Chemical contamination threatens marine biodiversity, but detecting chronic sublethal effects on early-life development remains challenging. In this study, we developed a chronic larval bioassay based on repeated monitoring of development in the shrimp species Palaemon serratus, which is representative of European coastal environments. The larvae were exposed to the insect growth regulator fenoxycarb (0-64 µg.L-1), and the following endpoints were quantified: survival, stage progression, metamorphosis, juvenile growth and energy reserves at metamorphosis. Our objectives were to: (i) define larval life-history and fitness traits that are informative for chronic testing; (ii) propose a statistical workflow suited to heterogeneous endpoint types (binary, continuous and censored) and non-monotonic dose-response relationships; (iii) and investigate the importance of inter-female variability in response. Fenoxycarb altered developmental dynamics, with stage-timing endpoints frequently exhibiting non-monotonic (U-shaped) patterns; development was accelerated at low doses and slowed at higher doses. In contrast, metamorphosis success decreased monotonically with increasing concentration. Juvenile mass declined by ∼20% at intermediate doses, with partial recovery observed at higher doses. Meanwhile, daily weight gain decreased progressively across the gradient. Energy stores at metamorphosis also shifted with exposure, suggesting effects on condition beyond simple delays in development. Several endpoints displayed significant variability between females, indicating that maternal effects can substantially modulate apparent sensitivity. Overall, the experimental design and analysis pipeline capture sublethal and non-monotonic chronic effects, providing a practical framework for incorporating maternal variability into marine chemical risk assessment.
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