Growth media change Eisenia fetida epithelium thickness: implications for improving earthworm welfare in...

Because the earthworm tegument represents their first barrier against environmental aggressions, we hypothesised that substrate-induced changes in earthworm growth would also alter the morphology of this protective layer. Therefore, a histological and morphometric study was performed on Eisenia fetida specimens that have been grown in five different organic residues: cow manure, horse manure, grape marc, coffee grounds, and a mixture of coffee grounds and cooking oil. The results showed that, across all treatments, both cuticle and epidermis were significantly thicker in the pre-clitellar region than after the clitellum, attributed to the fact that the former region is responsible for breaking up the soil. Notably, the growing media significantly altered the thickness of these two integumentary layers, and the greatest thicknesses were measured in grape marc and in the mixture coffee grounds + cooking oil, which makes them suitable organic residues for vermicomposting. By contrary, a significant epidermal thinning was observed when growing on coffee grounds alone, clearly indicating that it is a hostile environment for earthworms. Overall, these results reveal that tegument thickness can be used not only as a trait indicator of substrate quality but also of earthworm welfare, and it should be integrated in future toxicity assessment and captive breeding guidelines.