M. Holmstrup, PHYSIOLOGY OF COLD-HARDINESS IN COCOONS OF 5 EARTHWORM TAXA (LUMBRICIDAE, OLIGOCHAETA), Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology, 164(3), 1994, pp. 222-228
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.