Intraspecific genetic variation modulates immune responses to acute heat exposure in an aquatic ectotherm

Climate change-induced heatwaves threaten ectotherms, whose physiology is tightly coupled to ambient temperature. Vulnerability assessments often rely on data from one or a few populations, implicitly assuming uniform thermal sensitivity across species genetic diversity. Quantifying such variation is especially important for traits with wider ecological consequences; our focus here is on immune function, which shapes disease dynamics. We addressed this knowledge gap using ten clonal lineages of the New Zealand snail Potamopyrgus antipodarum exposed to ambient (17{degrees}C) or heatwave conditions (27{degrees}C) for 4 or 8 days. We measured two complementary innate immune traits: general phenoloxidase-like (PO-like) activity, which integrates the activity of multiple phenoloxidase enzymes, and laccase activity, which targets a specific PO enzyme subclass important in mollusc immunity. Heat exposure suppressed both traits, but patterns differed across clones. While PO-like activity declined uniformly, laccase activity showed substantial among-clone variation in heatwave responses at day 4, though these differences converged by day 8. Heat-induced immune suppression is thus trait-specific, depends on genetic background, and varies with exposure duration. Together, these results demonstrate that studies limited to a single genotype, population, or timepoint risk miscalculating species-level vulnerability.