A big task for a miniature species: understanding Fluviphylax simplex (Cyprinodontiformes: Fluviphylacidae) distribution

Dhállyth Zaínny Pereira Silva¹, Felipe Polivanov Ottoni^2,3 , Cleonilde da Conceição Silva Queiroz⁴ and Pedro Henrique Negreiros de Bragança^1,3,5

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Associate Editor: George Mattox

Section Editor: William Crampton

Editor-in-chief: José Birindelli

Abstract​

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Introduction​

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The Amazon River basin is the world’s most biodiverse region, with about 3000 freshwater fish species, and many new species being discovered every year (Dagosta, de Pinna, 2019). One of the most intriguing aspects about fishes in the Amazon River basin, besides its extraordinary diversity, is to understand the historical events and ongoing processes that underlie their distributions. Within this attempt, the species’ ecological information, which is often scarce, has a major importance in interpreting the impacts of physical and physiological barriers in limiting and defining a species distribution range (Schneider et al., 2012; Pires et al., 2018).

Historically, the different water types found in the Amazon River tributaries are considered to play a major role in defining freshwater fish species’ distribution due to their different physico-chemical aspects, probably acting as barriers (Wallace, 1853; Sioli, 1984; Schneider et al., 2012; Pires et al., 2018; Borghezan et al., 2021). Basically, rivers draining old geologic formations, such as the Brazilian and the Guiana Shields are characterized by low sediment input and are classified as black-water and clear-water. The main difference between them is the water colour, where black-water bodies are characterized by dark tea-color waters, whereas clear-water bodies are clear and transparent. Besides that, they can be differentiated by the presence of a high concentration of humic acids in black-waters, because of plant decomposition, resulting in an extremely low pH, between 3.8 and 4.9, and the dark tea colour, whereas in clear waters the pH is between 4.4–7.8 (Borghezan et al., 2021). On the other hand, rivers draining recent geologic formations such as the Andes, are popularly known as white-water rivers, and are recognized by their high sediment load, fast flowing waters, and alkaline pH, between 6.2–7.2 (Borghezan et al., 2021). Generally, fish species adapted to one water type struggle to keep osmotic regulation and ionic balance when exposed to different water types (Borghezan et al., 2021). Thus, in the Amazon River basin, the confluence between rivers with different physico-chemical water parameters does not necessarily facilitates communication between different fish fauna, but might represent a major barrier to fish movement.

Vagility, the capacity of an individual to move within the species distribution range, connecting and maintaining gene flow with individuals from different populations, is also critical in understanding species distribution. In general, the main variable limiting a species vagility is its size, with large species presenting much broader foraging and distribution ranges, whereas smaller species have more limited distributions (Roff, 1991; Griffiths, 2012). A larger fish, for example, can more easily swim against strong currents, and are probably not affected by high sediment waters as small fish species. Thus, it is not expected that small fish species have wide distribution ranges, especially if the only way to keep the genetic flow between the different populations implies transposing major physico-chemical barriers (Washburn et al., 2020).

Dagosta, de Pinna (2019), relying on distribution data of Amazon freshwater fish species, information on historical events, and the different water types in the Amazon, defined 20 distinct distribution patterns for its fish fauna. Among them, the Central Black-water Amazon distribution pattern, could not be defined by a historical event but by the presence of black-water dwelling species in the central part of the Amazon River basin from the lower Tapajós River in the east to the Juruá, and Lake Tefé to the west, including the Negro River. According to Dagosta, de Pinna (2019), many river systems within this region are recognized as having white-water (e.g., Madeira, Branco River) or clear water (e.g., Tapajós) but they also have many small black-water tributaries, especially on their lower sections. The presence of black-water dwelling species within this mosaic of water types was previously considered as an unexpected distribution pattern of difficult interpretation. For example, the distribution of the characiforms Creagrutus maxillaris (Myers, 1927) and Chalceus macrolepidotus Cuvier, 1818, both disjunctly distributed in the black-water environments of the Negro and Madeira River drainages, could not be easily explained considering the physico-chemical barrier represented by the Madeira and the Solimões rivers white-waters (Vari, Harold, 2001; Zanata, Toledo-Piza, 2004). The presence of black-water dwelling fish species showing a similar disjunct distribution pattern, however, is more common than previously thought, and an extensive list of species showing a similar distribution is provided in Dagosta, de Pinna (2019). However, no comment was made on the mechanisms that possibly allow species with low vagility capacity and restricted to low pH black-water environments to overcome fast flowing and high sediment load waters.

Within this knowledge gap scenario on how small-sized species with a Central Black-water Amazon distribution pattern can overcome physico-chemical barriers, the present study provides a first insight into the distribution of one of the smallest fish species in the Amazon, the killifish Fluviphylax simplex Costa, 1996. The cyprinodontiform genus Fluviphylax Whitley, 1965 includes eight miniature species, belonging to the family Fluviphylacidae, which are characterized by an extremely reduced body size in adults with individuals rarely surpassing the 20 mm of standard length (Costa, 1996; Costa, Le Bail, 1999; Bragança, 2018). The miniaturization process is generally associated with morphological simplification and even the loss of bone structures, as have already been reported for Fluviphylax species (Costa, 1996; Costa, Le Bail, 1999; Bragança, 2018). Most of the species are distributed within the Central Black-water Amazon region, with the highest number known for the Negro River, the major black-water tributary of the Amazon: F. zonatus Costa, 1996 is known from the lower Negro River and black-water tributaries of the Branco River; F. obscurus Costa, 1996 and F. wallacei Bragança, 2018, are known from the middle Negro River; F. gouldingi Bragança, 2018 and the recently described F. rubens Huber, 2024 are known from the upper Negro River (Costa, 1996; Bragança, 2018; Huber, 2024). The genus diversity in the Orinoco is not well known, but likely some of the species from the middle and upper Negro River might also be present in this system, which is connected to the upper Negro through the Casiquiare River. The species F. palikur Costa & Le Bail, 1999 is known from clear and blackwater rivers draining to the coast between the Amapá and Pará states in Brazil; F. pygmaeus (Myers & Carvalho, 1955) is only known from the black-water tributaries of the Madeira River; and F. simplex is found in black-water tributaries of the Amazon and Solimões, between the mouth of the Tapajós River and the region west to Tefé, representing the more widespread species in the genus (Costa, 1996). This species distribution nearly matches the Central Black-water Amazon distribution pattern, except by the absence of F. simplex in the middle and upper Negro River. The species has been recorded for the lower Negro River, nearby the city of Manaus (Souza et al., 2011), close to its confluence with the Solimões River, but never in the middle and upper reaches of the Negro River.

This unusual distribution for a miniature species becomes even more intriguing when considering the genus’ ecology. Fluviphylax species inhabit high acidic slow flowing waters and are surface dwellers (Weitzman, Vari, 1988; Costa, 1996). They feed uniquely on small arthropods floating in the water surface, a feeding habit known as neustophagia (Leitão et al., 2016). Probably, the capacity to feed on those small arthropods is critical for the incorporation of biomass in nutrient poor environments such as black-waters. Recent studies suggest that miniature species have specific and important roles for the ecosystem functioning (Leitão et al., 2016; Perkin et al., 2022). During field expeditions directed to sample species of Fluviphylax between 2011 and 2019 by PHNB and FPO, no specimen was ever found in the high sediment load Amazon or Solimões rivers main channel. That’s enigmatic considering that there are reported populations of F. simplex in both northern and southern tributaries of the Amazon and Solimões rivers.

This study aims to provide a first insight into the impacts of white-water rivers over the communication between Fluviphylax simplex populations, a Central Black-water Amazon distribution pattern species which so far has only been sampled in black-water. Phylogeographic patterns within this broadly distributed miniature species were investigated, and its distribution range updated. Also, the species coloration in life is herein described.

Material and methods

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Samples.Twenty-one specimens and/or tissues of Fluviphylax simplex from different localities along the species distribution range were made available through loans by the following Brazilian institutions: Universidade Federal do Rio de Janeiro, Rio de Janeiro (UFRJ); Universidade Federal de São Carlos, São Carlos (LISO); Coleção Ictiológica do Centro de Ciências Agrárias e Ambientais, Universidade Federal do Maranhão, Chapadinha (CICCAA); Laboratório de Biologia e Genética de Peixes, Departamento de Morfologia, Instituto de Biociências, Universidade Estadual Paulista, Botucatu (LBP-UNESP), and Museu de Zoologia da Universidade de São Paulo, São Paulo (MZUSP) (Fig. 1). One Fluviphylax simplex sequence (MF497420.1) was retrieved from NCBI database and added to this dataset. The distribution map was made in the software QGIS v. 3.34.7 and includes both the localities of the specimens included in the molecular study and specimens examined for the compilation of the map shown in Bragança, Costa (2018) which are listed in S1.

FIGURE 1| Fluviphylax simplex haplotypes distribution: blue, Tibarrá-Santa Isabel do Rio Negro; brown, Amanã-Solimões River; red, Alvarães-Solimões River; grey, Tefé, Solimões River; yellow, Beruri-Purus River; purple, Manaus, Amazon River; green, Parintins, Uaicurapá River; pink, Alter do Chão, Tapajós River. Dark grey dots represent additional examined specimens and the large black dot, Fluviphylax pygmaeus haplotype from Borba, Madeira River.

Fluviphylax pygmaeus (NCBI accession number MG451923.1), the sister species of F. simplex according to both morphological (Costa, 1996; Costa, Le Bail, 1999) and molecular analyses (Bragança, Costa, 2018), was included as outgroup. Specimens’ localities, collection numbers and NCBI accession numbers are provided in Tab. 1.

TABLE 1 | Specimens’ localities, catalogue numbers and National Center for Biotechnology Information (NCBI) accession numbers of Fluviphylax simplex.

Species	Catalogue number	Tissue no	Haplotype	Distribution and coordinates	NCBI
Fluviphylax simplex	LBP 26131	9596 .6	H9	Alvarães, Solimões River, AM, Brazil 03°20’00.0”S 64°49’08.0”W	PV688167
Fluviphylax simplex	LBP 26131	9596.7	H9		PV688168
Fluviphylax simplex	LBP 26131	9596.8	H9		PV688169
Fluviphylax simplex	LBP 26131	9596.9	H9		PV688170
Fluviphylax simplex	CICCAA 05287	05287.1	H6	Lake Tefé, Tefé, Solimões River, AM, Brazil 03°17’38.0”S 64°46’05.1”W	PV688174
Fluviphylax simplex	LISO 1012	LISO 2019.011501.1	H10	Lake Ayapuá, Beruri, lower Purus River, AM, Brazil 04°25’25.88"S 62° 9’21.46"W	PV688171
Fluviphylax simplex	LISO 1012	LISO 2019.011501.2	H10		PV688172
Fluviphylax simplex	LISO 1012	LISO 2021.110401.1	H11		PV688173
Fluviphylax simplex	UFRJ 9639.1		H3	Alter do Chão, Santarém, lower Tapajós River, PA, Brazil 02°31‘24.5”S 54°55‘32.2”W	PV688159
Fluviphylax simplex	UFRJ 9639.2		H4		PV688160
Fluviphylax simplex	UFRJ 9639.4		H3		PV688161
Fluviphylax simplex	UFRJ 8342.1		H5	Lake Amanã, Maraã, Solimões River, AM, Brazil 02°30’1.09"S 64°42’52.39"W	PV688162
Fluviphylax simplex	UFRJ 8342.2		H6		PV688163
Fluviphylax simplex	UFRJ 9085.1		H2	Uaicurapá River, Parintins, AM, Brazil 02°45’50.3”S 56°46’32.2”W	MF497420
Fluviphylax simplex	UFRJ 9085.2		H1		PV688158
Fluviphylax simplex	UFRJ 9085.3		H2		PV688157
Fluviphylax simplex	UFRJ 9085.4		H1		PV688156
Fluviphylax simplex	UFRJ 9085.5		H1		PV688155
Fluviphylax simplex	UFRJ 9192.1		H8	Tibarrá River, Santa Isabel do Rio Negro, Negro River, AM, Brazil 00°24’46.8”S 64°56’57.3”W	PV688165
Fluviphylax simplex	UFRJ 9192.2		H8		PV688166
Fluviphylax simplex	UFRJ 9209.1		H7	Igarapé Tarumã-Mirim, Manaus, Negro River, AM, Brazil 03°01’24.3”S 60°10’40.0”W	PV688164

DNA extraction, PCR and sequencing. Molecular data was obtained from muscular tissue taken from the right side of the caudal peduncle using the commercial extraction kit Extract-N-Amp™ Tissue PCR (Sigma), reducing the regent’s volume by ¼ of the original protocol. The quality of the extracted DNA was verified through gel electrophoresis stained with GelRed (Sigma). The mitochondrial gene cytochrome oxidase subunit I (COI) was amplified through PCR using the following primers: LCO1498 5’- GGT CAA CAA ATC ATA AAG ATA TTG G- 3’ and HCO2190 5’- TAA ACT TCA GGG TGA CCA AAA AAT CA – 3’ (Folmer et al., 1994). Polymerase chain reactions (PCR) were performed in 10 μl reaction mixtures containing 4.2 μl of pure water, 4.0 μL of RedMix (Sigma), 0.4 μL of each primer (final concentration of 1 μM) and 1 μL of genomic DNA. The thermocycling profile was as follows: (1) 1 cycle of 3 min at 94°C; (2) 30 cycles of 1 min at 94°C, 1 min at 48°C, and 90 sec at 72°C; and (3) 1 cycle of 10 min at 72°C. In all PCR reactions, negative controls without DNA were used to check contaminations. Amplified PCR products were quantified and sent for sequencing at ACTGENE Ltda., Porto Alegre, Brazil.

Sequences editing and alignment. Sequences were edited manually by checking the sequencing quality electropherograms on BioEdit (Hall, 1999) and aligned using ClustalW (Thompson et al., 1994). The alignment was translated into proteins to check for the presence of stop condons. After alignment, sequences extremities were trimmed (= 502 bp) and this final matrix was used to establish phylogenetic relationships and to estimate the haplotype network.

Bayesian analysis and haplotype network. The phylogenetic analysis was conducted through Bayesian inference (BI), using the program MrBayes v. 3.2.5 (Ronquist et al., 2012). The best-fit evolution model was estimated on JModeltest v. 2.1.10. (Darriba et al., 2012). The BI reconstruction included two Markov chain Monte Carlo (MCMC) runs of two chains each for 3 million generations, with a sampling frequency of 1000. The quality of the MCMC chains were evaluated in Tracer 1.6 (Rambaut et al., 2014), and the first 25% of samples removed as burn-in in TreeAnnotator v. 1.8.4 (Drummond et al., 2012). A haplotype network was inferred using the median joining network method in PopArt 1.7 (Leigh, Bryant, 2015).

Colour pattern of Fluviphylax simplex. Photographs taken in the field of live individuals from different populations of F. simplex along its distribution were used for the description of the species life coloration pattern.

Results​

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Bayesian Inference analysis. The best fit-evolution model for the dataset was GTR+I and the intrapopulational relationships of Fluviphylax simplex recovered three clades (Fig. 2). The first clade was weakly supported as sister to all other populations [posterior probability (PP) = 63] and include the haplotype from Santa Isabel do Rio Negro (H8), in the middle Negro River, representing the first record of the species in this region. The second clade (pp = 86) include two haplotypes, one from Uaicurapá (H1), near Parintins, close to the type-locality of F. simplex, and the other from the lower Negro River, near Manaus (H7). The third clade (pp = 52), albeit weakly supported, includes three well supported subclades: one with haplotypes from Uaicurapá (H2), near Parintins, from Alter do Chão in the lower Tapajós (H3, H4), and from the lower Purus River (H10, H11) (pp = 94); the second including haplotypes from the middle Solimões River in Amanã (H5, H6) and Tefé (H12) (pp = 100); and the third including a haplotype from Alvarães (H9), another locality in the middle Solimões.

Fluviphylax simplex genetic diversity and haplotype network. The mitochondrial COI matrix including 502 bp of the twenty-one individuals of Fluviphylax simplex have twenty-six polymorphic sites, which two are singleton and twenty-four parsimony informative. A total of thirteen mutations were found, eleven of them transitions, and two transversions. There was a total of twelve haplotypes, with a genetic divergence of Hd = 0.9381 and π = 0.0150 (Fig. 3).

FIGURE 2| Mitochondrial COI bayesian inference analysis of Fluviphylax simplex haplotypes. Posterior probability values are shown above nodes.

FIGURE 3| Mitochondrial COI haplotype network of Fluviphylax simplex (502 bp).

There was only one shared haplotype within the dataset, it was between the Solimões River populations of Tefé and Amanã Lakes, which are in opposite margins of the Solimões River. The population from Alvarães, which is close to Tefé and Amanã, had its own unique haplotype with nine nucleotide substitutions relative to the Tefé and Amanã cluster. The population from Uiacurapá near Parintins was the only to show haplotypes that are not grouped in the same cluster, one of them being more similar to the haplotype from Manaus, lower Negro River, from which they differ by only three nucleotide substitutions, and the other haplotype clustered with haplotypes from the Purus River, from which they differ by only one nucleotide substitution.

Life colour pattern. The coloration in life of Fluviphylax simplex is herein described based on photographs from males and females belonging to different populations along the species distribution (Fig. 4). Males colouration in life (Figs. 4A–F): side of body yellowish gray. Dorsum dark brown. Venter white to light gray between head and region anterior to pelvic fin insertion, with metallic green ventral line between point below pectoral fin insertion to the caudal peduncle, more conspicuous from insertion of the anal fin. Side of head gray to brownish gray, dorsal portion brown. Jaws pale orange to gray. Iris yellow. Eye bright silver on dorsal portion. Dorsal fin yellow, with few scattered black dots in some males. Anal fin pale yellow. Caudal fin yellow, scattered with small black dots in some males, black distal margin. Pectoral fin hyaline. Pelvic fin pale yellow, with a bright yellow tip. Females colouration in life (Fig. 4G): similar to the pattern described for the males, but with the pelvic, dorsal, anal, and caudal fins hyaline and without black dots.

FIGURE 4| Fluviphylax simplex coloration in life: A. UFRJ 9389, male, Alter do Chão, Tapajós River; B. UFRJ 10077, male, Ariaú, Lower Negro River; C. UFRJ 10381, male, Beruri, Purus River; D. UFRJ 10373, male, Manacapuru, Solimões River; E. UFRJ 9213, male, Tibarrá, Middle Negro River; F. UFRJ 9085, male, Parintins, Uaicurapá River; and G. UFRJ 10381, female, Beruri, Purus River. Photos taken by PHNB.

Discussion​

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The analysis of Fluviphylax simplex haplotypes supports that it is the most widespread species in the genus, showing a high haplotype but a low nucleotide diversity, based on COI gene. Besides the low number of specimens sequenced, genetic structuring suggests that communication between populations is not that often and probably constrained by the different water types in the Amazon River basin. In only one locality, Uaicurapá near Parintins, there was more than one non-related haplotype (Figs. 2–3).

Before this study, in the Negro River, Fluviphylax simplex was only known from its lower portion near Manaus (Souza et al., 2011; Bragança, Costa, 2018). The lack of F. simplex specimens in the middle and upper Negro is enigmatic considering the extensive expeditions carried in that area since the 1970s’, thus its absence cannot be attributed to the lack of knowledge on the regions’ fish community. However, populations were found in tributaries draining to the middle Negro River, in the Tibarrá and Daraã rivers, near Santa Isabel do Rio Negro, indicating that the species has a non-continuous distribution in that system (Fig. 4). This record, which is about 750 km from the nearest sites in Manaus, is better explained by a recent dispersal event or an ongoing interchange between populations in the headwaters of black-water forested tributaries of the Japurá River, itself a tributary of the Solimões River and of the Urubaxi and Uneiuxi rivers, both tributaries of the middle Negro River. Beltrão et al. (2019) indicated that there might be other fish interchange routes between the Solimões and the Negro rivers through the headwaters of the Quiuini River, which in periods of high-water level show an increase in the presence of turbid waters. Other species such as the cichlid Laetacara thayeri (Steindachner, 1875), and the trichomycterid Potamoglanis hasemani (Eigenmann, 1914) (Ottoni, 2018; E. Henschel, pers. comm.) that are present in the Solimões and in the Negro rivers might have their connection facilitated by the same interchange routes.

The only study so far indicating historical river captures and avulsions between the Pleistocene and Holocene in the central Amazon is the one by Ruokolainen et al. (2018). They indicated that due to the many cases of river avulsions, there were major changes in river connections within the region, one of them is the recent capture of the Japurá River, that used to be a tributary of the Negro River, by the Amazon/Solimões at about 1000 years ago. According to Dagosta, de Pinna (2019) such river avulsion mechanism in the central Amazon probably contributed to the establishment of a Central Black-water Amazon distribution pattern.

Despite information on Fluviphylax simplex distribution and genetic diversity suggests that communication between populations do occur occasionally, little is known about how this Black-water Amazon distribution pattern species keeps gene flow and overcomes physico-chemical barriers. Here we hypothesize that Fluviphylax simplex individuals can expand their distribution through contact zones between black-water tributaries headwaters during flood periods, through a pattern we define here as lateral movement. The presence of many black-water streams and tributaries in the lower portion of central Amazon rivers, even in systems considered typically white-waters (e.g., Madeira River), probably allows this lateral communication between populations. During the flood period, the headwaters of black-water streams, temporarily connect, allowing typical black-water fish fauna exchange. The capacity to cross high sediment load waters, however, is probably linked to a combination of other variables.

Goulding et al. (1988) study the ecology of the Negro River fishes, and recorded the presence of 56 species inhabiting floating meadows in the Anavilhanas Archipelago, lower Negro River, with the predominance of Fluviphylax species and small characids. Floating meadows are constituted by floating macrophytes with complex reticulated root structure extending to the water column, providing shelter for small fish species. During periods of high-water level, known as flood pulses, the floating vegetation is more abundant, and is characterized by the presence of mainly autochthonous food resources such as aquatic invertebrates, representing a preferred nursery habitat for many species (Correa et al., 2007). Despite the apparent absence of Fluviphylax species in the main channel of the high sediment load Amazon or Solimões, floating meadows, which are much more abundant in nutrient rich waters, might contribute to the cross of white-waters by Fluviphylax. Here we recognize the speculative nature of this hypothesis given that no Fluviphylax species have been sampled in white-waters floating meadows or in associated marginal vegetation. However, we hypothesize that the absence of Fluviphylax records in white-water environments could be related to the rarity and opportunistic crossing of white-water by black-water dwelling species. Additionally, the use of inappropriate gear (e.g., wide-mesh size nets), the difficulty in identifying miniature species, which are sometimes mistaken as juveniles of larger species, and the genus sampling effort bias towards black-water environments, probably contribute to this knowledge gap. Further studies directed to understand the seasonal variation within the fish community composition of white-water floating meadows and vegetation areas are needed to better understand the Central Black-water Amazon species distribution pattern. Recent studies indicate the importance of Amazon floating vegetation in facilitating the dispersion of fish and amphibian species through rafts of this vegetation (Schiesari et al., 2003; Fonte et al., 2021). The aquatic roots of plants provide shelter for small species (Schiesari et al., 2003; Correa et al., 2007) and we hypothesize that it also helps in decreasing the sediment concentration within the vegetation, likely improving the impacts of sediments on small fish (Kjelland et al., 2015; Borghezan et al., 2021).

Alternatively, it is also possible that Fluviphylax simplex specimens can cross white-water bodies during exceptional drought periods when there is an increase in the proportion of black-water input into the Amazon and Solimões relative to the white-waters (Park, Latrubesse, 2015; Espinoza-Villar et al., 2018). During the extreme low water levels around November, at the confluence between the Negro and the Solimões rivers, the Negro River water plume can predominantly affect the water type downstream, almost reaching the adjacent margin (Park, Latrubesse, 2015). Both the drift through vegetation rafts and the crossing during extreme droughts might contribute to the dispersion of small black-water fish species, but continuous sampling during the flood pulse extremes is needed to test both hypotheses.

There are few studies on the ecology of miniature freshwater fish species (Perkin et al., 2022), but there is a common sense that they are more abundant in clear and high acidic black-water streams (Weitzman, Vari, 1988). Burns, Bragança (in press) provide an updated list of Neotropical miniature species indicating their respective environmental and habitat information. According to this study, about 85% of the miniature species in the neotropics are found in clear and black-waters, whereas the few exclusive white-water miniatures represent about 9% of the miniatures’ diversity. However, the higher number of miniature species in clear and black-waters has never been investigated in detail as well as how they can overcome major physico-chemical barriers represented by white waters. Different from what is known from the other Fluviphylax species, here we show that Fluviphylax simplex can cross white-waters and is probably more tolerant to physico-chemical stress. Recently, Mattox et al.(2024) described the ninth species of Priocharax Weitzman & Vari, 1987, a characiform genus including only miniature species. The species, Priocharax phasma Mattox, Lima, Britz, Souza & Oliveira, 2024, unlike all its congeners except P. britzi Mattox, Souza, Toledo-Piza & Oliveira, 2021, which was described from the Purus River, inhabits white-water floodplain lakes in the Amazon River basin near the mouth of the Tapajós River. Until the discovery of these two species, Priocharax species were thought to be restricted to black-waters. These records of pelagic miniature species in white-waters are rare and raise many questions about the capacity of miniature species to survive in such an environment. However, despite the genus Priocharax showing a typical Central Black-water Amazon distribution pattern, most species have constrained distributions, whereas Fluviphylax simplex distribution mostly matches Dagosta, de Pinna (2019) Central Black-water Amazon distribution pattern.

This study was one of the first attempts to understand and provide a preliminary insight on the genetic structure within a miniature species with a typical Central Black-water Amazon distribution pattern. Subsequent research should be directed to investigate the presence of Fluviphylax simplex in the inner most floodplain lakes of the Amazon and Solimões, and the use of vegetation rafts by this species during the high flood. Alternatively, the facilitated dispersion and crossing of white-water rivers during periods of extreme droughts should be evaluated as an alternative communication route between Fluviphylax simplex populations.

Acknowledgments​

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We are grateful to all the following curators and researchers from institutions that shared specimens and tissues for this study: Wilson Costa (UFRJ); George Mattox (UFSCar); and Claudio Oliveira (UNESP). Marcelo Ândrade (UFMA), Axel Katz (UFRJ), and Pedro Fasura (UFRJ) gave valuable suggestions during the development of this research.

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Authors

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Dhállyth Zaínny Pereira Silva¹, Felipe Polivanov Ottoni^2,3 , Cleonilde da Conceição Silva Queiroz⁴ and Pedro Henrique Negreiros de Bragança^1,3,5

^[1]    Programa de Pós-Graduação em Biodiversidade e Conservação, Universidade Federal do Maranhão, Av. dos Portugueses, 1966, 65085-580 São Luís, MA, Brazil. (DZPS) dhallythsilva.20180003338@uemasul.edu.br.

^[2]    Universidade Federal do Maranhão, Centro de Ciências de Chapadinha, Laboratório de Sistemática e Ecologia de Organismos Aquáticos, BR-222, km 4, s/n, Bairro Boa Vista, Chapadinha, MA, Brazil. (FPO) fpottoni@gmail.com (corresponding author).

^[3]    NRF-South African Institute for Aquatic Biodiversity (NRF-SAIAB), P. Bag 1015, Makhanda 6140, South Africa.

^[4]    Centro de Ciências Exatas, Naturais e Tecnológicas, Universidade Estadual da Região Tocantina do Maranhão, Rua Godofredo Viana, 1300, 65901-480 Imperatriz, MA, Brazil. (CCSQ) cleo@uemasul.edu.br.

^[5]    Department of Ichthyology, American Museum of Natural History, 10024 New York, USA. (PHNB) pedrobra88@gmail.com.

Authors’ Contribution

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Dhállyth Zaínny Pereira Silva: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing-original draft, Writing-review and editing.

Felipe Polivanov Ottoni: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing-original draft, Writing-review and editing.

Cleonilde da Conceição Silva Queiroz: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing-original draft, Writing-review and editing.

Pedro Henrique Negreiros de Bragança: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing-original draft, Writing-review and editing.

Ethical Statement​

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Not applicable.

Competing Interests

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The author declares no competing interests.

Data availability statement

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The authors confirm that the data supporting the findings of this study are available within the article.

Funding

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This study was partially funded by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq): CNPq project number 441189/2023–7, and CNPq grant number 307974/2021–9 to FPO.

How to cite this article

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Silva DZP, Ottoni FP, Queiroz CCS, Bragança PHN. A big task for a miniature species: understanding Fluviphylax simplex (Cyprinodontiformes: Fluviphylacidae) distribution. Neotrop Ichthyol. 2025; 23(2):e250067. https://doi.org/10.1590/1982-0224-2025-0067

Copyright​

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This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Distributed under

Creative Commons CC-BY 4.0

© 2025 The Authors.

Diversity and Distributions Published by SBI

Accepted May 27, 2025

Submitted April 9, 2025

Epub September 08, 2025