Parental competition history shapes offspring responses to thermal stress across generations.

Predicting how populations respond to environmental change remains a central challenge in ecology. While temperature is widely recognized as a major determinant of organismal performance, population responses to thermal stress are also shaped by biotic interactions. Intraspecific competition, in particular, can impose energetic and density-dependent constraints that influence life history traits underlying individual and population fitness. Yet, despite its ecological importance, little is known about whether competition history alters offspring responses to subsequent environmental stress across generations. Here, we investigated how parental competition history shapes offspring responses to cool thermal stress across multiple generations in the marine nematode Litoditis marina (Pm III). Populations were maintained for nine generations under contrasting competition regimes and offspring were subsequently evaluated at either an optimal temperature (20°C) or a cool suboptimal temperature (15°C), representing a deviation below the species' thermal optimum. Cool thermal stress consistently reduced offspring performance, but its effects depended strongly on parental competition history. Under optimal conditions, offspring originating from high-competition populations exhibited slower development but partially compensated through increased fecundity. Under cool thermal stress, however, this compensatory response disappeared: Offspring from high-competition populations showed lower fecundity and reduced population growth relative to offspring from low-competition populations, with these differences becoming increasingly pronounced across generations. Across all measured traits, offspring originating from high-competition populations exhibited slower development, lower reproductive output, and reduced demographic performance under cool thermal stress. These findings demonstrate that parental competition history can strongly influence offspring responses to subsequent environmental challenges and contribute to persistent multigenerational differences in population performance. Our results identify biotic environmental history as an important determinant of population responses to environmental change and highlight the need to integrate both ecological interactions and abiotic stressors when predicting population resilience under future environmental conditions.