Diferenciação de nicho alimentar de larvas de duas espécies potamódromas neotropicais

Abstract

Determining the differences in the use of the niche between species that coexist in the same environment is essential to understand how they interact. Coexisting species tend to segregate the use of food resources and space, as a way of reducing the effects of interspecific competition. This research used larvae from two potamodroma species, Salminus brasiliensis and Brycon orbignyanus, with similar morphological and dietary characteristics, to understand how these species use food resources in the early stages of life, in order to allow coexistence between them. Sampling was carried out with conical-cylindrical plankton nets, from October 2018 to February 2019, on a 650 km stretch of the Middle Uruguay River, RS, Brazil. This section was separated into four sample sites, where the collections were carried out in the main river and tributary river in all sample sites and in site 4 the mouth and lagoon were inserted as it is the only one that presents these biotopes. The larvae that were used in the analysis of stomach contents were captured mainly at site 4, with 98% of the larvae of S. brasiliensis and all of B. orbignyanus. Stomach analysis indicated that both species feed mainly on autochthonous items, the most representative being fish larvae, followed by insects and microcrustaceans. In addition, there were interspecific differences in the composition of the diet. Brycon orbignyanus fed almost exclusively on Characiformes larvae, with the families Characidae and Prochilodontidae, together with the alien insects, being the most representative of their diet. S. brasiliensis consumed both Characiformes and Siluriformes larvae, with emphasis on the consumption of larvae of the Characidae and Heptapteridae families. Both species proved to be specialists throughout the larval stage, with a slight increase in the niche width of S. brasiliensis larvae at the end of the larval stage. There were spatial differences in the use of food resources, in the lotic environment S. brasiliensis consumed more fish larvae of the Heptapteridae family, and B. orbignyanus consumed more alien insects. In Lagoa, both species preferred larvae of Characidae. Although the larvae of the analyzed species have similar eating habits, it is concluded that there is partitioning of resources between them, and that this process is facilitated by the exploration of different food items, represented by inter and intraspecific variations, as well as spatial variation in the use of resources. These variations allowed these species, predominantly piscivores in the larval stage, to coexist in the Middle Uruguay River.