The marine bivalve Macoma balthica has disappeared from the Spanish part of the Bay of Biscay during the past four decades. Investigating the possible role of climate change in forcing this species up north, we translocated it down south, back into two Spanish estuaries, and followed the thermal acclimatization of the metabolic rate during spring, summer and autumn. Our results reveal that in natural populations the respiratory response to temperature becomes down-regulated during summer and autumn. The respiration rate in the southward translocated populations became down-regulated to a safe level at very high temperatures (31 °C), but remained high at average temperatures instead. These translocated populations showed a gradual reduction of the condition-index, down to the level of starvation at the end of summer. Combined with an increased metabolic rate this indicates that the translocated specimens suffered from elevated maintenance rates. We conclude that short-term but frequent exposure to > 30 °C in the Spanish estuaries, induces elevated maintenance rates in M. balthica, and ultimately starvation. M. balthica indeed disappeared from the northern Spanish coast due to increasing summer maxima during the last decades. We prospect that the southern distribution limit of M. balthica will shift further north, and that the clam will eventually disappear from the entire Bay of Biscay with future increases in summer temperatures.