1. Biological capacities to respond to changing environments dictate success or failure of populations and species over time. The major environmental feature in this context is often temperature, and organisms across the planet vary widely in their capacity to cope with temperature variation. With very few exceptions, Antarctic marine species are more sensitive to temperature variation than marine groups elsewhere, having survivable temperature envelopes between 5degreesC and 12degreesC above the minimum sea temperature of -2degreesC. 2. Our findings show that in biological functions important to long-term survival these animals are even more tightly constrained. The Antarctic bivalve mollusc Laternula elliptica and limpet Nacella concinna both survive a few days in experiments at 9-10degreesC, but suffer 50% failure in essential biological activities at 2-3degreesC and complete loss at 5degreesC. The Antarctic scallop Adamussium colbecki is even more sensitive, and loses the ability to swim as temperature approaches 2degreesC. 3. These failures of activity are caused by a loss of aerobic capacity, and the animals investigated are so sensitive that a 2degreesC rise in sea temperature could cause population or species removal from the Southern Ocean.