Physopyxis lyra is a member of the Doradidae, a South American catfish family that is easily recognized by the presence of spiny bony plates along the lateral line. The genus Physopyxis included just a single species, P. lyra, until Sousa and Rapp Py-Daniel (2005) redescribed this species and described two new species in 2005. This is is a tiny catfish, with adults not known to exceed 31 mm standard length (i.e., excluding tail). Physopyxis lyra differs from the other species of the genus in possessing a single row of spines on the lateral plates and a strong scapular girdle with long, broad coracoid processes having distal tips that are enlarged and divergent. This species is known from the rio Ampyiacu (and lowland portions of other tributaries to the upper Amazon in northeastern Peru) to the rio Uatumã, a left bank tributary to the Amazon in eastern Amazonas State, Brazil. No reliable records exist for the rio Negro basin. (Sousa and Rapp Py-Daniel 2005)

Physopyxis lyra are usually found in sites with accumulated organic debris, such as the dense tangles of roots of floating macrophytes that are abundant in rivers with turbid water. They can also be found in submerged leaf litter and among root mats of riparian plants, such as Symmeria paniculata (Polygonaceae). Observations of captive specimens suggest that P. lyra are predominantly nocturnal and spends most of the day hidden among submerged root mats or buried in sand.

188 measured individuals ranged between 17 and 40 mm total length (including tail) and 13 to 31 mm standard length (excluding tail) (Sousa and Rapp Py-Daniel, 2005).