Assessment of acquired protection levels against the parasite Neobenedenia girellae (Monogenea) between body surface sites including fins of amberjack Seriola dumerili (Carangidae) and the skin in response to the parasite infection

This study investigated acquired protection against the parasite Neobenedenia girellae (Capsalidae) at multiple body surface sites and fins of the amberjack Seriola dumerili (Carangidae). We also assessed histopathology of skin between naïve and primed fish. Fish, which were previously infected with N. girellae and treated by freshwater bath, was referred to as “primed fish”. The number of N. girella was significantly lower on primed S. dumerili than on naïve fish at 4 days and 8 days after exposure to N. girellae oncomiracidia when they were exposed in the same aquarium. Parasite numbers on pectoral and pelvic fins and on the dorsal (region 2) and belly side of fore body (region 3) were also significantly lower on primed fish than on naïve fish. In addition, the length of parasites was significantly shorter from primed S. dumerili fins than from naïve fish. However, in the head region (region 1), parasite number and length were similar. These results show that acquired protection was high in pectoral and pelvic fins and in regions 2 and 3 but low in region 1. Epidermis from regions 2 and 3 of oncomiracidia-exposed naïve fish at 4 days and 8 days after exposure was significantly thinner than the skin of an uninfected naïve fish, whereas epidermis of oncomiracidia-exposed primed fish at 8 days was significantly thicker than skin of uninfected primed fish. Furthermore, mucus cell numbers in regions 2 and 3 of primed fish was markedly increased at 4 days post-exposure as compared to naïve fish. Taken together, these results suggest that increased numbers of mucus cells and increased mucus secretion in fish skin may have contributed to acquired protection against N. girellae infection.