Seagrasses such as Cymodocea nodosa are crucial to coastal ecosystems, but little is known about their chemical defenses against herbivory. This work evaluated two hypotheses: (1) herbivory triggers the production of certain secondary metabolites in C. nodosa, and (2) these chemicals structurally reflect defensive mechanisms seen in terrestrial and freshwater plants. Feeding studies with mesograzers, coupled with LC-MS-based metabolomic profiling, resulted in the identification of 43 potential secondary metabolites (Level 2 confidence). Among them, 79% (n = 34) were solely identified in grazed (induced) samples, whereas just 21% (n = 9) were present in ungrazed controls. Samples triggered by cues (Sample 32) revealed no distinctive metabolites, suggesting that direct tissue injury is necessary for chemical induction. Induced substances comprised phenolic acids (e.g., caffeic, rosmarinic, ferulic acids), flavonoid glycosides (e.g., quercetin-3,4′-diglucoside), and nitrogenous compounds (e.g., putrescine, methylguanidine), all of which were missing in controls and functionally confirmed via grazer avoidance behaviour. Chi-square analysis indicated no significant difference in the proportionate distribution of compound classes (χ² = 0.376, p = 0.829); nevertheless, absolute compound richness was markedly greater in grazed tissues. The findings indicate the existence of inducible, functionally significant chemical defences in C. nodosa, which have implications for seagrass resilience and marine plant-herbivore interactions.
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