DEHYDRATION OF EARTHWORM COCOONS EXPOSED TO COLD - A NOVEL COLD-HARDINESS MECHANISM

Mechanisms involved in cold hardiness of cocoons of the lumbricid eart hworm Dendrobaena octaedra were elucidated by osmometric and calorimet ric studies of water relations in cocoons exposed to subzero temperatu res. Fully hydrated cocoons contained ca. 3 g water.g dry weight(-1); about 15% of this water (0.5 g.g dry weight(-1)) was osmotically inact ive or ''bound''. The melting point of the cocoon fluids in fully hydr ated cocoons was - 0.20 degrees C. Exposure to frozen surroundings ini tially resulted in supercooling of the cocoon fluids, but over a perio d of 1-2 weeks the cocoons dehydrated (as a result of the vapour press ure difference at a given temperature between supercooled water and ic e) to an extent where the vapour pressure of water in the body fluids was in equilibrium with the surrounding ice. This resulted in a profou nd dehydration of the cocoons, even at mild freezing exposures, and a concomitant slight reduction in the amount of osmotically inactive wat er. At temperatures around -8 degrees C, which cocoons readily survive , almost all(> 97%) osmotically active water had been withdrawn from t he cocoons. It is suggested that cold injuries in D. octaedra cocoons observed at still lower temperatures may be related to the degree of d ehydration, and possibly to the loss of all osmotically active water. The study indicates that ice formation in the tissues is prevented by equilibrating the body fluid melting point with the exposure temperatu re. This winter survival mechanism does not conform with the freeze to lerance/freeze avoidance classification generally applied to cold-hard y poikilotherms. Implications of this cold hardiness mechanism for oth er semi-terrestrial invertebrates are discussed.