Taxonomy & naming
Hemipsilichthys nimius was described by Pereira, Reis, Souza and Lazzarotto in 2003, in Zootaxa (issue 285, pages 1–10). The Catalog of Fishes (Eschmeyer, CAS) carries a valid species link for the name (spid 67484).
The genus Hemipsilichthys is placed by FishBase in the subfamily Delturinae of the Loricariidae — a small subfamily of southeastern Brazilian armoured catfishes. The specific epithet nimius refers to the species' supernumerary dorsal-fin rays.
Morphology
Hemipsilichthys nimius reaches a maximum length of about 4 in standard length. The diagnostic feature is the dorsal fin, which bears an unusually high ray count of 7–9 soft rays — typically eight branched rays, and in some specimens nine, the excess that gives the species its name. The dorsal-fin membrane is expanded posteriorly, and the teeth cusps are equal.
No data on sexual dimorphism is published in the sources consulted.
Habitat
The species occurs in the Perequê-Açu and Taquari river basins, among the southern coastal rivers of Rio de Janeiro state, southeastern Brazil, at altitudes of roughly 328–1214 ft above sea level. It inhabits small coastal streams within well-preserved Atlantic forest, over rocky substrate, in fast-flowing clear water exposed to direct sunlight, at depths of about 4–19.5 in.
Feeding
Underwater observation indicates that Hemipsilichthys nimius is a nocturnal bottom grazer, feeding on the stream bed at night. Consistent with its loricariid body plan and rocky-stream habitat, its diet is understood to consist of algae and aufwuchs rasped from the substrate, though no detailed dietary analysis is published in the sources consulted.
Mating
No data on courtship or mating behaviour has been published for this species.
Breeding
No breeding data has been published for Hemipsilichthys nimius in the sources consulted.
In the aquarium
Hemipsilichthys nimius is not an aquarium species and has no record of hobby maintenance in the sources consulted. As a range-restricted, Near Threatened fish of well-preserved Atlantic-forest streams, it is not an appropriate subject for the aquarium trade, and any inference about captive care would be speculative.
Conservation
The IUCN Red List assessed Hemipsilichthys nimius as Near Threatened (NT) under criterion B2b(iii), with the assessment dated 7 November 2018.
The listing reflects the species' restricted distribution in the Perequê-Açu and Taquari coastal basins and its dependence on well-preserved Atlantic forest — one of the most fragmented and threatened biomes in South America. Degradation of forest cover and stream quality in its narrow range is the principal concern.