Monocystis sp. As Possible Bias in the Use of Eisenia fetida for Ecotoxicological and Ecopathological Studies

<i>Eisenia fetida</i> is one of the soil invertebrates most used in ecotoxicological and ecopathological studies. To date, the potential contribution of naturally occurring parasites to the variability of ecotoxicological endpoints has been poorly investigated. In this study, we provide a detailed histological description of the male reproductive system of <i>E. fetida</i> and report the occurrence and histological alterations associated with <i>Monocystis</i> sp. infection in laboratory-reared individuals. Uninfected individuals exhibited complete spermatogenesis, with all developmental stages from spermatogonia to mature spermatozoa and normal sperm storage within the spermathecae. Meanwhile, infected earthworms displayed marked reproductive alterations, including reduced sperm production and diminished sperm retention within spermathecae. Multilayered encapsulations, inflammatory nodules and melanization were detected within the seminal vesicles, in contrast with the immunological evasion observed in <i>Lumbricus terrestris</i>. These findings suggest species-specific differences in immune response and indicate that <i>Monocystis</i> sp. infection can induce reproductive impairment and activate energy-consuming immune responses. Because these parasite-induced changes closely resemble pollutant-driven ecotoxicological effects, <i>Monocystis</i> infections may act as a potential bias in ecotoxicological studies. We therefore recommend implementing parasitological screening of laboratory cultures to ensure the reliability of studies employing <i>E. fetida</i> as a bioindicator.