Heat hardening in Antarctic notothenioid fishes

Many ectotherms rapidly acquire a short-lived increase in heat tolerance following a heat shock. This capacity to heat harden has been noted in a number of temperate fishes, but it is unknown whether it can also be found among the stenothermal Antarctic notothenioid fishes. To investigate, specimens of six notothenioid species were first brought to their critical thermal maxima (CTMax), and then following a recovery period of 4-24 h, a second CTMax was determined for each species to test for an increase in heat tolerance. All six species showed a significant increase over their initial CTMaxs, providing evidence for the existence of heat hardening in notothenioids. The magnitude of this increase ranged from 0.6 +/- A 0.29 to 1.8 A degrees C +/- A 0.45, comparable to previously reported values from several temperate fishes and amphibians. This suggests that the heat hardening of Antarctic notothenioids remains undiminished despite their limited heat tolerance and provides further evidence that these fishes retain plasticity in their responses to heat despite their historical residence in a constant cold environment.