PHYSIOLOGY OF COLD-HARDINESS IN EARTHWORMS

Physiological, biochemical and behavioral mechanisms involved in the w inter survival of earthworms and earthworm cocoons are reviewed. Overw intering strategies of cold hardy invertebrates are commonly divided i nto two groups. In freeze-avoiding animals, ice formation in the tissu es is lethal and winter survival is dependent on prolonged and extensi ve supercooling. Freeze-tolerant animals tolerate extracellular freezi ng up to a certain fraction of the body water. Hatched earthworms belo ng to the first group, having a behaviorally-based strategy, escaping frost in the soil by migration. The cold hardiness mechanism of earthw orm cocoons is based on a protective dehydration of cocoons occurring when cocoons are exposed to subzero temperatures in a frozen environme nt. Due to the lower vapour pressure of ice compared to supercooled wa ter at a given temperature, water evaporates from the cocoon surface a nd condenses onto ice in the surroundings. Dehydration of cocoons cont inues until vapour pressure equilibrium between cocoon fluids and surr ounding ice is attained. At equilibrium the cocoons cannot freeze sinc e the melting point of cocoon fluids equals ambient temperature. Becau se the melting point of cocoon fluids is high, -0.2 to -0.4 degrees C, the cocoons lose substantial amounts of water, even at mild freezing exposures. As a response to dehydration the embryos of the cocoons acc umulate a polyol, probably sorbitol, which may prevent deleterious eff ects of the extensive water loss coupled with vapour equilibration. Si nce dehydration is a key event when earthworm cocoons are exposed to l ow temperatures it is likely that tolerance to drought is important fo r cold survival. Substantial support for this hypothesis has been foun d in Dendrobaena octaedra which is far more tolerant to drought and co ld than other investigated species. It is suggested that the described ''protective dehydration mechanism'' of cold hardiness should be cons idered in cold hardiness studies of other soil animals with high cutic ular evaporation rates.