Dynamics of serum IgM level during the growth of juvenile Japanese amberjack Seriola quinqueradiata

Immunoglobulins (Igs), also termed antibodies, orchestrate host-acquired immune responses against foreign antigens, including invasive pathogens. In fish, IgM, which is present predominantly circulating in the blood, is particularly important for humoral systemic immunity and protecting the host from pathogens. The efficacy of inactivated vaccines, a major type of vaccine commonly used worldwide in fish, is directly linked to the serum antibody level; however, the timing of the appearance of systemic IgM circulating in the blood has not been determined in fish. In the present study, we examined the dynamics of serum IgM levels in juvenile Japanese amberjack Seriola quinqueradiata, using a highly sensitive sandwich enzyme-linked immunosorbent assay (ELISA) that we developed for IgM. We found that serum IgM concentration in young fish up to 72 days post-hatching (d.p.h.) (mean ± standard error of the mean [SEM]; body weight: 5.73 ± 0.38 g, standard length [S.L.]: 72.2 ± 1.94 mm) was sustained at a low level, but that the level significantly increased from 79 d.p.h. onward, reaching a mean of 84.76 ± 9.23 μg/mL at 85 d.p.h. (body weight: 14.05 ± 0.92 g, S.L.: 101.1 ± 2.07 mm). These results suggest that systemic immunity mediated by IgM is only partially matured in the early growth stage of juveniles. The present findings could help establish effective vaccination programs for infectious diseases in young fish.