História natural de poraquês (electrophorus spp.), gymnotiformes: gymnotidae

Abstract

The electric eel is the largest representative of neotropical electric fish (Gymnotiformes), reaching more than two meters in length and 20 kg, and capable of emitting the higher electrical discharge known in the animal kingdom (up to 860 volts), enough to stun large vertebrates. These peculiar characteristics have always fascinated people since their discovery by European naturalists more than 250 years ago. Throughout that time, the electric eel has served as a model animal for morphological, anatomical, physiological and biological studies. Laboratory detailed studies with electric eels have developed from technological innovations (e.g. creation of the electric battery) to the elaboration of drugs to treat neurodegenerative diseases (e.g. Alzheimer's disease). However, their biological peculiarities still inspire many questions about how they evolved or how they are used in a natural environment. These questions evidence the lack of field studies that explain the basic biological aspects of such an important species in the history of scientific knowledge. The present study addresses the natural history of electric eels (Electrophorus spp.) at different locations in the Brazilian Amazon, seeking to consolidate dispersed information on population structure, reproductive and feeding behavior. In the first chapter, we carried out a population study of E. varii in a small forest stream, Igarapé Bolivia, at the Adolpho Ducke Forest Reserve (Manaus/AM). A population census was conducted over a year using an electric-discharge detector, visual detection and a telemetry system (PIT tags), producing an array of data on abundance, patterns of distribution and movement of individuals. The results show that there is an estimated population of 25 adult specimens in that stream. At the beginning of the reproductive period (June to August), adult electric eels adopt a gregarious behavior and make small migrations to the headwaters of the stream to form couples, occupy spawning sites (cavities under the ravine bank) and reproduce. The low number of young specimens in the following months and low recruitment of adults in the population in the following years, suggest a high mortality rate soon after the end of parental care in upland’ streams. In the second chapter, we investigated the reproductive strategy of electric eels in different Amazonian aquatic habitats. The presence of eighteen new nests / presence of juveniles were recorded in different spots, widely distributed in the Amazon basin. These records demonstrated that the lowland electric eel (E. varii) is an equilibrium strategist, always reproducing in the dry season, and despite its considerable plasticity concerning different environmental conditions, it always follows some specific behavioral features for its reproductive success, such as: air-breathing, use of specific spawning sites, construction of bubble nests, parceled spawning, deposition of trophic oocytes, and high investment in parental care (4 months). The few records of reproductive behavior in highland species (E. voltai and E. electricus) indicate a shorter period of parental care compared to E. varii, suggesting a different reproductive strategy and adapted to the dynamics of river rapids (lotic systems). In the third chapter we presented an unrecorded hunting activity performed by a large group of individuals (social predation) of E. voltai in a population of the Iriri River, Xingu River basin. Observations obtained through different years have shown that more than a hundred electric eels conduct, always in the twilight period (dawn and dusk), large shoals of small tetra fishes (Characidae) to shallow areas on a lake' shore, delivering 5 to 7 attacks with synchronize discharges of high voltage to capture its prey, which jump and fall stunned on the water's surface. After a series of attacks for about an hour, the electric eels return to the bottom of the lake. This strategy is rare in fish, but common in aquatic marine mammals (e.g. whales and dolphins), being highly effective against pelagic prey that exhibit collective anti-predatory behaviors. Finally, a synthesis is presented with the main results of the thesis and the prospects for future studies that will increasingly help in the understanding of this unique group of species.