Characterizing antioxidant defense and energy metabolism proxies in a neglected Mediterranean ecosystem engineer: Sabellaria spinulosa (Leuckart, 1849)

Littoral environments are highly productive systems that commonly face intense anthropogenic pressures. Polychaete sabellariid (Annelida) reef ecosystems play crucial roles in coastal environments by providing essential habitat, sustaining biodiversity, mitigating erosion and, thus, conveying valuable ecosystem services. Despite their ecological importance, these sedentary organisms and their physiological state are poorly studied, representing a real gap for assessing ecosystem health and resilience. This study aims at providing valuable baseline data by investigating size-related variations in antioxidant defense mechanisms and energy metabolism in the reef-building polychaete Sabellaria spinulosa (Leuckart, 1849). Specimens were collected from veneer-type bioconstructions growing along the northern Adriatic coast of Italy (Mediterranean Sea) during spring seasons in a two-year field survey. We analyzed key enzymatic biomarkers in adult specimens of S. spinulosa characterized by different sizes. Small-size individuals displayed higher activities of antioxidant enzymes (CAT, GPx, SOD, GSR, and GST) and pyruvate kinase (PK). Conversely, larger specimens exhibited elevated phosphofructokinase (PFK) activity, indicating a metabolism probably more prone to energy storage and stress resilience. The consistency of the observed enzymatic patterns across two sampling years highlights robust, size-dependent physiological differences in S. spinulosa. By establishing this clear physiological baseline, our findings underscore the suitability of this species as a sentinel for environmental monitoring. Its widespread distribution allows for largescale comparative assessments, while its ecological tolerance enables the detection of the sub-lethal stress responses that are crucial for early-warning biomonitoring programs.