Arnold's Pacu

Myloplus arnoldi can be told apart from other pacus by its vivid yellow and red fins, and by the highly elongated hook-shaped anal fin. This species has a maximum published total length of 33 cm (13 in) and a maximum published weight of 1.3 kg (2.9 lb).

Myloplus arnoldi is endemic to Brazil, where it is found in the drainages of the Xingu, Tocantins, Trombetas, Jari and Juruena rivers. This species also occurs in the Caiapó River in the Araguaia River system. M. arnoldi is found in the states of Amapá, Goiás, Mato Grosso and Pará. The juveniles prefer clear water, and the adults occur in the vicinity of rapids.

Myloplus arnoldi is classified within the genus Myloplus. This taxon was originally proposed as a subgenus of Myletes, this being a name which has been suppressed by the International Commission on Zoological Nomenclature as a synonym of Alestes. The proposer of this genus, Theodore Gill, did not explain its etymology. It may be a combination of mylo-, a reference to the disk-shaped body, and a prefix used for similarly-shaped fishes in the same family, with plus, which means "more", i.e., another Myleus-like genus. The specific name honours the German aquarist Johann Paul Arnold, who donated the type specimens to the Natural History Museum, Berlin.