Morphology and molecular characterization of Acanthogyrus (Acanthosentis) bilaspurensis Chowhan, Gupta & Khera, 1987 (Acanthocephala: Gyracanthocephala: Quadrigyridae) from the common carp Cyprinus carpio Linnaeus (Cypriniformes: Cyprinidae) in Pakistan.

Acanthogyrus (Acanthosentis) bilaspurensis Chowhan, Gupta & Khera, 1987 is a poorly known acanthocephalan species reported from the reba carp Cirrhinus reba (Hamilton) (Cypriniformes: Cyprinidae) in India. In the present study, the detailed morphology of A. (Acanthosentis) bilaspurensis were studied using light microscopy and, for the first time, scanning electron microscopy (SEM), based on newly collected specimens from the common carp Cyprinus carpio Linnaeus (Cypriniformes: Cyprinidae) in Pakistan. The SEM observations revealed the outer shell of eggs with numerous remarkable protuberances and the gonopore of female located at a discoid protrusion surrounded by some very small spines. This is the first time that A. (Acanthosentis) bilaspurensis was reported in Pakistan, and the common carp represents a new host for this species. In addition, the molecular characterization of the 18S and 28S rDNA of A. (Acanthosentis) bilaspurensis is provided for the first time. There was no intraspecific genetic variation detected in the 18S and 28S regions between different individuals of A. (Acanthosentis) bilaspurensis, but high level of interspecific nucleotide divergence was found in the 18S (4.15-16.7%) and 28S regions (3.60-15.4%). Phylogenetic results based on the 18S, 28S and 18S + 28S sequence data, respectively, all revealed the Quadrigyridae, Pallisentinae and the genus Neoechinorhynchus are not monophyletic groups, and the genera Acanthogyrus and Pallisentis have far relationship. The systematic status of the genus Acanthogyrus and some species of Neoechinorhynchus need further clarify based on different mtDNA and nuclear DNA data including broader representatives of the class Eoacanthocephala.