Effect of host competence on infection of G. kobayashii
Given that decreases in density of susceptible hosts may lead to a dilution effect (Keesing et al. 2006; Johnson & Thieltges 2010; Johnson et al. 2013), goldfish densities were maintained constant in the experiments. Based on different competences of the 12 fish species (Table 1), three were selected for the mixed experiment with goldfish, including the silver crucian carp (Carassius auratus gibelio ) with low establishment success, the swordtail (Xiphophorus helleri ) and the grass carp (Ctenopharyngodon idellus ), both without establishment success. Then these fish were randomly assigned to five groups, including a control group (10 goldfish), a silver crucian carp group (10 goldfish and 10 C. auratus gibelio ), a grass carp group (10 goldfish and 10 C. idellus ), a swordtail group (10 goldfish and 10 X. helleri ) and a multi-species group (10 goldfish, 3 C. auratus gibelio , 4 C. idellus and 3 X. helleri ), which is an additive design. Each group was maintained at a constant density of goldfish and replicated five times. Fish were maintained in a round tank [60 (diameter) × 60 (depth) cm] containing 50-L dechlorinated tap water at 20 ± 1 °C, and fed twice daily with commercial pellet feed. To keep the water in good condition, feces and uneaten food were removed regularly, and one-third of the water was changed every two days.
After five days of acclimation, a donor goldfish (marked with particular body color) heavily infected with G. kobayashii (n = 170-180) on its fins was introduced into each tank. After 10-days, gyrodactylids were counted on fins of each fish anesthetized with MS-222 except for the donor goldfish using a stereomicroscope. Since the majority of G. kobayashii were found on fins of goldfish in the preliminary experiments, only fins were included in the parasitological examination to reduce potential effects of manipulation on the fish. Mean abundance was calculated on each infected species, while total mean abundance was determined at the host community level.