Overwintering adaptations in earthworms

摘要

The aim of this paper is to give a mini-review of the physiological adaptations to frost in earthworms. Low temperature is one of the most important environmental parameters determining the distribution of earthworms. In temperate areas many species avoid frozen soil by migrating to deeper soil layers during winter. Earthworm species of cold regions where the soil is frozen in winter have been shown to be freeze-tolerant, i.e. they endure ice formation in extracellular body fluids. Eisenia nordenskioldi and Dendrobaena octaedra are freeze-tolerant species that rapidly accumulate high concentrations of glucose as a response to freezing. The accumulation of glucose may slow down the freezing process, and even reduce the amount of ice formed. Furthermore, glucose preserves the structure and function of membranes and proteins during the freezing-induced dehydration of tissues. Earthworm cocoons are exposed to frost because they are often deposited in superficial soil layers. Cocoons are not freeze-tolerant but become dehydrated in frozen soil. The dehydration occurs because water potential (water activity) of ice is lower than water potential of supercooled water, if held at the same temperature. These physical properties of ice and supercooled water result in a passive transport of water out of cocoons causing them to become substantially dehydrated. Cocoons Eire well adapted to dehydration. If dehydrated, cocoons will not freeze, even at low sub-zero temperatures, and thus winter survival is ensured.