Seasonality in the immune defence of invertebrates can coincide with environmental variation but whether it is endogenously regulated, via biological clocks, or affected by previous immune challenges remains unclear. Using the native noble crayfish (Astacus astacus) held under constant laboratory conditions for a year, we explored (1) potential endogenous seasonal variation in immune defence, i.e. the encapsulation response, (2) the potential positive effect of repeated challenges with a standardized immune insult in subsequent seasons, i.e. long-lasting immune priming, and (3) whether long-lasting tune priming is dependent on endogenous seasonality. Independent measurements of the encapsulation response in different seasons revealed significant variation and a decrease in autumn. This result indicates previously undetected endogenous seasonal variation in invertebrate immunity. The weaker immune defence observed in autumn, i.e. the reproductive season of crayfish, might be caused by a circannual clock. When corrected for endogenous seasonality, we found no evidence for long-lasting immune priming.