Grazing Unworried: Environmental Changes in a Dammed River Favor the Diet of the Invasive Metynnis Lippincottianus (Characiformes, Serrasalmidae)

Abstract

We evaluate the natural diet of the invasive silver dollar Metynnis lippincottianus in the Lower São Francisco River, Brazil. The species' diet was composed almost exclusively of Zygnemataceae algae, a food item consumed by almost all individuals (Frequency of Occurrence (FO) = 99.80%). The remaining consumed items were plant fragments (FO = 3.57%), mites (FO = 2.38%), gastropods (FO = 1.19%), thalloid algae (FO = 1.19%), ants (FO = 1.09%), and invertebrate eggs (FO = 1.09%). The high alimentary importance of Zygnemataceae algae (Alimentary Index = 99.992%) highlights the lack of a great intraspecific variation in the diet of M. lippincottianus and a high trophic specialization. The herbivorous diet, with a tendency to consume algae, is supported by high values of intestinal coefficient (4.71 ± 1.45) that favor the digestion of these items and are commonly related to low trophic levels. The specialized diet of M. lippincottianus is supported by the proliferation of algae following the damming of the river. In addition, the diet composition of the species changed between the rainy and the dry season (PERMANOVA, p < 0.001), displaying a broader niche by the consumption of alternative items (e.g., thalloid algae, gastropods) in the rainy season. The species had an herbivorous diet, with a predominance of filamentous algae, throughout the study, with small variations between seasons.