Abstract
Long-term parasitological surveys were conducted on the isopod Ceratothoa verrucosa infecting red seabream Pagrus major caught in Iyo-Nada, Ehime Prefecture during the period of 2003–2007. The isopods were almost always in a pair of a male and a female, attached to the buccal cavity wall. The earliest infection occurred when seabream was 0+ year-old and the isopods grew as host fish grew. Prevalence of infection ranged from 12.2% to 21.2% when the host was 0+ to 5-year-old and decreased to 3.4% at 6-year-old and no infection among 7-year-old and older fish. These results suggest that infection was established when the host was 0+ year-old, and their life span of C. verrucose was estimated to be up to 6 years. Infected fish sometimes showed deformation in the upper jaw and growth retardation, but there was no evidence of infection-related mortality. The main reproduction season of the isopod was in summer, estimated by the presence of manca larvae in the marsupium of the female. Sometimes the monogenean Choricotyle elongata attached to the pleotelson of C. verrucosa. Prevalence and abundance of the monogenean were higher in the hyperparasitized seabream than in the fish without isopod infection, suggesting that the isopod favored the monogenean infection.
