Grazing preferences of Sarpa salpa on temperate habitat-forming macroalgae

Herbivory is intensifying on many temperate reefs under ocean warming and tropicalisation, raising concerns for canopy-forming seaweeds. Along the Portuguese coast, the obligate herbivore Sarpa salpa is abundant and frequently observed grazing on kelps and fucoids. We tested whether wild S. salpa shows consistent feeding preferences among four habitat-forming brown macroalgae— Laminaria ochroleuca , Fucus spiralis , Sargassum muticum and Cystoseira humilis —and whether simple stoichiometric signals (C and N) align with observed choices. Using outdoor, submerged mesocosms at Sines region (SW Portugal), we ran four sequential no-choice trials conducted over a 12-day period (with 48 h intervals between session) followed by choice assays (days 5–6). Consumption was recorded as 24-h wet-mass loss, standardised to grazer biomass (g·kg⁻¹), and elemental C and N were measured in algal tissues and fish faeces to infer assimilation. Fish readily consumed all species in no-choice trials, indicating no gross non-palatability. Under simultaneous availability, however, S. salpa consistently consumed more F. spiralis , L. ochroleuca and S. muticum than C. humilis , revealing non-uniform selection. Tissue composition differed among species: L. ochroleuca had the highest N and lowest C:N ratio, which aligned with the highest apparent N assimilation by the fish. This clear link between simple stoichiometry and assimilation suggests that species-specific nutritional payoffs are a key mechanistic driver of preference. Given the warming and tropicalising context of NE Atlantic reefs, this trait-based selectivity is significant: it may amplify canopy vulnerability (particularly of high-quality species) and favour turf-dominated states at warm range margins. We recommend integrating herbivory metrics (selectivity, bite rates, grazer biomass) into monitoring, conservation and restoration to safeguard Portuguese marine forests.