The Swimbladder Parasite Anguillicola Crassus in Native Japanese Eels and Exotic American Eels in Taiwan

To understand differences in infection patterns of the swimbladder parasite Anguillicola crassus between habitats and eel species in Taiwan, the prevalence and intensity of the parasite were examined based on specimens collected from wild and cultured Japanese eel Anguilla japonica and from exotic cultured American eel A. rostrata in the Kaoping River and culture ponds in southwestern Taiwan in 2006-2007. The prevalence of Aco. crassus in wild Japanese eels was lower in winter compared with summer/autumn, varying 33%-58%, with a mean intensity of 1.5-4.4. The prevalence and intensity were sizedependent and increased with eel size. In cultured Japanese eels, the prevalence and mean intensity varied greatly at 3%-68% and 1.0-29.0, respectively. In cultured American eels, the prevalence and intensity were very high in ponds without drug treatment. In contrast to wild eels, the mean intensity of larval and adult worms showed a size-dependent decreasing trend in cultured eels. The mean body mass of Aco. crassus in American eels was significantly larger than that in Japanese eels. The external morphology, condition factor, and hepatosomatic index showed no significant differences between infected and uninfected groups, indicating a low pathogenic effect of Aco. crassus on these 2 eel hosts. Our results showed that both native Japanese eels and naive American eels are highly susceptible to Aco. crassus, but it causes little pathogenicity under good pond management.