Global drivers of size variation in patellid limpets and their implications for marine protection and conservation outcomes

Coastal ecosystems support high biodiversity and provide essential habitats for marine organisms. Limpets are key intertidal grazers that regulate algal dynamics, and their size structure is highly sensitive to harvesting pressure, habitat characteristics, and local environmental conditions, making them useful indicators of human impact and management effectiveness. We conducted a cross-regional, multi-ocean comparison of limpet shell length to evaluate how protection status (No-take vs. Harvested), substrate type, and spatial scale influence size distributions. The dataset comprised 95,809 individuals from 12 species in three genera (Cymbula, Patella, and Scutellastra), spanning Madeira (Portugal), the Canary Islands (Spain), the western Mediterranean and Strait of Gibraltar (Alborán Island, Ceuta, La Línea), the Pacific coast of Mexico (Jalisco and Nayarit), and South Africa (Southern Benguela and Agulhas bioregions). Generalized Linear Mixed Models with hierarchical spatial structure revealed an overall positive effect of protection, with individuals significantly larger in No-take areas across the global model, although responses varied among species and regions. Harvested species most consistently attained larger sizes under protection, whereas non-targeted taxa exhibited weak or context-dependent responses, reflecting differences in exploitation history and ecological interactions. Substrate effects were generally smaller but contributed to local variation in size structure. Variance partitioning indicated that most size variability occurred among sites rather than among regions, highlighting the dominant role of fine-scale environmental conditions, harvesting intensity, and enforcement effectiveness in shaping population size structure. These findings demonstrate that while No-take areas contribute to maintaining larger individuals, conservation outcomes depend strongly on site-level ecological and management contexts. Effective conservation of intertidal assemblages therefore requires site-specific strategies that integrate protection measures with local habitat conditions and enforcement capacity.