To Freeze or Not to Freeze? Invertebrate Survival of Sub-Zero Temperatures

Low temperature tolerance in invertebrates is complex, and is compounded by a great diversity of response to cold temperatures even within a single taxon such as the insects. Invertebrates inhabit a very wide range of terrestrial environments, in which the physical conditions may fluctuate both daily and over a longer time span, and hence their response to a single physical variable such as temperature is far from uniform. Block (1990) emphasized the need to distinguish between the responses of insects to temperatures just above 0'C (non-freezing) and sub-zero temperatures (potentially freezing). This paper is concerned with the latter suite of responses. Ecophysiologists have attempted to synthesize information on low temperature tolerance of invertebrates, particularly for arthropods. For example, from the ecological perspective this has been attempted by Downes (1964), Danks (1978), Bale (1989) and S0mme (1989), whilst Baust & Rojas (1985), Zachariassen (1985), Cannon & Block (1988), and Storey & Storey (1988, 1989) have incorporated a physiological-biochemical approach. There is not, at present, a universally accepted theory of cold tolerance applicable to invertebrates generally, but several recent developments suggest that a, reassessment is timely. In this paper one particular aspect of low temperature tolerance is highlighted; if an invertebrate can tolerate and survive the effects of freezing. The phenomena of freezing tolerance and intolerance are briefly discussed together with their perceived advantages and disadvantages. The question of switching from one to the other is addressed and proposals are made for future experimental work to resolve some of the outstanding problems.