Orthon

The smallest and most delicate of the nominal hatchetfishes. The rounded keel is shallower than related species, giving this fish a distinctive elongate profile. The family Gasteropelecidae is separated from other Characiformes by the following combination of characters: front...

It exhibits slightly different behaviour to the majority of congeners in that it tends to swim in midwater and spends a large proportion of its time away from the substrate. Its morphology exhibits corresponding adaptations towards a pelagic existence with a relatively large eye, a more terminal mouth position, more strongly-forked caudal-fin, and more symmetrical body shape than most other Corydoras species.

The majority of records pertain to the rio Guaporé (Río Iténez in Bolivia), the main affluent of the rio Mamoré, which drains the vast tropical wetland known as the Pantanal, and the upper rio Paraguai, which also has its headwaters in the Pantanal but flows in the opposite direction. The Guaporé and Paraguai are connected due to rising water levels during the annual wet season.

This species appears endemic to the upper rio Madeira basin in Bolivia and Brazil. Records exist from the Río Mamoré and Río Guaporé drainages in Bolivia (where the Guaporé is known as the Iténez) and Brazil plus the upper Río Orthon (ríos Tahuamanu and Manuripi). Type locality is 'sand-bank in the Rio Marm...