Morphological aspects in the ontogeny of Salminus hilarii Valenciennes , 1850 ( Ostaryophysi : Characidae )

This study compared various morphological characteristics of S. hilarii, in order to verify possible variations over the life of the fish. It was used individuals collected from the Tietê river basin (sub-basins of Sorocaba, Jacaré Pepira and Corumbataí rivers) and from the Rio Grande basin (sub-basins of Pardo and Mogi Guaçu rivers). For the analysis we used 15 relative values of morphological attributes. Measures of S. hilarii were taken from samples with a range of size ranging between 49-430 mm standard length. The relative values were first analyzed by an array of Principal Component Analysis of correlation (PCA). Subsequently the most important attributes of PCA were correlated with standard length, using the Pearson correlation coefficient. The PCA revealed eight most important characteristics, and six showed a significant correlation with standard lengths (relative height, depth ratio, relative area of caudal fin, relative head length, eye diameter relative height and relative mouth height), indicating common variations along the fish’s growth besides indicating an adaptation to the conditions of the environment it occupies. On the other hand some characteristics have not changed, maintaining typical morphological patterns of fish that occupy the middle water and piscivorous predators.


Introduction
Included within the Characiformes order, Salminus hilarii Valenciennes, 1850 is similar to S. brasiliensis (Cuvier, 1816), differing by the white and silver color besides being smaller in size.It is a typical predator species, preferably found in running waters of medium and small rivers.It is considered as rheophilic fish with the reproductive period between late spring and summer (Agostinho et al., 2004;Andrade et al., 2004;Godoy, 1975;Honji, 2007;Honji et al., 2011;Takahashi, 2006;Villares Junior et al., 2007;Villares Júnior, 2014).Currently, the species is threatened in the region of the Upper Paraná River Basin by industrial and domestic pollution and dam construction (Honji, 2007;Graça and Pavanelli, 2007).
Morphologically, S. hilarii has as a typical configuration of predatory fish with elongated fusiform body, a terminal type mouth with a wide opening and the jaw slightly protruding before in relation to the pre-jaw.The teeth are sharp and facing the inside of the mouth (Godoy, 1975), relating to the evasive preys (Aleev, 1969;Teixeira and Bennemann, 2007).
The study of morphology may reflect integration between environmental influences with the organism behavior, life history and feeding habitats.In fish, studies of this nature are conducted with adult fish, generally focusing on the existence of correlations between morphological and ecological variables (Chao and Musick, 1977;Singer, 1985).Besides, examining how functional morphology change during the ontogeny is another interesting research question that is rarely considered (Mullaney Júnior and Gale, 1996).
There is a high diversity of forms of embryos, larvae and juvenile fish (Balon et al., 1986), but the ecological correlations have not been fully clarified.Research on intra-specific morphological variations in neotropical fish stocks are scarce (Shibatta and Garavello, 1993).The study of the functional anatomy related to fish nourishment can bring basic information about the occupation of ecological niches and their adaptations to this occupation.
As little is known about morphological changes in ontogeny of neotropical freshwater fish, this study aims to evaluate how morphological traits of S. hilarii changes during its relating them to swimming and feeding.

Morphological attributes
For this study it was analyzed 58 individuals of Salminus hilarii originated from the upper Paraná river basin rivers, being 42 collected in the Tietê rivers basins (sub-basins of the Sorocaba, Jacaré Pepira and Corumbataí rivers) and 16 in the Rio Grande basin (sub-basins of Mogi Guaçu and Pardo rivers).The measurements were taken from samples with a size ranging from 49 to 430 mm (standard length).

Data analysis
It was first performed a multivariate analysis using a Principal Component Analysis (PCA) using the matrix of correlation of the morphological attribute in order describe of the principal sources of morphological variations.
Subsequently the most important attributes (attributes with the higher loads) of the first two axes of the PCA were correlated with standard length using Pearson correlation coefficient considering α = 0.05.For the statistical test it was used the PAST 2:15 software (Hammer et al., 2001).

Results
The first component (PC1) explained 27.73% of the variance, with the relative height having the highest negative loading and depth ratio, relative area of caudal fin, relative mouth height and relative eye diameter the largest positive loading.The second component (PC2) explained 16.42% thus adding 44.15% of explained variance.In this axis the principal components were the relative caudal peduncle height with the highest negative loading and the relative head length and relative jaw length with the largest positive loading, (Figure 1, Table 2).
The Pearson correlation coefficient (Figure 2) showed significant correlations among standard length and six of the most attributes highlighted in the PCA (relative height, depth ratio, relative area of caudal fin, relative head length, eye diameter relative height and relative mouth height), indicating that several morphological attributes showed allometric variations along the fish growth.Although highlighted in the PCA, the relative caudal peduncle height and the relative length of the jaw showed no significant correlation with the length, indicating no signs of allometry.

Discussion
The fish fins allow a sudden interaction with water, revealing a strong adaptability to the environment (Breda et al., 2005;Videler, 1993), determined by the type of swimming required for optimal survival and ability to explore diverse niches (Breda et al., 2005;Videler, 1993;Jobling, 1995).Caudal fins with large area and low-aspect ratio values indicate that fish produce quick starts (impulse) (Aleev, 1969;Sambilay Júnior, 1990;   Webb and Weihs, 1986) showing better performance in heterogeneous sites (Gosline, 1971) and more ability to perform maneuvers (Jobling, 1995;Videler, 1993;Casatti and Castro, 2006).In S. hilarii a gradual reduction was observed in the caudal fin relative areas.With a smaller fin area, the fish can maintain a higher frequency of beats of this fin, allowing higher speed and lower energy consumption resulting from the drag force of the rapids.
On the other hand the aspect ratio of the caudal fin showed no difference, showing that differentiation does not occur in the form of caudal fin (Figure 3) in the development of this species and potentially in its function.The caudal fin characteristics observed for S. hilarii give an indication for better efficiency for locomotion, indicating the swimming behavior with continuous swimming and occupying the middle water.The fish body shape can be interpreted as the result of evolutionary adaptations to environmental pressures.Thus the resistance that water has on the body should be known due to the high correlation it presents to hydrodynamic aspects (Winston, 1995).The relationship between the shape of the body and the swimming performance influences the ability the fish has to keep itself in the water column, the acquisition of food and avoid predators (Videler, 1993).Fish with higher bodies have better ability to produce lateral movements (Aleev, 1969;Gatz Júnior, 1979a, b) besides affecting the maintenance of the fish speed (Webb and Weihs, 1986), once the value that most favors the maintenance of fish speed is the intermediate (Pauly, 1989).Fish with higher bodies have higher performance in habitats where the maneuvers are more important, as in full of obstacles locations (Breda et al., 2005).Fish like these are more efficient in start and maneuvers in relation to fish with more elongated bodies, showing better performance in structurally complex environments (Neves and Monteiro, 2003).This increase in height may indicate an ability to overcome strong rapids and obstacles such as rocks and waterfalls besides compensating and/or minimizing the prey escape strategies.The allometric increase of body height was compensated in proportion to the values of flatness index, maintaining a standard cross-section of the fusiform type.The fusiform form is characteristic by having the oval cross section and maximum height of the body located approximately one third of its previous total length and relatively thin, elongated caudal peduncle (Lagler et al., 1977), this being the most common form of fish with regular and sustained swimming (Keast and Webb, 1966) and related to fish swimming in waters with stronger current (Gatz Júnior, 1979a, b), concomitant factors to the lifestyle of S. hilarii.A higher body height added to a smaller area of the caudal fin and consequent low value of the caudal fin aspect ratio, enables the largest individuals greater start capacity and maneuvers to move and hunt in an environment full of obstacles besides helping in the escape from predators.
The values for mouth height are directly related to the opening capacity of the mouth, and this opening limiting the size of the prey to be ingested (Werner, 1974).Both the relative height of the mouth is also related to the hydrodynamics of the body (Keast and Webb, 1966;Werner, 1974;Gatz Júnior, 1979a).In fish species with ontogenetic changes, the dimensions of the head and mouth may vary due to ecological characteristics of the environment or within proportional relationships among the body, the head and the jaw, with the biggest ratio in the smaller fish.In S. hilarii this differentiation was evident and strongly influenced by an increase in fish length, concomitant to proportional decrease of the head.
In comparisons between head relative length and relative diameter of the eye were noticed a gradual reduction of these values as the fish size increases.This is a common fact in the ontogeny of the later stages of many species of fish (Aleev, 1969).In the larval and juvenile stages freshwater fish tend to stay in the shallow, calm waters, as close to shore or in ponds, where sediments settle out and water transparency increases (Rincon, 1999).In these places the capture of the food can be based on visual orientation in predatory species, with larger eyes and heads being very important.In the process of growth, the fish tend to live in waters that may be deeper or turbid.Thereafter visual guidance can gradually give way in importance to other types of guidance, as the lateral line system and the smell, both in search of food, and in intraspecific communication.Thus it is understandable the gradual reduction in the size of the eye and head.Although a negative allometry in the relative values of the head and eyes is observed, S. hilarii has relatively large head, regardless the size, indicating that it feeds itself on large prey (Gatz Júnior, 1979a;Teixeira and Bennemann, 2007) and the eyes are relatively large and laterally positioned predators typical of fish occupying the middle water (Lincon et al., 1995).Many of the features observed in S. hilarii comply with the description given by Webb and Weihs (1986) with a fusiform and tubular body, narrow caudal peduncle, forked fins and efficient propelling surface, coupled with big mouths and powerful jaws are specializations of piscivorous fish.
In our study, we observed that some of the analyzed morphological attributes showed variations along its growth.Morphological variations related to body height, the caudal fin area and dimensions of the head occurred due to allometric changes in own proportional ratios of the fish body structures, which may also indicate adjustments to the conditions of the environments where individuals occupy at various stages of development.On the other hand, others characteristics are not changed indicating the swimming behavior and the lifestyle of the species, keeping a typical morphological pattern of fish with continuous swimming and occupying the middle water, besides piscivorous predatory.

Figure 2 .
Figure 2. Scatter plot of the principal morphological attributes in relation to their standard lengths.r = Pearson correlation coefficient; p = probabilities.

Table 1 .
Description of the 15 relative values calculated to Salminus hilarii with their respective formulas and definitions.
Area of the pectoral fin divided by the product of the standard length and the maximum height.Relative mouth width (RMW) = (LB/SL) Open mouth width divided by the standard length.Relative mouth height RMH = HB/SL Open mouth height divided by the standard length.Relative jaw length RJL = CM/SL Jaw length divided by the standard length.Relative head length RHL = Ccb/SL Head length divided by the standard length.Relative eye diameter RED = DO/SL eye diameter divided by the standard length.Morphological aspects in the ontogeny of Salminus hilarii

Table 2 .
Loading values between the 15 morphological attributes for the two principal axis of PCA calculated for the length classes of Salminushilarii.