endangered B. longipinnis. Similar distress extends to other species which occur sympatrically in the area or occasionally occupy these rapids. Moreover, our study reveals that rapids in the Lower TARB host a rich fish diversity. Though, the rapids in this area still have some degree of environmental integrity, they also face significant threat by the possibility of implementation of planned dams, such as the Serra Quebrada and Marabá Hydroelectric Plants (Akama 2017). These projects can result in hydrological changes to the whole river course, as already documented in portions of the Middle and the Upper Tocantins River basin (Estreito and Lajeado Hydroelectric Plants) (Akama 2017, Lucinda et al. 2007, Medeiros et al. 2014, Pelicice et al. 2021). Investigations about the effect of those impacts on the TARB’s rapid-flowing environments have indicated important ecological alterations on fish populations, diversity decay and changes in community structure (Agostinho et al. 2007, Lucinda et al. 2007, Araújo et al. 2013, Medeiros et al. 2014, Arantes et al. 2019, Perônico et al. 2020, Pereira et al. 2021, Keppler et al. 2022).

The persistence of some taxa in the remaining rapid stretches suggests that their populations may endure despite habitat alterations. Anthropic alterations on freshwater ecosystems can occur rapidly over a short time scale, but in some cases, fish communities can thrive despite these impacts by migration if there are no barriers to dispersal (McFandden et al. 2022). However, these models may not be adequate for rheophilic species, as these organisms are intrinsically associated to the substrate and the conversion of the rapids to lentic habitats by damming tend to result in a decline in species richness and abundance, and boost genetic homogenization on long term, especially given the lack of consistent evidence for population connection via gene flow in this community (Hrbek et al. 2018, Magalhães et al. 2021, Keppler et al. 2022, Americo 2022, Leonardo 2023). In this context, an increase in population genetics studies on these taxa could reveal how populations are structured and give an appraisal on their viability. Considering that Tocantins downstream remnant rocky rapid habitats are vulnerable to disappear due to the intent to construct mega projects in an area that seems to be a refuge for rheophilic species is a cause of concern, as many endangered species can disappear.

An example of an endangered species that inhabits this area, is the batrachoidid species, Potamobatrachus trispinosus, whose type locality was submerged for the operation of Tucuruí Power plant (Collet et al. 1995). In our survey, we found specimens occurring in the rapids of São João do Araguaia (Tocantins downstream), suggesting that this species still has a population in this relatively well-preserved stretch of rapids. Therefore, it can provide arguments to support that these remaining rapids are priority areas for conservation. A number of TARB´s threatened taxa were not sampled in our campaigns, including Rhyncodoras xingui and Mylesinus paucisquamatus. The lack of successful collections may also reflect environmental changes impacting the TARB’s ichthyofauna. The absence of M. paucisquamatus, was particularly alarming as this is an endemic species to the region and its population decline has already been reported, with only a few individuals being recorded in small flowing areas of the Tocantins River (Vitorino-Júnior et al. 2016). Similarly, the absence of R. xingui, which co-occurs in the TARB and Xingu drainages highlights the need for further investigations. Further studies should incorporate long-term seasonal data and encompass a broader geographic range of its known distribution. This comprehensive approach would help elucidate the processes potentially driving their disappearance.

Conclusions

Despite the significant loss of major lotic environments in this River basin over the past four decades due to development projects, counting approximately 920 km² over the whole TARB’s drainage (2450 km) (Akama 2017, ICMBio 2018), the remaining rapids seems to be surprisingly stable environments, supporting a rich fish fauna, especially those in the transitional section (Tocantins downstream). Therefore, its maintenance is essential for the conservation of this diversity. To support this, different spheres of government need to re-evaluate development plans that directly impact this unique freshwater habitat.

Supplementary material

The following online material is available for this article:

Voucher codes of fish collected in this study in the rapids from the Tocantins-Araguaia River basin deposited in the fish collection of the Goeldi Museum – Belém, Pará, Brazil.

Acknowledgments

This project received grants from the Fundo de Defesa de Direitos Difusos program of the Brazilian Ministry of Justice and Public Safety (FDD 24/2019 – 0800.012635/2019-80). We are indebted to numerous people for their help in collecting specimens including: Alany Gonçalves, Ariel Romeiro, Cleonice Lobato, Cristina Fernandes, Erys Amorim, Gabriel de Jesus, Ithalo Castro, José Nolasco, Mayllon Moura, Manuela Dopazo, Mark Sabaj, Naraiana Benone, Romina Batista, Thiago Vieira, and Yeda Rocha. We thank in special the fisherman “Dani”, Damilton da Costa, for his inestimable expertise in diving and collecting specimens in the depths of the TARB in the expeditions for this study. Additionally, we thank Andrés Montes-Correa and Joudellys Andrade-Silva for valuable assistance on statistical analysis. The authors are supported by: Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001 (FA, IM, LJ, ML, JN), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPQ), RL #Proc. 140641/2020-3), WW (#Proc. 307988/2021-0), Financiadora de Estudos e Projetos (FINEP), WW (Projeto 25661), and Fundação Amazônia de Amparo a Estudos e Pesquisas – FAPESPA (AC).

Data Availability

The dataset includes the list of specimens collected during this study are available in https://doi.org/10.48331/scielodata.TPIRIK.

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Edited by

• Associate Editor
  Juan Schmitter-Soto

Publication Dates

• Publication in this collection
  14 Mar 2025
• Date of issue
  2025

History

• Received
  06 July 2024
• Accepted
  19 Dec 2024