German ichthyologist Axel Zarske has formally named the pencilfish previously known in the aquarium trade as Nannostomus marginatus Purple or N.
marginatus Red (not to be confused with N. mortenthaleri, which was previously imported as Nannostomus Peru Red).
Naming the new pencilfish Nannostomus rubrocaudatus in the most recent issue of the journal Vertebrate Zoology, Zarske distinguishes it from congeners by its unusual sexual dichromatism.
Nannostomus rubrocaudatus is very similar to N. mortenthaleri, but differs from it by the extent of the red colour in the adult males, which extends to the belly in N. rubrocaudatus but not in N. mortenthaleri.
The new species, named after its red caudal fin (Latin rubro=red, caudatus=tail), is known from the Marano River in Peru, where the author states that it is collected commercially for the aquarium trade one by one (instead of in large shoals as for other pencilfishes), making this a relatively expensive fish.
For more information, see the paper: Zarske, A (2009) Nannostomus rubrocaudatus sp. n. " ein neuer Ziersalmler aus Peru (Teleostei: Characiformes: Lebiasinidae). Vertebrate Zoology 59, pp. 11"23.