Effects of the parasitic isopod Ichthyoxenus japonensis Richardson, 1913 (Isopoda, Cymothoidae) on the growth and gonad development of the goldfish Carassius auratus Linnaeus, 1758

A total of 336 specimens of the goldfish Carassius auratus Linnaeus, 1758 were examined in the Baihe River, Henan Province, China, for the presence of the parasitic isopod Ichthyoxenus japonensis Richardson, 1913. Only 20 goldfish (5.95%) were infected with single and paired isopods, and seven fish were previously infected. There was no significant difference in mean body condition (as K = (EW/L-3) x 100, where EW is the eviscerated body weight and L is fork length) between infected (mean 1.450) and uninfected fish (mean 1.448; P = 0.95). The weight of gonads of the infected fish (N = 20) was calculated as zero because the gonads were too small to weigh or to identify the sex. There was also no significant difference in the GSI (gonadosomatic indices) between the infected fish (mean 2.66) and the uninfected individuals (mean 4.80; P = 0.30). The results suggest that the reproductive ability of the host was severely reduced by infection of the isopod, but that the gonads of the castrated fish could recover after the loss of the isopods.