A day-night rhythm of immune activity during scale allograft rejection in the gulf killifish, Fundulus grandis.

The circadian variation of scale allograft rejection was studied in teleost fish maintained on 12-h daily photoperiods (LD 12:12). Immune activity, measured by melanophore breakdown, was two to three times greater during the dark than during the light whether scale allografts were transplanted at light onset or light offset. Because rejection occurred predominantly at night, survival times of both primary and secondary allografts were about 0.4 days longer when the transplants were made at light onset. Immune activity undergoes a robust daily variation in a teleost fish.