Occurrence and Effects of the Rhizocephalan Parasite,Polyascus gregarius,in the Chinese Mitten Crab,Eriocheir sinensis,Cultured in a Freshwater Pond,China

An epizootic outbreak of the parasitic barnacle, Polyascus gregarius, was reported for the first time in cultured commercial mitten crab, Eriocheir sinensis. Observations were made in a freshwater culture pond in Shenjia Dang, Zhejiang province, China, from June to October 2007. The surface morphology and appearance of the interna, externa, or scar of P. gregarius were recorded in 1464 crabs, including 871 males and 593 females. Overall prevalence of infection was 37.8 and 31.7% in male and female crabs, respectively. The monthly variation of infection was mainly dependent on the mortality of the host, with the highest prevalence (58.0% of males and 51.3% of females infected) in June, and the lowest (13.4% of males and 6.3% of females) in October. A host with multiple externae or scars was encountered frequently, with the maximum of 28 externae or 19 scars on a single crab. Being restricted in the culture pond, the hosts could not escape and return to the brackish water of the estuary; therefore, the externae were never exposed to male cyprids and remained unfertilized. As a result, all the externae degenerated and dropped off finally after 2–3 mo. This parasite caused castration, anecdysis, and increased mortality in the host, and therefore, had both direct and indirect effects on the commercial culture of E. sinensis. Infected males were castrated and displayed morphological modifications of secondary sexual characters. The size of infected crabs was significantly smaller than that of uninfected crabs because the molting was reduced by infection. After all the externae dropped off, the scarred host crabs finally died. A high mortality (possibly >50%) due to parasitism was observed in this study.