Plasticity and superplasticity in the acclimation potential of the Antarctic mite Halozetes belgicae (Michael).

The plasticity of an organism's phenotype may vary spatially and temporally, and across levels of physiological organisation. Given the adaptive value of plasticity in heterogeneous environments, it might be expected that it will be expressed most in a phenotype's most significant adaptive suites; at high latitudes, one of these is low temperature adaptation. This study examines the phenotypic plasticity of cold acclimation in the Antarctic mite, Halozetes belgicae (Michael). Both plastic and 'superplastic' (extreme plasticity) acclimation responses were found. Plastic responses were evident in responses to laboratory acclimation and field acclimatisation. 'Superplasticity' was found in its ability to rapidly cold harden (RCH) at 0, -5 and -10 degrees C. For example, after just 2 h of acclimation at 0 degrees C, mites acclimated at 10 degrees C shifted their supercooling points (SCPs) by approx. 15 degrees C. In terms of the combined speed of induction and lowering of lethal temperature, this is the most potent RCH response yet reported for a terrestrial arthropod. RCH was also expressed in thermal activity thresholds. Mechanisms responsible for significant differences in recovery from chill torpor are unknown; however, analysis of gut nucleator abundance suggest that the dynamic management of supercooling potential is largely achieved behaviourally, via evacuation. Comparisons with the literature reveal that plasticity in this species varies latitudinally, as well as temporally. The high degree of plasticity identified here is coincident with H. belgicae's occupation of the most exposed spatial niche available to Antarctic terrestrial arthropods.