Worms from the cold: Lumbricid life stages in boreal clay during frost

The vertical distribution and activity of earthworm life stages were studied in an arable field during 0.5 m deep frost. The anecic Lumbricus terrestris L. were below the frost at the bottom of their home burrows (max. depth 1.0 m) and remained there apparently active. Their burrows were open, free of ice and water. The endogeic Aporrectodea caliginosa Sav., mainly small juveniles, were aestivating in the frost layer, which confirms freeze-tolerance in this species. Large A. caliginosa individuals were actively burrowing below the frost down to 1 m depth at soil temperatures close to +1 °C, frost evidently triggering much deeper burrowing than summer droughts. Demonstrating cold-hardiness, viable cocoons of both A. caliginosa and L. terrestris were obtained within a 0–0.25 m layer, frozen for ca. one month prior to sampling. These two common earthworms of boreal soils seem to over-winter in all life stages and remain active below the frost, potentially contributing to the maintenance of subsoil processes during the winter months.