Plasticity in arthropod cryotypes.

Low-temperature acclimation and acclimatization produce phenotypic changes in arthropods at multiple levels of biological organization from the molecular to the behavioural. The role and function of plasticity - where a constitutive, reversible change occurs in the phenotype in response to low temperature - may be partitioned hierarchically at evolutionary scales according to cryoprotective strategy, at macrophysiological scales according to climatic variability, and at meso- and micro-scales according to ecological niche and exposure. In correspondence with these scales (which are interdependent rather than mutually exclusive), a hierarchical typology of interaction between thermal history and organism is proposed, descending, respectively, from what we define as 'cryotype' (class of cryoprotective strategy) to genotype and, ultimately, phenotype. Alternative (and sometimes complementary) strategies to plasticity include specialization, generalization, bet-hedging, cross-resistance and convergence. The transition of cryotypes from basal to derived states is a continuum of trait optimization, involving the fixation of plasticity and/or its alternatives.