The response of Allolobophora chlorotica to drought stress in four soils

Earthworms are key contributors to healthy and productive soils, yet their reliance on water makes them vulnerable to the increased frequency and severity of droughts predicted under climate change. To avoid desiccation, some earthworms induce aestivation, a period of reduced metabolism during which they coil up and seal themselves into a chamber until conditions improve. However, the environmental conditions that trigger aestivation remain poorly understood. Here the responses of Allolobophora chlorotica, a common UK earthworm, to gradual air drying (at 15 {+/-} 1 {degrees}C) were examined in four soils differing in texture. Earthworm activity (active or aestivating) and mass change relative to initial values (pre- and post-24 hours hydration) were measured at three gravimetric moisture contents (~19.7, 15.55 and 12.39 wt%) and three water potentials (~pF 1.59, 2.92 and 4.05). Water potential, rather than bulk water content, was the strongest predictor of behaviour. All earthworms remained active and gained ~36-53 % mass at the highest water availability (~pF 1.59), but 100 % aestivated at the lowest (~pF 4.05) in all but the sandiest soil. In contrast, responses at equal gravimetric moisture contents varied by soil type. All individuals in the clay aestivated and lost up to ~45 % mass, whereas those in sand and sandy loam soil largely remained active and gained mass. Differences likely reflect textural constraints on movement and the construction of aestivation chambers, which were fragile in sandy soils but more robust in clay-rich soils. After 24 hours of hydration, all earthworms had increased beyond their starting mass, indicating changes in mass were largely due to reversible water loss. However, some residual mass differences between control and drying treatments suggest differences in tissue mass, potentially attributable to suspended feeding and clitellum regression, characteristic features of aestivation. Overall, these findings show that Al. chlorotica is highly desiccation tolerant, but that soil texture strongly modulates both the onset and viability of aestivation, with implications for predicting earthworm resilience under future drought regimes.