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CO 2 -driven ocean acidification (OA) affects many aspects of sea urchin biology. However, even in the same species, OA effects are often not univocal due to non-uniform exposure setups or different ecological history of the experimental specimens. In the present work, two groups of adult sea urchins Paracentrotus lividus from different environments (the Lagoon of Venice and a coastal area in the Northern Adriatic Sea) were exposed to OA in a long-term exposure. Animals were maintained for six months in both natural seawater (pH T 8.04) and end-of-the-century predicted condition (−0.4 units pH). Monthly, physiological (respiration rate, ammonia excretion, O:N ratio) and behavioural (righting, sheltering) endpoints were investigated. Both pH and time of exposure significantly influenced sea urchin responses, but differences between sites were highlighted, particularly in the first months. Under reduced pH, ammonia excretion increased and O:N decreased in coastal specimens. Righting and sheltering were impaired in coastal animals, whereas only righting decreased in lagoon ones. These findings suggested a higher adaptation ability in sea urchins from a more variable environment. Interestingly, as the exposure continued, animals from both sites were able to acclimate. Results revealed plasticity in the physiological and behavioural responses of sea urchins under future predicted OA conditions. [Display omitted] • Long-term exposure to reduced pH was performed with sea urchins from different sites. • Seawater acidification affected sea urchin physiological and behavioral parameters. • The effects of reduced pH were less evident in lagoon sea urchins than in coastal ones. • Sea urchin responses change over time possibly related to the gametogenic cycle. • Overall results suggested adaptability of P. lividus to future pH levels. [ABSTRACT FROM AUTHOR]