PAGOTTO, JPA; GOULART, E; OLIVEIRA, EF and YAMAMURA, CB. Trophic ecomorphology of Siluriformes (Pisces, Osteichthyes) from a tropical stream. Braz. J. Biol. [online]. 2011, vol.71, n.2, pp. 469-479. ISSN 1519-6984. http://dx.doi.org/10.1590/S1519-69842011000300017.
Does body shape predict feeding habits? An ecomorphological study with stream fishes
The study analyzes the relationship between body shape (morphology) and the use of feeding items by species that compose the Siluriformes Order, a relatively wider group which embraces distinct types of fishes, such as the popularly known catfishes (bagres) and suckermouth armored catfishes (cascudos). The study, in general, demonstrated that in the same stream, the occurrence and coexistence of Siluriformes tend to be established by a natural selection of those with the most adapted morphology to the use of different available resources; such fact enables the prediction of the ecology of each species (the way that each one uses the environmental resources) from the species morphology, that is, to presume what is the consumed feeding item and the preferentially used sites to obtain it, based on the body form.
However, the ecomorphology has been a source of intense discussions. One of the reasons is that some analyses performed within relatively wider groups of species (regarding their evolutionary lineages), such as Orders and Classes, indicate that the most similar species in morphological terms are the evolutionarily closest ones, and not necessarily those that use the same resources. Thus, according to these analyses, the morphological similarity is related only to a closer relationship among species and not to an adaptation to a specific resource. Note that species with a high degree of relatedness do not always use the same resources, as in the classic example of Darwin's finches in the Galapagos Islands. Therefore, the implementation of new ecomorphological studies in different ecosystems is important to investigate the possibility to identify adaptive morphological patterns in wider groups, and thus to test whether the morphology can be used as an efficient tool to predict the ecology, or whether the ecomorphology is flawed in this aspect and only could be used for narrower groups.
Based on that, the reserch tested the hypothesis that body shape of Siluriformes is intimately related to the exploited feeding resource, independent of their evolutionary distance. Measures of some morphological structures related to the head, eyes, mouth and fins of the collected fishes were taken, and the stomach contents were analyzed.
A total of six Siluriformes species were sampled and analyzed: Hypostomus ancistroides and Hypostomus strigaticeps (see Figure 1), popularly known as suckermouth armored catfish, basically consumed organic detritus and some kind of algae (periphyton) which stay adhered to submerged rocks in stretches of runs. For this reason they were classified as detritivores. The other species were Cetopsorhamdia iheringi, Imparfinis schubarti, Phenacorhamdia tenebrosa and Rhamdia quelen, popularly known as catfish (see Figure 1). The first three species of catfishes consumed terrestrial insects that fall on the water surface and also insect larvae that remain on the bottom, and they were classified as insectivores. Rhamdia quelen, which fed on a large diversity of food resources, ranging from terrestrial plants (grasses, fruits and seeds) that fall into the water until the remains of insects and fish, was classified as omnivore.
Figure 1. Siluriformes from the Caracu Stream: (A) Hypostomus ancistroides; (B) Hypostomus strigaticeps; (C) Cetopsorhamdia iheringi; (D) Imparfinis schubarti; (E) Phenacorhamdia tenebrosa; (F) Rhamdia quelen. The scale shown in each picture corresponds to 1cm from the actual size of the fishes.
Considering this clear difference in the used feeding resource, a question needs to be investigated: which factors are responsible for the variation in the diet of the Siluriformes species? The difference between the suckermouth armored catfishes and the catfishes may be explained by the evolutionary distance that separates both kinds of fishes. The first ones show a body form that permits them to perform short displacements and stabilize their bodies while they move through the flowing water. It provides more advantage to graze the periphyton and the encrusted organic material on the surface of rocks. On the other hand, the catfish have more compressed bodies and possess a greater capacity for maneuverability, that is, more ability to perform small-angled maneuvers. The species with this feature exhibit a superior performance in the exploitation of structurally complex habitats with low current velocity, where there are a large deposition of insects and plants.
Yet, what explains the differentiation between the diet of R. quelen and the other catfish species, since the first one is evolutionarily closer and, therefore, morphologically similar to the others? Here we can say that there was an ecomorphological divergence, where closest evolutionarily species diverge in some of their morphological and ecological characteristics, probably due to the process of natural selection, which tends to generate new evolutionary adaptations that may promote an effective sharing of the available resources. Rhamdia quelen have a more compressed body than the other catfishes and possesses more capacity for maneuver. Thus it can inhabit marginal backwater zones of the streams, where the flooded vegetation presents a great deal of structuring and spatial complexity. In these environments, this fish assumes an omnivorous profile and can be considered a bottom sweeper. On the other hand, the insectivores prefer to inhabit areas near stretches of rapids and runs (e.g., small spaces among the rocks, where the deposition of both terrestrial and aquatic insects is intense). In these environments, these organisms actively swim among rocks, where they capture their prey individually by excavating (while moving) and turning over the substrate.
Therefore, this study demonstrated that the body form is related to the way that the Siluriformes from the Caracu Stream use the feeding resource. Consequently, the morphology can be used as a tool to predict the ecology of species. Even with distinct kinds of fishes, with different evolutionary history, the detection of an adaptative divergence among the highest relatedness species permit us to argue that the evolutionary distance do not limit the analyses. In fact, these analyses show that it is possible to relate the use of the feeding resource with the body form, independent of the evolutionary distance among the species. Thus, studies aiming to analyze the ecological organization of wider groups, such as Order, Class or Community, would be performed together with the ecomorphology, since the predictive character of this approach would provide greater precision in the analyses and significantly contribute to our understanding of the mechanisms that allow the coexistence of species.
The research in question was funded by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES/Ministério da Educação) and it was linked to Programa de Pós-Graduação em Ecologia de Ambientes Aquáticos Continentais (PEA), with the support of Núcleo de Pesquisas em Limnologia, Ictiologia e Aqüicultura (Nupélia), the latter two belonging to Universidade Estadual de Maringá (UEM). The analyses were performed with fishes collected from the Caracu Stream (22º 45' S/ 53º 15' W), a tributary of the left bank of the Paraná River, near Porto Rico-PR, Brazil.
Contato: João Paulo Alves Pagotto