For the first time, the structure, dimensions, and composition of the epidermis of an invasive earthworm species that has successfully colonized hostile conditions in actively volcanic soil on São Miguel (Azores) have been measured. Metal concentrations in actively volcanic (Furnas) and volcanically inactive (Fajã) soils were similar; however, Furnas soil was characterised by elevated temperature (10°C differential), relative hypoxia, extremely high CO2 tension, and accompanying acidity. The epidermis of earthworm's resident at Fajã was approximately twice the thickness of the epidermis of conspecifics resident in Furnas soil. Reference worms transferred to Furnas soil for 14 days experienced an epidermal thinning of approximately 51%. In comparison, when Furnas earthworms were transferred to mesocosms at the relatively benign Fajã site, their epidermal thickness increased by approximately 21% over 14 days. Earthworms resident in Furnas soil had higher goblet cell counts than the residents of volcanically inactive soil on a neighbouring island (S. Maria). Transferring worms from S. Maria to mesocosms at Furnas induced a significant increase in goblet cell counts. Clearly, the active volcanic environment at Furnas poses a multifactorial stress challenge to the epigeic A. gracilis colonizer.