Seasonality affects key physiological pathways in Pontastacus leptodactylus during an Aphanomyces astaci epidemic outbreak.

Freshwater crayfish Pontastacus leptodactylus is an indigenous European species of considerable ecological and commercial significance. However, it is highly susceptible to the oomycete Aphanomyces astaci, the causative agent of crayfish plague. Although its importance is widely recognized, little information exists on how seasonality affects the physiology and welfare of this species. The present study was designed to (i) investigate seasonal effects on the physiological responses of P. leptodactylus in relation to abiotic factors and crayfish plague outbreak, and (ii) identify the most suitable period for restocking in temperate regions. To achieve this, we characterized for the first time the mRNA expression of hsp70, hsp90, hif-1α, ubiquitin, and gapdh (as endogenous control gene), alongside protein levels of Hsp70, Hsp90, hydroxylated Hif-1α, ubiquitin conjugates, cleaved caspases, Bax/Bcl-2 ratio, and interleukin-6 (IL-6). Our findings revealed that cold stress triggered both the heat shock response and apoptosis. Interestingly, despite the activation of apoptosis, which generally suppresses inflammation, IL-6 levels increased, suggesting a possible association with A. astaci infection. Moreover, elevated Hif-1α levels indicated hypoxic stress during warmer months, likely linked to reduced oxygen availability and turbidity. Overall results suggest that crayfish plague epidemics may intensify with rising temperatures, whereas winter appears to be the optimal period for restocking. Interestingly, in addition, freshwater crayfish in temperate latitudes are organisms that face both cold and heat stress seasonally in the same ecosystem.