Ecomorfologia e uso de recursos por quatro espécies sintópicas de peixes (Characidae) em igarapés amazônicos de terra firme, Amazonas, Brasil

Abstract

tetras (Characidae) compose a group of fishes that share a generalized morphology and a superficial similarity that is apparently associated with a generalized diet based on the opportunistic consumption of a wide variety of food items. This morphological and functional similarity, coupled with the known occurrence of several species of tetras in the same aquatic environment, has raised the hypothesis that there would be a high ecological niche overlap between simpatric species of this family, especially relevant in environments where food resources are limited, as is the case of the oligotrophic upland forest streams of the Central Brazilian Amazon. To test this hypothesis, we analyzed the morphological characteristics, diet and hábitat use of four syntopic species of characins (Iguanodectes geisleri, Bryconops giacopinii, B. inpai, and Hyphessobrycon melazonatus) in streams of the Adolpho Ducke Forest Reserve, Manaus. We evaluated the degree of overlap in the use of food resources and space, and the morphological and behavioral characteristics that allow the coexistence of these four species (in different combinations) in the streams. Fish were collected by seine nets in 50-m stretches of eight streams, selected on the basis of previous records of occurrence of these species. The use of space in the streams, recorded by underwater observations, was compared between species, as well as the frequency of foraging events. We analyzed 18 ecomorphological attributes of 22 specimens of each species, which were compared by means of ordination techniques (PCA). The diet of the species was analyzed by means of the alimentary index (AI), and information about the relative volume of the food items consumed were used to assess the amplitude and degree of overlap of the food niche between them. The different combinations of co-occurrence of species of minnows in streams (without co-occurrence; with a co-occurrence (alone, with one additional species, or with two other species) were tested using niche breadth data calculated for the individuals of H. melazonatus and B. giacopinii. We detected a significant difference in the use of the horizontal space by the species, separating H. melazonatus (that predominantly occupyed the margins) from the other species (that inhabited mainly the stream channel). All species used predominantly the middle third of the water column, but I. geisleri exhibited a higher proportion of use of the lower third, near the stream bottom. The analysis of ecomorphological characteristics showed significant interspecific differences, especially in body shape, size and orientation of the mouth, despite the high overlap in the diet of the species. However, when analyzing the composition of the diet on co-occurrence with one or two syntopic H. melazonatus and B. giacopinii exhibited narrowing of trophic niche. In short, despite the minnows are considered phylogenetically related and morphologically similar, the integrated analysis of information on habitat use, behavior, diet, and ecomorphological characteristics found significant differences between these species, which can act as mediators of coexistence in Amazonian lowland forest streams , through the partition of available resources.