In a recent paper, Marshall and Rashid show how relationships between attributes of the calcium carbonate shells of intertidal gastropods can be used to understand organismal responses to coastal acidification. They found that shell surface erosion increased in naturally acidified coastal waters, and that shell growth rate and shell roundness were, respectively, negatively and positively correlated with shell erosion. Because shell erosion is an extrinsic passive marker of acidification, and shell roundness and growth rate are intrinsic performance responders, their interrelationships infer predictive power and improve data interpretation confidence under natural field conditions. This approach of studying shell traits is a step towards overcoming the limitations associated with laboratory (or mesocosm) assessments of organismal responses to marine environmental acidification.