Sandy beaches are being threatened by a changing climate. However, the effects of this changing environment, including warming, on these ecosystems, have hitherto been tentative and qualitative. Using concurrent long-term (1984−2007) observations on abundance and individual size, together with laboratory examinations of body abnormalities (morphological anomalies and epibionts), we provide evidence that the sandy beach yellow clam Mesodesma mactroides of the Uruguayan coast has responded to climate change.