Seasonal lipid and fatty acid profiles of the invasive blue crab Callinectes sapidus and the native ditch shrimp Palaemon varians in the Bay of Cádiz salt marshes.

Salt marshes support aquatic species of ecological and commercial importance, including the invasive Atlantic blue crab Callinectes sapidus and the native Atlantic ditch shrimp Palaemon varians, both of which are widely consumed in southern Europe. However, year-round comparative information on how co-occurring native and invasive decapods adjust their lipid reserves and fatty acid (FA) composition under natural estuarine conditions remains limited. Here, we compared seasonal variation in lipid classes and FA composition in adult males of both species sampled monthly from salt marsh ponds in the Bay of Cádiz (SW Spain) from February 2023 to January 2024. Environmental conditions in the water varied across seasons, and both species showed seasonal changes in lipid classes and FA composition, but with contrasting patterns. In C. sapidus, triglycerides declined during summer, whereas unsaturated fractions, particularly polyunsaturated FAs, were generally more prominent in late winter and spring. In P. varians, triglycerides increased in autumn, saturated FAs dominated from winter to spring, and monounsaturated and polyunsaturated FAs increased during autumn and early winter. The blue crab showed a more diffuse seasonal biochemical pattern, whereas the ditch shrimp showed a more structured seasonal organization, with broader statistical associations between lipid traits and environmental variables. Seasonal effects were evident in both species, but more pronounced in P. varians. Both analysed matrices contained eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) throughout the year, indicating the presence of nutritionally relevant long-chain n-3 PUFA, although these relative FA percentages should not be interpreted as direct estimates of EPA/DHA intake or food safety. This study provides a year-round biochemical baseline showing that co-occurring native and invasive decapods can display distinct matrix-specific seasonal biochemical trajectories in the Cádiz Bay salt marsh ponds.