Sternarchogiton nattereri

S. nattereri is restricted to large river channels in the lowlands of the Amazon basin. Its habitat ranges from high-conductivity, sediment-rich whitewater rivers such as the Rio Solimões, to low-conductivity, sediment-poor blackwaters such as the Rio Negro. It is one of the most common gymnotiforms in Rio Solimões. ==Description==

S. nattereri has a laterally compressed, knife-shaped body. The dorsal profile of the head is sharply curved from the nape to the snout. The mouth is terminal, with the upper jaw longer than the lower. The dentary is as long as deep, bearing a row of 7–8 conical teeth on the posterior portion, and there are upper and lower pharyngeal tooth plates bearing 9–11 and 7–9 teeth respectively. The eyes are small and covered by translucent skin. The long anal fin contains 180–198 rays. The pectoral fins are broad and pointed with 13–15 rays each, and the lanceolate caudal fin contains 11–15 rays and is almost completely covered with scales. There is a whip-like dorsal appendage anchored into a groove along the back by many fine threads of tendon. with males tending to be larger than females. ==Etymology==

The fish is named in honor of Johann Natterer. ==Biology and ecology==

The diet of S. nattereri consists of freshwater sponges growing on submerged trees, stumps, and other woody debris. Before they were recognized to be the males of S. nattereri, "Oedemognathus exodon" was speculated to use its dentition to feed on fish scales. The external teeth of male S. nattereri are now thought to be used in combat for territory or mates; scars and scratches have been found on the napes of several specimens. In the Tefé region of Brazil, breeding occurs during the rising water period, with adults spawning in floating meadows along the edges of paraná channels (side branches of whitewater rivers that run through adjacent várzea floodplains). As water levels rise further and dissolved oxygen levels drop, the growing juveniles eventually disperse out of these floodplains within floating rafts of vegetation. ==References==