Humoral and mucosal defense molecules rhythmically oscillate during a light–dark cycle in permit, Trachinotus falcatus

• Circadian rhythm imposes regulatory control over many physiological systems.
• The interplay of circadian rhythm and immunity in fish is barely explored.
• Selected defense factors were more active in serum than mucus in permit fish.
• Defense molecules of permit exhibited robust daily rhythmicity.
• Daily profiles of serum and skin mucus showed distinct circadian features.

Circadian rhythm provides organisms with an internal system to maintain temporal order in a dynamic environment. This is typified by a 24-h cycle for a number of physiological processes, including immunity. The present study characterized the humoral and mucosal defense molecules and their dynamics during a light–dark (LD) cycle in juvenile permit, Trachinotus falcatus. All studied defense molecules were constitutively identified in serum and skin mucus. Serum generally exhibited higher levels of these defenses than skin mucus, with the exception of anti-protease (ANTIPRO). The difference in ANTIPRO, lysozyme (LYZ), esterase (ESA) and catalase (CAT) levels between serum and skin mucus was not affected by the phase of the daily cycle. However, a clear phase-dependent difference was observed in protease (PRO), globulin (GLOB), myeloperoxidase (MPO), alkaline phosphatase (ALP) and glutathione peroxidase (GPX) levels. Activities of ALP and GPX displayed significant daily rhythmicity in both serum and skin mucus. Circadian profile of ALP was identical in both biofluids, but an antiphasic feature was exhibited by GPX. GLOB and MPO levels also exhibited significant daily oscillation but only in serum with acrophases registered at ZT 14.5 and 6.15, respectively. Mucus PRO and serum ANTIPRO demonstrated significant temporal variations during a daily cycle albeit not rhythmic. Cluster analysis of the defense molecules in serum and skin mucus revealed two different daily profiles suggesting a possibility of distinct circadian control between humoral and mucosal immunity. These observations indicate that LD cycle had a remarkable impact in the defense molecules characterizing the humoral and mucosal immunity in permit. Daily rhythmic patterns of these defense molecules contribute to our understanding of the barely explored interplay of immunity and circadian rhythm in teleost fish. Lastly, the results could be useful in developing aquaculture practices aiming at modifying the immune functions of permit for improved health.