[1] As its scientific name suggests, S. preto is distinguished from all other Sternarchogiton species by its entirely dark brown to purplish black color.

The head is laterally compressed with a convex dorsal profile and small eyes that are covered by a membrane.

The tail is frequently missing or regenerated; when intact it is compressed and short, with a small lanceolate caudal fin containing 13-20 rays.

Reproduction is believed to occur early in the rising water period, with individuals in breeding condition in October.

Juveniles have been caught in floating meadows alongside the Tefé River throughout the flooding season (December to April).