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Insect herbivory exerts strong selective pressure on plants, yet no study has documented its effects on the halophytic Apiaceae Crithmum maritimum L. (sea fennel). Here, we present the first evidence of natural insect attack on this species, based on five Tunisian coastal populations distributed along a transparent bioclimatic gradient—from sub-humid to semi-arid—and exposed to different levels of herbivory. Using an integrative, multi-trait approach—combining morphological, biochemical, mineral, and lipophilic traits with multivariate analyses (ANOVA with Tukey HSD, PCA, PLS-DA with VIP scores, correlation matrices, HCA and heatmaps, variance partitioning, and dbRDA)—we identified two distinct defense syndromes. Populations from low-herbivory, sub-humid sites (Tabarka, Bizerte) showed higher levels of phenolics, tannins, antioxidants, sterols, PUFA, and structural robustness, indicating a tolerance strategy. Conversely, high-herbivory, semi-arid sites (Haouaria, Monastir) were marked by elevated apiol and terpene levels, sodium and phosphorus accumulation, and reproductive adjustments, reflecting a resistance strategy. The transitional site (Cap Negro) displayed an intermediate profile, demonstrating adaptive flexibility. These findings show that herbivory intensity and bioclimatic conditions jointly influence the defense syndromes of C. maritimum, emphasizing its remarkable phenotypic plasticity and providing the first ecological evidence of insect herbivory in sea fennel.