Taxonomic studies of Piaractus brachypomus (Cuvier, 1818) (Serrasalmidae): a non-native species collected from Polish water bodies and laboratory culture

Piaractus brachypomus is highly valued as an aquaculture and ornamental fish species. However, it is frequently considered as a non-native or invasive species. In Polish waters P. brachypomus is an incidental non-native species, most likely released by aquarists, and it is unable to overwinter or reproduce. Outside its native distributional range, this species has been recorded nearly world-wide. In regions where exist it is often confused with other species such as Piaractus mesopotamicus and Piaractus orinoquensis, possibly with Colossoma macropomum and their hybrids, and Pygocentrus nattereri. In this study, we provide a detailed taxonomic description and the most useful discriminative features for the species, of five specimens of P. brachypomus caught in water bodies in northwestern Poland (in 2002-2010), and a sample of 11 specimens obtained from laboratory culture (2012-2019) that originated from South America. The examinations included describing metric and meristic characters, the neurocranium structure, the dentition formula, parasite fauna, and species identification based on a mitochondrial COI sequence. The phylogenetic tree inferred using maximum likelihood showed that pirapitinga samples were nested within the Amazon Basin clade of P. brachypomus. This is the first molecular study in Europe and first detailed morphometric study (including the neurocranium) of this species from a non-native area of its distribution. Characters such as dentition can be used to discriminate among the genera Colossoma, Piaractus and Pygocentrus, while meristic characters such as lateral line scale counts, numbers of scale rows above and below the lateral line, pectoral and pelvic fin rays counts, and ventral-keel spine counts can be used to discriminate among species within the genus Piaractus. Nevertheless, morphometric characters do not seem to be an appropriate tool to distinguish closely related species in the genus Piaractus.

Automated summary

Piaractus brachypomus is a valuable aquaculture and ornamental fish species, but often considered as a non-native or invasive species. This study provides a detailed taxonomic description and discriminative features of P. brachypomus specimens from Poland, using measurements, dentition, parasite fauna, and mitochondrial COI sequence. The study also reveals the phylogenetic relationship and suggests that morphometric characters may not be suitable for distinguishing closely related species within the genus Piaractus.