Freshwater environments harbor an estimated 12,000 species of strictly freshwater fish (Nelson 2006). The great diversity of species is reflected in a variety of reproductive behaviors, morphological plasticity, trophic plasticity and sensitivity to various environments (Gatz 1979a, Wootton 1991, Vazzoler 1996, Luiz et al. 1998, Collen et al. 2014). Knowing the biology of fish species, their ecological relationships, and their different responses to biotic and abiotic variations is necessary to understand and to mitigate the impacts of anthropic actions. Anthropic changes often degrade the environment, causing homogenization of the aquatic physical environments and loss of habitat (Lowe-McConnell 1975, Miller 1984, Jackson et al. 2001, Rosenfeld 2003, Chapman et al. 2014).

Ecological morphology, or ecomorphology, is the branch of ecology that studies the relationships between morphology and ecological aspects among individuals, populations, guilds, and communities (Karr and James 1975, Gatz 1979b, Winemiller 1992, Chapman et al. 2015). For a long time, ecologists have been interested in the relationship between the morphology of organisms, their ecological performance and the evolutionary consequences of this relationship for the selection and maintenances of adaptative traits in populations (Gatz 1979a, Wikramanayake 1990, Winemiller 1991, Leal et al. 2011, Saraiva and Pompeu 2016). Ecomorphological studies of fish aim to understand the importance of certain attributes in the ecology of the species and how they influence their adaptation to different habitats (Douglas and Matthews 1992, Norton et al. 1995, Casatti and Castro 2006, Lailvaux and Husak 2014).

The first studies on the swimming capacity of fish focused mainly on physiological aspects and how it is influenced by water characteristics such as dissolved oxygen, temperature and pH (Brett 1964). In Brazil, investigations have been generally descriptive and comparisons have been made with species from other countries (Santos et al. 2008), focusing on establishing constructive parameters for fish ladders (Santos et al. 2012). Even today, studies that experimentally test the correlation between swimming capacity and morphological aspects and/or species ecology are rare for Neotropical species (Sampaio et al. 2012). The few studies that have advanced in this direction have attempted to understand how morphological attributes possibly associated with swimming influence the use of the hydraulic habitat by a species (Casatti et al. 2001, Chapman et al. 2015).

In the present study, we compared the swimming capacity and morphological aspects of two congeneric sympatric species for which spatial segregation in different hydraulic habitats is known (Leal et al. 2011). Three hypotheses were tested: (i) the species occurring in environments with faster water velocity has greater swimming capacity; (ii) the two species differ with respect to some morphological parameters; (iii) the morphological parameters that differentiate the species are correlated with their swimming performance and explain intraspecific differences in their ability to remain positioned on the substrate despite the flow.

A significant difference was observed in the relative velocity measured from lengths per second between individuals of both species. Higher velocities (swimming capacity) were registered for Characidum fasciatum (14.51 lengths × s^-1; Min: 5.24 lengths × s^-1; Max: 24.86 lengths × s^-1; Std. Dev: 4.97) than for C. cf. zebra (12.78 lengths × s^-1; Min: 6.11 lengths × s^-1; Max: 15.69 lengths × s^-1; Std.Dev: 2.44) (Fig. 2). Body size (standard length) was positively correlated with the ability of both species to resist flow (p < 0, 01; R² = 0.1543; F (1.42) = 7.66) (Fig. 3).

The variables that presented higher values of loadings were RLH in the first main component axis, RACdF in the second axis and CICP, CI and RAPtF respectively third, fourth and fifth axis (Table 1). The Discriminant Analysis confirmed that the species differ significantly in the ecomorphological space, VFI (Ventral Flattening Index), RAPtF (Relative area of the pectoral flank), ARCdF (Aspect ratio of the caudal fin), RACdF (Relative area of the caudal fin) being the attributes that have the greatest influence on the distinction between the ecomorphological space of the two species (Table 2). Overall, specimens of C. fasciatum have dorso-ventrally flattened bodies and pectoral fins longer and narrower than C. cf. zebra specimens, which have in turn have longer caudal fin than C. fasciatum specimens (Fig. 4).

The multiple regression between the residuals of the regression between the critical velocity (m × s^-1) and the standard length (cm) and the attributes responsible for the differences in ecomorphological space between each species showed that the relative area of the pectoral fin (RAPtF) explained 18.22% of the velocity variation not explained by the individuals length [(N = 44), p < 0,05, R² = 0,2110, R² adjusted= 0,1301, F (4,39) = 2,60] (Fig. 5).

Our data show that C. fasciatum and C. cf. zebra have different swimming capacity and differ in the ecomorphological space. Collectively, our results support the three hypotheses tested. First, the species that thrive in the environment with greater hydrodynamism (C. fasciatum) has greater capacity to withstand the flow of water. Second, four ecomorphological attributes account for the differences between both species in the ecomorphological space. Third, swimming capacity is correlated with the morphological characteristics (pectoral fin morphology) and the swimming activity of the fish or with the capacity the fish has to resist the water flow by adhering to the substrate.

Characidium fasciatum has the greatest capacity to withstand the water flow. This allows them to inhabit areas where the water velocity is higher, as documented in a previous study of the habitat use and morphology of these species (Leal et al. 2011). Differences in swimming capacity between individuals are easy to observe when comparing species with different swimming styles, position in the water column or different preferences for substrates (Castro et al. 2010, Santos et al. 2012). Therefore, the information on the differences between individuals of the same species or congeneric species can give insights on species diversification.

Several studies, both in the northern and southern hemispheres, have showed a strong positive correlation between swimming capacity and body size, often higher than 40% (Brett 1964, Jones et al. 1974, Santos et al. 2007, Castro et al. 2010, Srean et al. 2017). Most of these studies were carried out on species that travel long distances to reproduce. Body size, in these cases, is important not only for fish locomotion but also for their reproductive success. Body size is particularly important for the maintenance of energy reserves. However, a strong correlation between body size and swimming capacity can be found even in some small fish species (Sampaio 2009, Sampaio et al.2012, Castro et al. 2010, Srean et al. 2017). This includes species that also stand still on the substrate, and can be found in Trichomycteridae and Loricariidae (Burguess 1989). Nevertheless, the correlation between the capacity to support the flow of water and body size was not strong in the species analyzed in this study, which can be explained by their style of swimming. Species of Characidium spend much of their time positioned on the substrate using their pectoral fins, and actively swim mostly when they are looking for food (Casatti et al. 2001, Casatti and Castro 2006). This example shows that the relationship between swimming capacity and body size is not standard for all species.

In this study, we identified significant differences between some ecomorphological attributes of C. fasciatum and C. cf. zebra, possibly correlated with their differential use of the habitat. The attributes are responsible for the morphological distinction between the two species and are mainly in the swimming movement and/or the capacity to support water flow, such as ventral flattening index and relative area of the pectoral fin (Gatz 1979a, Watson and Balon 1984). The discriminant analysis also indicated that the relative area of the caudal fin and aspect ratio of the caudal fin were responsible for the morphological distinction between the two species. These two ecomorphological attributes are also linked to the capacity to perform active and fast movements. Therefore, the different flow velocity in the natural environment faced by the two studied species are strongly correlated with their morphology differences.

We provide information on the relationship between swimming capacity and morphological aspects, possibly correlated with the differential habitat use by two Characidium species. The studied species are found in drainages at the Brazilian savanna biome (also known as Cerrado), a global biodiversity hotspot (Myers et al. 2000) where agriculture is fast growing. Such land use change often jeopardizes streams by causing sedimentation, a contribution of fine sediment above the carrying capacity of the watercourse (Chapman et al. 2014). The deposition of sediment acts directly on the instream habitats by reducing both depth and substrate variability, homogenizing the streambed. Therefore, understanding the relationship between swimming capacity, morphology and habitat use may be important for identifying species that are prone to extinction by silting processes.