Knifefishes · waters

The waters behind the fish

Knifefishes are creatures of specific water — a flooded Amazon forest, a clear blackwater channel on the Xingu, a warm oxygen-poor Orinoco backwater, a Central American swamp. Knowing which system a species comes from — its temperature, chemistry and whether it must breathe air — is the foundation of keeping it well, and explains why a soft-water shield knifefish and a hardier Central American Gymnotus want different tanks. Here are the freshwater systems that shaped 275 of the 279 species in this atlas.

The Amazon mainstem & white-water tributaries

186 species

Greater Amazonia · the Solimões, Amazon, Madeira & Purus

The vast, sediment-rich white-water heart of the Amazon — the Solimões and lower Amazon and their great tributaries the Madeira, Purus and Juruá, with their floodplain lakes and flooded forest. The centre of knifefish diversity: deep-channel ghost knifefishes, glass knifefishes in the open water, and Gymnotus and bluntnose knifefishes among the flooded margins.

The clearwater & blackwater shield rivers

21 species

Amazonia · the Xingu, Tapajós, Tocantins & Rio Negro

The clear and blackwater rivers draining the ancient Brazilian and Guiana shields — the Xingu, Tapajós and Tocantins to the south, the Rio Negro to the north. Warm, soft, acidic, low-sediment water over rock and sand. Dark, tannin-stained rivers suit the nocturnal, electrolocating knifefishes especially well, and several ghost and sand knifefishes are narrow-range shield endemics.

The Orinoco & the llanos

27 species

Northern South America · Venezuela & eastern Colombia

The Orinoco and its vast seasonal grasslands, the llanos of Venezuela and eastern Colombia. Sheet-flooded in the wet season and shrinking to channels and pools in the dry — a stronghold of Gymnotus, Eigenmannia and Brachyhypopomus, and one of the classic habitats where the electric eel is found gulping air in warm, oxygen-poor backwaters.

The Guiana Shield rivers

6 species

The Guianas · Suriname, Guyana & French Guiana

The forest rivers draining the Guiana Shield directly to the Atlantic — the Maroni, Corantijn, Essequibo and their kin. Clear, soft, tea-stained waters that hold their own knifefishes, including bluntnose Brachyhypopomus and coastal Gymnotus and Eigenmannia of the black-water creeks.

The Paraná & Paraguay

10 species

Southern South America · the lower Paraná, Paraguay & Pantanal

The southern river system — the Paraná, Paraguay and the great Pantanal wetland of Brazil, Bolivia, Paraguay and Argentina. Cooler and more seasonal than Amazonia, with sandy channels and floodplain lagoons. The range of southern Gymnotus and Sternopygus, and the southern limit of the order.

The Magdalena & Maracaibo

16 species

Trans-Andean Colombia & Venezuela

The isolated rivers west and north of the Andes — the Magdalena–Cauca of Colombia and the Maracaibo basin of Venezuela. Cut off from Amazonia by the mountains, each holds its own trans-Andean knifefishes, including endemic Gymnotus and Apteronotus that evolved in isolation.

Central America & the Pacific slope

9 species

The isthmus · Panama to Guatemala & southern Mexico

The northern edge of the order — the rivers of the Central American isthmus and the Pacific and Caribbean slopes from Panama up to Guatemala and southern Mexico. Only the banded knifefishes (Gymnotus) reached this far, crossing after the isthmus closed; they are the sole gymnotiforms north of Colombia.

Other Neotropical waters

4 species

Tropical South & Central America

Knifefishes whose recorded range falls outside, or spans, the named systems above — including widely-distributed species carried across several basins and a few little-collected fish known from a handful of localities. Knifefish distributions are incompletely mapped and the order's taxonomy is still being resolved.

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