The first report of Dactylogyrus formosus Kulwiec, 1927 (Monogenea: Dactylogyridae) from goldfish (Carassius auratus) in central China.

Dactylogyrus formosus Kulwiec, 1927 (Monogenea: Dactylogyridae), widely distributed across Eurasia, is considered as one of the most frequently encountered ectoparasites of goldfish (Carassius auratus). In the present study, D. formosus on gills of cultured goldfish was reported in central China for the first time, using the methods of morphological characters and molecular markers. It was characterized by "Anchoratoid-Wegeneri" type anchor with elongate inner and unobvious outer root. The morphology of the opisthaptoral hard parts of D. formosus superficially resembles species of D. arcuatus Yamaguti, 1942 parasitized crucian carp, using light and scanning electron microscopy. However, the anchor total length of D. arcuatus (range 90-102) is almost double the size of D. formosus (range 43.7-57.1) which allows for their rapid discrimination from each other; on the other hand, the two species also can be easily separated by the shape of the male copulatory organ. The morphological description of D. formosus is supplemented with phylogenetic analysis of a reference DNA sequence spanning 18S rDNA and ITS1. What is more, this study also redescribes comprehensively D. formosus, providing for the first time a full ten point-to-point morphometric measurements of the hard parts, and importantly, a photographic record of the armature of the haptor and the male copulatory organ.