Published September 4, 2020
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Feeding ecology of electric eel Electrophorus varii (Gymnotiformes: Gymnotidae) in the Curiaú River Basin, Eastern Amazon
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Mendes-Júnior, Raimundo Nonato Gomes, Sá-Oliveira, Júlio César, Vasconcelos, Huann Carllo Gentil, Costa-Campos, Carlos Eduardo, Araújo, Andrea Soares (2020): Feeding ecology of electric eel Electrophorus varii (Gymnotiformes: Gymnotidae) in the Curiaú River Basin, Eastern Amazon. Neotropical Ichthyology (e190132) 18 (3): 1-17, DOI: 10.1590/1982-0224-2019-0132, URL: http://dx.doi.org/10.1590/1982-0224-2019-0132
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- urn:lsid:plazi.org:pub:522FFF92FD2FFFA5FF915D112C0DFFDE
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- Figure: 10.5281/zenodo.12719696 (DOI)
- Figure: 10.5281/zenodo.12719698 (DOI)
References
- Considering that the ratio between intestine size and fish size reflects their eating habits, ranging from short intestines of carnivores to long-intestines of herbivores, and intermediate-sized intestines of omnivores (Fryer, Iles, 1972; Barbieri et al., 1994; Gerking, 1994; Ward-Campbell et al., 2005), the short intestine of electric eels reflects their carnivorous eating habits. The average IQ of E. varii is short when compared to that of the piscivore Hoplias malabaricus (Bloch, 1794), with IQ values above 0.53 (Mazzoni, Costa, 2007; Peretti, Andrian, 2008), but the small visceral cavity in relation to the body size of Gymnotiformes do not allow a comparison of IQ values of this group with those of other Teleostei fish. The mean IQ of E. varii is lower than the IQ of the generalist carnivore Sternopygus macrurus (Bloch & Schneider, 1801) (IQ = 0.60) (Olaya-Nieto et al., 2009) and is higher than that observed in invertivorous electric fish such as Eigenmannia trilineata Lopez & Castello, 1966 (IQ = 0.25), Brachyhypopomus bombilla Loureiro & Silva, 2006 (IQ = 0.27), and Brachyhypopomus gauderio Giora & Malabarba, 2009 (IQ = 0.29) (Peretti, Adrian, 1999; Giora et al. , 2012; Giora et al. , 2014), which contradicts the idea that the IQ of fish-eating predators tends to be lower than that of invertivores. The values observed in E. varii might be higher due to its larger body cavity (covering more than 30 vertebrae) compared to the 16 to 19 vertebrae in Brachyhypopomus Mago-Leccia, 1994 and Eigenmannia Jordan & Evermann, 1896 (Peixoto et al., 2015; Crampton et al., 2016). If the intestine is a good indicator of the diet of electric fish, and since Gymnotus, the putative sister group of Electrophorus (e.g. Alda et al., 2019), consists of voracious predators of fish and insects (Campos-da-Paz, 2003), their intestine is likely intermediate in length between the ones observed in Electrophorus (IQ = 0.40) and those documented in Eigenmannia and Brachyhypopomus species (IQ = 0.25).
- Similar to the observed for diet composition, the feeding dynamics of E. varii did not differ among size classes and between seasonal periods in the Curiau River EPA, which may reflect the high capacity of electric eels to locate and subdue prey using weak and strong electrical discharges, respectively. The electrical organ of E. varii individuals with a length of at least 40 cm can produce discharges greater than 300 volts (from de Santana et al., 2019), thus, electric eels of all size classes analyzed in the present study were equally lethal to prey fish. In addition, the hunting tactics of electric eels described by Catania (2019) allow them to find and control mobile or sedentary prey, regardless of the time of year. Visually oriented piscivorous predators, such as species of Hydrolycus Muller & Troschel, 1844, have different feeding dynamics than those observed in electric eels. They modify their feeding activity according to the seasonal period (Barbosa et al., 2018), since the flooding of river banks in the rainy season provides new refuges for prey, making it difficult for predators to hunt (Lowe-McConnell, 1999), which probably does not occur with E. varii. The present study reports the initial findings of the feeding dynamics of electric eels, due to limitations in the use of stomach weight to measure feeding activity in fishes species (Elliasen, Jobling, 1985; Bromley, 1994).
- In conclusion, our findings reveal that electric eels are piscivorous predators and that fishes are the main prey of young individuals (Lt 45-85 cm) and adults (Lt > 85 cm), regardless of the time of year. Future analyzes of the stomach contents of 10-40 cm individuals may indicate in what size class there is a change from a diet rich in crustacean larvae (Assuncao, Schwassmann, 1995) to a piscivorous diet (present study). Our initial findings also indicate that the feeding dynamics of electric eels are not influenced by ontogeny and seasonality. However, future studies with more robust methods than the analysis of stomach weight are needed to more effectively measure the feeding dynamics of electric eels. Protection and management strategies and actions are essential to ensure that future generations can experience the celebrate electric eel not only in zoos, public aquariums, wildlife books and documentaries, but in their natural habitat.