Seasonal changes in the humoral immune response and the lymphoid tissues of the marine teleost, Sebastiscus marmoratus.

Seasonal modulation in antibody production in relation to the state of lymphoid tissue development was studied in the ovoviviparous fish, S. marmoratus. Animals were kept in tanks with running seawater at 23 +/- 1 degrees C for a minimum 2 week acclimatization period and then immunized three times at 2 day intervals with 20% SRBC (5 microliters/g body weight) intraperitoneally. Immunized fish were bled 2 weeks after the first injection at the time of peak response and plasma were analyzed for titration. The thymus and pronephros were weighed and the number of leukocytes counted in the cell suspension. Antibody levels in fish immunized in summer were higher than those in fish immunized in winter, even if the environmental temperature was held constant. Furthermore, the reactivity of mature females to SRBC was lower than that of males or immature females in the spawning season (winter). The antibody titre was inversely related to the weight of the thymus with the exception of females during the spawning season, while the weight of the pronephros did not show any significant change throughout the year. In addition, the thymus of pregnant, and especially post-spawning females, was entirely involuted, showing a marked decrease in the number of lymphocytes in both the cortex and medulla. Preliminary experiments to examine the effect of photoperiod on antibody production indicated a slight increase in titre among adult fish under a long photoperiod regimen. The present data show that both the season and stage of sexual maturity affect the humoral immune response and lymphoid tissues, and suggest that the thymus might have a function other than that concerned with immunity.