Ocean acidification - the decrease in seawater pH from absorption of atmospheric CO2 - has profound welfare implications for marine animals. Calcifying organisms (corals, molluscs, echinoderms, some crustaceans) experience physiological stress as carbonate ion availability for shell and skeleton formation decreases. Research documents behavioural changes in fish exposed to acidified water including impaired predator avoidance, altered olfactory responses, and disrupted navigation - behaviourally relevant welfare effects. Coral bleaching - driven by both warming and acidification - destroys reef ecosystems supporting millions of species. Shellfish aquaculture faces direct production impacts from acidification, with oyster larvae showing 60-80% mortality at projected 2100 pH levels in some studies. The welfare status of invertebrates affected by acidification is uncertain but precautionary concern is warranted given their ecological roles and the scale of impact. Climate mitigation is the only long-term solution; local interventions include alkalinity addition in shellfish hatcheries.