When the Soil Turns Hostile: A Comparative Study of Aestivation in Riparian and Terrestrial Earthworms.

In the Iberian Peninsula, intensifying summer aridity poses a significant threat to soil fauna. Earthworms, as key ecosystem regulators, often rely on aestivation-a dormancy state involving metabolic depression-to survive these hostile conditions. However, little is known about this strategy in riparian species like Allolobophora molleri, whose reliance on waterlogged soils and its narrow distribution make it particularly vulnerable to climate change compared to cosmopolitan species like Aporrectodea trapezoides. Here, we investigate the aestivation response of A. molleri in a comparative framework with A. trapezoides, identifying the specific temperature and moisture thresholds that trigger dormancy. We further assessed the associated physiological costs through changes in weight, tissue water content and lipid peroxidation to understand their resilience to abiotic stress. Our results highlight soil moisture as the primary driver of aestivation, though temperature also acted as a constraint for A. molleri. Both species suffered significant biomass loss, however, lipid peroxidation levels remained stable, indicating no measurable oxidative damage. Aestivating individuals of A. molleri retained slightly higher tissue water content compared to active individuals. Morphological changes, such as a differentiated region in the posterior segments, were observed in aestivating individuals of both species. Finally, moisture restoration triggered a rapid recovery of activity, confirming the facultative nature of this dormancy. These findings show the different ecology of both species and provide key insights into their resilience under future climate change scenarios.