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Neotropical Ichthyology

Print version ISSN 1679-6225On-line version ISSN 1982-0224

Neotrop. ichthyol. vol.17 no.1 Maringá  2019  Epub Mar 11, 2019 

Original article

Geographic variation of Moenkhausia bonita (Characiformes: Characidae) in the rio de la Plata basin, with distributional comments on M. intermedia

James Anyelo Vanegas-Ríos1

Ricardo Britzke2  3

Juan Marcos Mirande4

1División Zoología de Vertebrados, Facultad de Ciencias Naturales y Museo, CONICET, UNLP, Paseo del Bosque S/N° B1900FWA, La Plata, Buenos Aires, Argentina., (corresponding author).

2 Departamento de Morfologia, Instituto de Biociências, Universidade Estadual Paulista, 18618-970 Botucatu, São Paulo, Brazil.,

3 Universidad Técnica de Machala, Av. Panamericana km 5½, Via Pasaje, Machala, El Oro, Ecuador.

4Fundación Miguel Lillo, UEL-CONICET, San Miguel de Tucumán, Argentina.,


Moenkhausia bonita occurs in numerous additional localities from the Bermejo, Paraná, Paraguay, and Uruguay river basins. Given that this finding greatly expands the distributional range of M. bonita, we carried out an intraspecific comparison, using multivariate methods for 18 morphometric and eight meristic characters taken from a comprehensive sample of 536 specimens. All localities were distributed in four major geographic groups as follows: Bermejo, Paraná, Paraguay, and Uruguay. Results of the morphometric comparisons showed significant differences among the studied groups except between the Paraguay and Uruguay groups. Statistical differences in meristic values were found for most between-group comparisons, especially in those resulting from discriminant canonical analyses (DCA). Specimens from the Bermejo basin were the most distinct group in most morphological comparisons. However, the overall subtle differences found in body morphology likely reflect intraspecific variation within M. bonita and seem to be mainly influenced by spatial and environmental features of drainages. As M. bonita was previously identified as M. intermedia in the río de La Plata basin, distributional comments on the latter species in that basin are provided.

Keywords: Allometry; Argentina; Moenkhausia; Morphological variation; Widespread species


Moenkhausia bonita es registrada en numerosas localidades adicionales de las cuencas de los ríos Bermejo, Paraná, Paraguay, y Uruguay. Dado que estos hallazgos expanden ampliamente el rango distribucional de M. bonita, nosotros llevamos a cabo una comparación intraespecífica, usando métodos multivariados para 18 características morfométricas y 8 merísticas que fueron tomados en una muestra exhaustiva de 536 especímenes. Todas las localidades fueron repartidas en cuatro grupos principales como sigue: Bermejo, Paraná, Paraguay y Uruguay. Los resultados de las comparaciones morfométricas mostraron diferencias significativas a través de los grupos bajo estudio, excepto entre los grupos Paraguay y Uruguay. Diferencias estadísticas fueron encontrados en la mayoría de las comparaciones entre los grupos, especialmente en aquellas obtenidas de los análisis discriminantes canónicos (ADC). Los especímenes de la cuenca del Bermejo fueron encontrados como el grupo más divergente en la mayoría de las comparaciones morfológicas. No obstante, estas leves diferencias encontradas en la morfología del cuerpo son consideradas dentro de la variación intraespecífica de M. bonita y parecen estar influidas por características ambientales y espaciales de los drenajes. Dado que M. bonita fue previamente identificada como M. intermedia en la cuenca del río de La Plata, comentarios distribucionales sobre esta última especie en esta cuenca son presentados.

Palabras clave: Alometría; Argentina; Moenkhausia; Variación morfológica; Especie ampliamente distribuida


With 5,160 valid species, Neotropical freshwater fishes represent approximately one-third of global freshwater fish diversity (Reis et al., 2016). In general, the distribution of continental fish species at regional scales can be separated into two broad patterns: 1) the majority of species have confined distributions circumscribed by regional or geographic boundaries with sporadic occurrences beyond; and 2) a few species have large spatial distributions, extending to multiple basins, even across different biogeographic regions (Albert et al., 2011). Numerous factors, in isolation or in combination, have been put forward to potentially explain how species managed to reach such geographic ranges (reviewed in Gaston, 2003). These factors include changes in niche breadth, demographic dynamics, body size, environmental variability, colonization and extinction dynamics, and dispersal ability, among others (Gaston, 2003; Lomolino et al., 2010; Albert, Reis, 2011; García-Vázquez, Ribera, 2016). In many cases, widespread species require comprehensive taxonomic analyses in order to understand their distributional patterns (Albert et al., 2011; Reis et al., 2016).

The genus Moenkhausia Eigenmann is composed of 90 valid species that are widely distributed in most important cis-Andean river basins in South America (e.g. Amazon, Orinoco, La Plata, and San Francisco). This genus reaches its greatest diversity in the rio Amazon basin (~ 73 species), followed by the coastal basins of Guyana, Suriname, and French Guiana (~ 14) (Lima et al., 2003; Britzke et al., 2018; Fricke et al., 2019). The morphological definition of Moenkhausia is based on a combination of non-exclusive characteristics (e.g. presence of five teeth in the inner tooth row of the premaxilla, presence of small scales covering the bases of the anal and caudal fins, and a complete lateral line, see Eigenmann, 1917 for further details) that are highly homoplastic and shared in part with other characid genera such as Astyanax Baird & Girard or Hemigrammus Gill. Additionally, the genus has been recognized as non-monophyletic by morphological and molecular analyses (Mirande, 2010; Mariguela et al., 2013).

Examples of broadly distributed members of Moenkhausia include M. dichroura (Kner), M. intermedia Eigenmann, M. lepidura (Kner), M. megalops (Eigenmann), and M. oligolepis (Günther) (Benine et al., 2009; Britzke, 2011; Marinho, Langeani, 2016; Soares et al., 2017). In the rio de la Plata basin, seven species of Moenkhausia have been recorded: M. australe Eigenmann; M. bonita Benine, Castro & Sabino; M. dichroura; M. forestii Benine, Mariguela & Oliveira; M. intermedia; M. lopesi Britski & de Silimon; and M. sanctaefilomenae (Steindachner) (Britski, Silimon, 2001; Benine et al., 2004; Benine et al., 2009; Fricke et al., 2019).

Moenkhausia bonita is a relatively small-sized species (up to 44 mm of SL) that, although described as endemic to its type locality (the rio Baía Bonita, a tributary of the rio Miranda), has been subsequently recorded in other localities from the Paraguay and Amazon river basins (Benine et al., 2004; Teresa, Romero, 2010; Teresa et al., 2010; Teresa et al., 2011; Castro, Vizzotto, 2013; Lima et al., 2013; Queiroz et al., 2013; Cordeiro et al., 2014). Recently, M. bonita has been phylogenetically placed into the “Moenkhausia clade” as defined by Mirande (2018), which also includes the type species of the genus.

Preliminary results obtained by examination of a large sample of specimens, previously identified as Moenkhausia intermedia or M. cf. intermedia, from the Bermejo, Paraná, and Uruguay basins allowed us to conclude that all these specimens correspond to M. bonita, based on the number of gill rakers (6-8 + 11-15 in M. bonita vs. 9-12 + 18-22 M. intermedia) (see Benine et al., 2004; Britzke, 2011). This finding greatly expands the occurrence range of M. bonita to a great portion of the rio de la Plata basin. In order to explore the possible existence of cryptic species within this widespread range (especially the Bermejo specimens that are most distantly located) and to examine the morphological heterogeneity of spatially distinct populations, we conducted a geographic and population comparison of M. bonita in the río de La Plata basin, based on a comprehensive morphological dataset. Additionally, we provide distributional comments on M. intermedia in that basin.

Material and Methods

Data collection. Five hundred and thirty-six specimens of M. bonita were examined in order to include a representative sampling of its distributional range in the rio de la Plata basin. From the total number of examined specimens, 220 were fully measured and 317 were only partly measured (because they were available only temporarily). Although those specimens with partial data were not included in the statistical comparisons and reporting tables, they were unequivocally identified as M. bonita to more completely examine the morphology of this species across its geographic range. Data for the holotype follows Benine et al. (2004). Additionally, specimens of M. dichroura (177), M. intermedia (29), and M. sanctaefilomenae (6) were examined for comparative purposes. Institutional abbreviations used in the text follow Sabaj (2016).

Morphological data. Meristic and morphometric characteristics were used to analyze the intraspecific variation of M. bonita and to corroborate the identification of all examined specimens. All measurements and counts were taken according to Fink, Weitzman (1974). The following 18 morphometric variables were taken: standard length (SL), depth at dorsal-fin origin, snout to dorsal-fin origin, snout to pectoral-fin origin, snout to pelvic-fin origin, snout to anal-fin origin, dorsal-fin origin to caudal-fin base, dorsal-fin length, pectoral-fin length, pelvic-fin length, anal-fin lobe length, caudal peduncle depth, caudal peduncle length, head length (HL), snout length, horizontal eye length, least interorbital width, and upper jaw length. Measurements were taken point to point with digital calipers under a stereomicroscope and are expressed as percentages of SL or HL for units of the head. Those meristic variables that varied most intraspecifically, were analyzed statistically: lateral line scales, scales below the lateral line, circumpeduncular scales, branched anal-fin rays, maxillary teeth, teeth in the outer premaxillary row, and gill rakers of the first gill arch (separate counts for the lower and upper limbs). Other counts such as the scales above the lateral line, predorsal scales, dorsal-fin rays, pectoral-fin rays, pelvic-fin rays, and teeth in the inner premaxillary row, which were not statistically analyzed due to their almost uniform distribution across the groups or non-significant sample size (only for vertebral counts), are provided to characterize the species for identification purposes. Total number of vertebrae were counted in cleared and counterstained (c&s) specimens, which were prepared following Taylor, Dyke (1985). Those counts include the first preural centrum plus first ural centrum (PU1+U1) counted as one element and all four vertebrae of the Weberian apparatus. Other osteological characteristics were compared but only substantial differences are reported if observed.

Sex identification was based on gonadal examination and/or the presence of secondary sexual characteristics if present. Not all samples were collected in the same seasons, and those specimens collected outside the breeding season lacked secondary sexual characteristics and were not assignable to one sex or the other. Sexually dimorphic characteristics found are reported. Sexual variation could not be statistically analyzed in detail, because significant samples of mature male specimens were not available for all groups.

Statistical analysis. All localities of occurrence of M. bonita were divided into four major geographic areas (named as groups: Bermejo, Paraguay, Paraná, and Uruguay), which represents the most important sub-basins of the rio de la Plata basin in which this species occurs. The selection of these groups was based on hydrogeographic and ichthyofaunistic differences that characterize each basin (Quirós et al. 2007; Albert, Reis, 2011). The geographic distribution of the analyzed samples within the rio de la Plata basin is plotted in Fig. 1. These localities ranges from 55 to 492 m above sea level.

Fig. 1 Map showing the distributional range of Moenkhausia bonita across the rio de la Plata basin (based on the examined specimens). Black arrow indicates type locality. 

Morphometric and meristic data were analyzed separately, because these type of variables differ statistically and biologically and because they may respond differently to environmental and genetic conditions (Lawing et al., 2008; Hair et al., 2010). The allometric coefficient or k (Huxley, 1932; Klingenberg, 1996) of all measurements was obtained for each analyzed group (reference size used: SL) using a least-square based regression line of base-10 log-transformed data (see Kilmer, Rodríguez, 2017 for details on the preferable use of the ordinary least square algorithm in comparison with other methods). The regression plots were used to evaluate whether allometries differed among groups, but only those plots showing pertinent differences are reported. In order to study the size-free shape differences among groups, the morphometric data were treated with Burnaby’s allometric correction (Burnaby, 1966; Humphries et al., 1981; Rohlf, Bookstein, 1987). In that method, the morphometric variables are log-transformed and then are projected onto a space orthogonal to the first principal component, which typically removes size-dependent shape variation from the dataset. The size-corrected morphometric data for each studied group were analyzed by means of a principal component analysis (PCA) and a discriminant canonical analysis (DCA), using the covariance matrix in both cases. For the PCA analyses, the number of significant components was determined using the broken-stick model (Frontier, 1976) and the scree plot method (Cattel, 1966), including the larger number of biologically meaningful axes if they disagreed. In the DCA analysis, the Mahalanobis distances were calculated from the pooled within-group covariance matrix to obtain a linear discriminant classifier. The confusion matrix obtained from these calculations indicates the number of specimens in each group that were assigned to the different groups by the classifier. The group assignment was cross-validated by a leave-one-out cross-validation procedure using jackknifing.

Meristic data were square-root transformed (Quinn, Keough, 2002) and then analyzed with PCA (using the correlation matrix) and DCA. Resultant axes of all the multivariate analyses that showed a great overlap among individuals are not depicted here. Additionally, Tukey box plots were used to graphically represent those counts showing major differences among the studied groups. To test the significance of the resulting shape and meristic scores of the most discriminative axes found in PCA and DCA, a Kruskal-Wallis test plus Mann-Whitney pairwise comparisons were performed. All such analyses used the Bonferroni-corrected p values. Finally, a Mantel test was used to assess possible correlation between the morphometric and meristic matrices using Mahalanobis distances.

For statistics methods used here see Marcus (1990), Reyment (1990), Quinn, Keough (2002), and Hair et al. (2010). Prior to all statistical procedures performed, the data were examined for departures from statistical assumptions (normality and deviation from equality of variances), and were adjusted when necessary (logarithmic and root square transformations). Multivariate normality was assessed using the omnibus test of Doornik, Hansen (2008). Statistical significance was assessed at p < 0.05. All statistical analyses were carried out in PAST 3.14 (Hammer et al., 2001) and, complementarily, Sigma Plot 12 (2011, Systat Software, Inc. Windows).


Morphometric data. The measurements are summarized in Tab. 1. Comparing the allometric coefficients (k), different combinations of negative and positive allometry, and isometry were found among the studied groups (Tab. 1). The depth at dorsal-fin origin (positive allometry: k = 1.1), snout to pectoral-fin origin (negative allometry: k ranging from 0.6 to 0.8), anal-fin lobe length (negative allometry: k ranging from 0.4 to 0.8), and head length (negative allometry: k ranging from 0.6 to 0.9) showed similar patterns of biometric growth relative to body size (SL) across the groups. The majority of the measurements expressed as percentages of standard length varied among the groups in their allometric coefficients, with the Bermejo and Uruguay groups often negatively allometric and the Paraguay group often positively allometric. In measurements expressed as percentages of head length, almost all the groups showed negative allometry, except the Paraguay group with isometry in the least interorbital width and positive allometry in the snout length. Regression plots for most measurements with differing allometric coefficients did not completely separate the groups. However, regressions of snout length, upper jaw length, and pelvic-fin length, differed slightly among the groups, thereby allowing the partial discrimination of many individuals of the Paraguay group (S1, available only in the online version). These same variables were also informative in the multivariate comparisons (mentioned below).

Tab. 1 Morphometric data of Moenkhausia bonita arranged into geographic groups. Abbreviations are explained in the text, except SD: standard deviation. Those examined specimens that were not completely measured are not included here but are reported in material examined. 

Variables Bermejo Paraguay Paraná Uruguay
n Range Mean±SD k n Range Mean±SD k n Range Mean±SD k n Range Mean±SD k
SL (mm) 21 26.4-34.5 30.3±2.3 - 94 14.9-49.4 28.1±6.5 - 178 19.7-41.1 30.4±3.3 - 61 25.4-37.0 31.5±2.6 -
Percentages of SL
Depth at dorsal-fin origin 21 32.3-37.6 35.2±1.5 1.1 93 25.5-35.7 31.2±2.5 1.1 47 28.2-36.2 32.1±1.9 1.1 61 30.8-38.0 33.2±1.6 1.1
Snout to dorsal-fin origin 21 49.9-54.9 54.2±1.5 0.9 93 49.1-60.4 53.2±1.9 1.0 47 51.1-56.6 53.3±1.3 0.9 61 50.9-57.9 53.5±1.4 1.0
Snout to pectoral-fin origin 21 25.9-30.5 28.0±1.2 0.6 93 23.6-33.4 28.6±1.7 0.8 47 26.2-32.1 29.1±.1.3 0.8 61 23.8-30.7 28.0±1.3 0.7
Snout to pelvic-fin origin 21 46.2-52.0 48.6±1.5 0.9 93 45.1-55.0 48.6±2.0 1.0 47 45.7-52.3 49.4±1.6 0.9 61 47.1-53.5 49.6±1.6 0.9
Snout to anal-fin origin 21 61.0-66.5 63.5±1.5 1.0 93 58.4-72.6 65.1±2.5 1.1 47 61.5-69.5 65.4±1.7 1.0 59 63.1-71.0 66.5±1.9 1.0
Dorsal-fin to caudal-fin base 21 49.7-54.5 52.2±1.2 1.1 93 47.3-54.4 51.9±1.5 1.0 47 46.7-53.3 49.7±1.4 1.1 61 47.4-54.2 50.9±1.8 1.0
Dorsal-fin length 21 27.2-33.4 29.5±1.9 1.1 93 22.1-32.7 27.3±2.1 1.1 47 22.2-30.2 26.7±1.7 1.1 61 24.5-30.7 28.3±1.2 0.9
Pectoral-fin length 21 20.5-26.5 19.2±1.6 0.5 92 17.5-26.0 21.4±1.9 1.1 47 18.6-24.8 21.4±1.3 1.0 61 18.5-25.0 22.1±1.5 0.8
Pelvic-fin length 21 16.6-21.9 23.0±1.5 0.4 93 12.1-19.8 16.0±2.0 1.3 47 13.7-20.9 16.9±1.6 1.1 61 15.3-20.1 17.4±1.2 0.8
Anal-fin lobe length 21 21.0-26.2 22.8±1.1 0.7 92 14.2-23.2 18.7±2.2 0.8 46 15.4-24.7 20.7±1.7 0.8 61 16.8-24.5 21.2±1.8 0.5
Caudal peduncle depth 21 10.3-13.9 11.2±0.9 0.9 93 6.8-12.3 9.5±0.9 1.1 47 8.4-11.9 10.0±0.6 1.2 61 8.6-12.3 10.5±0.8 0.8
Caudal peduncle length 21 11.0-15.4 14.0±1.0 0.8 91 6.0-16.3 8.5±2.2 1.2 47 8.7-12.8 10.8±0.9 0.9 61 7.5-15.0 11.1±1.9 0.6
HL 21 25.0-29.7 27.3±1.2 0.6 93 22.8-28.8 25.4±1.2 0.9 47 23.6-28.6 26.3±.0.9 0.8 61 23.2-28.5 25.5±1.1 0.7
Percentages of HL
Snout length 21 21.8-26.7 24.2±1.1 0.7 92 18.2-28.9 22.2±2.5 1.2 47 23.1-29.8 26.9±1.5 0.6 61 21.3-30.1 25.4±2.4 0.6
Horizontal eye length 21 41.2-47.6 44.1±1.7 0.5 93 38.3-49.2 42.8±2.1 0.8 47 40.2-49.0 44.2±2.1 0.6 60 37.9-49.1 42.7±2.1 0.5
Least interorbital width 21 29.6-37.4 32.8±1.7 0.8 93 26.4-37.8 32.9±2.6 1.0 47 30.0-37.5 33.4±1.9 0.8 61 28.3-39.8 32.9±1.9 0.9
Upper jaw length 21 41.6-47.2 44.3±1.6 0.7 93 33.2-48.1 38.3±3.3 0.9 47 40.5-49.4 44.8±1.8 0.7 61 37.6-47.8 43.5±3.1 0.6

According to the scree plot method, between four and six relevant components should be selected, because the curve flattens after the fourth component but still shows a break point between the fifth and sixth eigenvalues (Tab. 2). Conversely, the broken-stick model suggested using only the first component (see S2, available only in the online version). In order to ensure that did not discard biologically relevant data, we chose a consensus between the two methods and extracted four eigenvector elements (of PCs) that accounted for 74.4 % of the total variance (Tab. 2). The plot of the first principal component (PC1) vs. the second principal component (PC2) (Fig. 2a, representing 59.3 % of the total variance) suggests that the individuals of the Bermejo group mainly separated from the individuals of the Paraná group along PC2, but not from those of the remaining groups. PC1 was loaded most heavily by the following measurements: negatively by caudal peduncle length (-0.75) and upper jaw length (-0.24); and positively by the dorsal-fin origin to caudal-fin base (0.23), snout to anal-fin origin (0.21), and snout to dorsal-fin origin (0.21). The positive loadings that most influenced PC2 were the snout length (0.64) and upper jaw length (0.34), whereas the negative loadings that most affected that component were pelvic-fin length (-0.37), caudal peduncle length (-0.29), pectoral-fin length (-0.24), and dorsal-fin length (-0.23). A table of the morphometric loadings can be found in S3, available only in the online version. The plot of the PC3 vs. PC4 (representing a 15.2 % of the total variance) does not separate the studied groups and for this reason, is not presented.

Fig. 2 Most discriminant axes obtained from the size-free multivariate analyses using the adjusted morphometric data of Moenkhausia bonita. a. principal component analysis; b. discriminant canonical analysis. 

Tab. 2 Results of the principal components analyses (PCA) and discriminant canonical analyses (DCA) of the adjusted morphological data of Moenkhausia bonita

Axes Morphometric data Meristic data
Eigenvalue % Variance Eigenvalue % Variance
1 0.0093 46.3 2.3896 29.9
2 0.0026 12.9 1.5275 19.1
3 0.0018 8.8 1.2475 15.6
4 0.0013 6.4 0.8448 10.6
5 0.0011 5.6 0.6754 8.4
6 0.0008 4.1 0.5496 6.9
7 0.0007 3.3 0.4220 5.3
8 0.0006 2.8 0.3437 4.3
9 0.0005 2.4 - -
10 0.0004 1.9 - -
11 0.0004 1.7 - -
12 0.0002 1.2 - -
13 0.0002 0.8 - -
14 0.0001 0.6 - -
15 0.0001 0.5 - -
16 0.0001 0.3 - -
17 0.0000 0.2 - -
18 0.0000 0.0 - -
1 1.2100 44.3 6.4836 77.3
2 1.1190 41.0 1.5130 18.0
3 0.4027 14.7 0.3912 4.7

Size-free DCA accounted for 85.3 % of the total variance in the first two canonical axes (CA) (Tab. 2). Along the plot of the first canonical axis (CA1) vs. the second canonical axis (CA2) (Fig. 2b), the individuals of the Bermejo group were well discriminated from the remaining individuals along CA1, while CA2 partly discriminated the individuals of the Paraná group. The most important loadings affecting CA1 was the caudal peduncle length (-0.03), snout to anal-fin origin (0.01), snout to dorsal-fin origin (0.01), and snout to pectoral-fin origin (0.01), while CA2 was most influenced by the snout length (0.02), upper jaw length (0.02), caudal peduncle length (0.01), dorsal-fin length (0.01), and dorsal-fin origin to caudal-fin base (0.01). A list of the morphometric loadings can be found in S3, available only in the online version. In the confusion matrix, 70.0 % of all the examined individuals were correctly classified to their given group (75.0 % if the data were not jackknifed), with those individuals of the Bermejo and Paraná groups being most frequently classified correctly (100 and 87.2 %, respectively) (Tab. 3).

Tab. 3 Confusion matrix showing