PHYSIOLOGY OF COLD-HARDINESS IN COCOONS OF 5 EARTHWORM TAXA (LUMBRICIDAE, OLIGOCHAETA)

Earthworm cocoons are mostly found in the uppermost soil layers and ar e therefore often exposed to low temperatures during winter. In the pr esent study, cocoons of five taxa of earthworms were investigated for their tolerance to freezing, melting points of cocoon fluids and dehyd ration of cocoons when exposed to a frozen environment. Embryos of the taxa investigated were freeze intolerant. The melting points of fully hydrated cocoon fluids were high (above -0.3 degrees C) and thermal h ysteresis factors were absent. Exposure to a frozen environment caused the cocoons to dehydrate drastically and dehydrated cocoons showed si gnificantly lower supercooling points than fully hydrated cocoons, red ucing the risk of freezing for dehydrated cocoons. It is proposed ther efore that the cold-hardiness strategy of the earthworm cocoons is bas ed on dehydration upon exposure to subzero temperatures in the frozen environment. Cocoons of three surface-dwelling taxa, Dendrobaena octae dra, Dendrodrilus rubidus tenuis and Dendrodrilus rubidus norvegicus h ad lower supercooling points and survived frost exposure better than c ocoons of two deeper-dwelling taxa, Aporrectodea caliginosa and Allolo bophora chlorotica. One of the investigated taxa, D. r. norvegicus, wa s collected from a cold alpine habitat. However, it was not more cold hardy than the closely related D. r. tenuis collected from a lowland t emperate habitat. D. octaedra was the most cold hardy taxon, its cocoo ns being able to withstand -8 degrees C for 3 months and - 13.5 degree s C for 2 weeks in frozen soil.