Habitats of Gymnotus
Gymnotus species are common in shallow freshwaters of the lowland Neotropics from Southern Mexico to Argentina. It is in the Amazon and Orinoco basins that Gymnotus reaches by far its highest diversity.
Shallow hypoxic swamps like this moriche palm grove from the Venezuelan Amazon are usually inhabited by Gymnotus. Gymnotus is an air breathing fish and can tolerate indefinite periods of anoxia. All species of Gymnotus are aggressive nocturnal predators. Adults of small species and juveniles of larger species feed on aquatic invertebrates. Adults of larger species also eat small fish and crustaceans. Gymnotus can be sampled in shallow swamps with dipnets and electric fish detectors or with fyke nets set before dusk and left overnight. Photo. Will Crampton (San Fernando de Atabapo, Amazonas, Venezuela).
Flooded forests are a rich source of food for fishes but floodplain waters are often hypoxic or even completely anoxic. Gymnotus forage among submerged branches and leaf litter banks – especially in newly inundated forest. Gymnotus can be easily captured with fyke nets in these habitats. Photo Will Crampton (Reserva Nacional Pacaya Samiria, Loreto, Peru).
Floodplain Floating Meadows:
Dense rafts of floating grasses and other macrophytes in the Amazon’s expansive whitewater ‘várzea’ floodplains support species-rich syntopic assemblages of Gymnotus. These habitats can be sampled very effectively with seine nets (15-30 x 4-6 m, 5 mm mesh) operated from canoes. The meadow is sampled by cutting a circular path through the meadow and surrounding it with the net. The vegetation and its root mat are then shaken in the water within the net to dislodge fishes. Photo Will Crampton (Reserva Nacional Pacaya Samiria, Loreto, Peru).
For additional notes on Whitewater floodplain habitats please visit:
Várzea Floodplain from the Air:
Várzea floodplains are a mosaic of channels, lakes and seasonally inundated forests and support species-rich assemblages off fishes, including Gymnotus. Photo. Luiz Claudio Marigo (Reserva de Desenvolvimento Sustentável Mamirauá, Brazil).
Deep River Channels
Deep river channels like the Amazon River contain rich assemblages of gymnotiform electric fishes – primarily Apteronotidae and Sternopygidae. However, Gymnotus does not form part of this specialized assemblage. Gymnotus species are absent from the bottom of deep river channels and found only occasionally in marginal vegetation or in quiet backwaters along the rivers edge. Photo W. Crampton (Rio Solimões near Tefé, Brazil).
Terra Firme Streams:
Gymnotus are common in terra firme streams. They live in hanging root tangles and undercut banks, and also in submerged leaf litter banks. Deep pools and undercut banks can host large specimens. Dipnetting with an electric fish finder is a productive method to catch these fish. Terra firme streams of lowland rainforests are usually typified by low conductivity water (less than 20 μS/cm), little sediment, low pH (less than 6), relatively high oxygen (> 3 mg/l) and are often dark-stained with tannic and folic acids.