Ocean acidification increased the rate at which sponges bored into scallop shells. At pH 7.8, sponges bored twice the number of papillar holes and removed two times more shell weight than at pH 8.1. Greater erosion caused by the lower pH weakened the scallop shells. A warmer water temperature had little effect on sponge growth, survival, or boring rates. Changes in predation associated with pH could have important implications for wild and farmed shellfish populations. (Laboratory study)