Identification of exotic sailfin catfish species (Pterygoplichthys, Loricariidae) in Taiwan based on morphology and mtDNA sequences

The sailfin catfish, a collection of several morphologically similar species belonging to the genus Pterygoplichthys of the Loricariidae from South America, has been widely introduced to tropical and subtropical freshwater environments worldwide and has caused serious ecological impacts. Although listed as one of the most serious invasive species, the taxonomic status of this fish in Taiwan has never been precisely described. In this study, we used morphological traits and mitochondrial DNA sequences to identify the fish which currently occur in Taiwanese rivers. Evidence from both datasets rejected the existence of P. multiradiatus, which was once the most widely applied name in local references. Pterygoplichthys gibbiceps and P. joselimaianus, 2 other species commonly appearing in the aquarium fish trade, were also not detected in the wild. Our results indicated that the fish occurring in Taiwanese rivers should be identified as P. pardalis and P. disjunctivus. Nevertheless, fish collected from the wild present a contiguous variation, among which roughly 28.3% could not be defined as either species and thus were grouped as an intermediate form. Morphological identification did not match the 2 mitochondrial clades. Compared to a native population of a pure-line P. pardalis collected from the Amazon River at Manaus, Brazil, exotic populations showed much wider morphological variations and higher genetic diversities. We put forth 2 hypotheses and 1 speculation to explain the current situation: (1) P. disjunctivus might just be a synonym of P. pardalis; (2) the exotic populations may have originated from hybridization between the 2 species or allopatric populations; and (3) superiority of the hybrid may have helped increase their fitness during invasions.