Bucephalid Metacercarial Infection in Wild Larval and Juvenile Ayu Plecoglossus altivelis

Abstract

Ayu Plecoglossus altivelis is a native amphidromous fish in Japan and an important target for farming and recreational fishing. ​We have found high infection of hitherto unknown digenean metacercariae in wild ayu larvae and juveniles captured from the sea and a river in Wakayama Prefecture. ​The infection prevalence was 95% and 100% in larvae and juvenile ayu, respectively with variable intensities up to 566 metacercariae per fish. ​The metacercariae were located primarily around fins and inside caudal fins, but appeared to cause no serious pathological features to the fish. ​The phylogenetic analysis based on the internal transcribed spacer 2 and 28S rDNA sequences revealed its identity as a member of Bucephalinae (Bucephalidae) with no identical genetic information on the database. ​Morphological characteristics of the larval trematode resembled those of the genus Prosorhynchoides. ​Genetically identical metacercariae were also found from Scomber japonicus and Engraulis japonicus captured at the same location as P. altivelis with variable infection levels, which suggested that it was a marine species. ​Experimental infection challenge of the metacercariae to several marine fish via oral intubation failed to obtain adult specimen and thus the precise identification of the parasite remains unknown.