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

## TL;DR

This paper shows that a parasite in earthworms can affect their reproduction and may skew results in soil toxicity studies.

## Contribution

The study reveals that Monocystis sp. infection in Eisenia fetida can bias ecotoxicological results by causing reproductive impairment.

## Key findings

- Monocystis sp. infection reduces sperm production and alters seminal vesicles in Eisenia fetida.
- Infected earthworms show immune responses like encapsulation and melanization in reproductive organs.
- Parasite-induced changes in earthworms resemble pollutant effects, potentially biasing ecotoxicological studies.

## Abstract

Earthworms are widely used to evaluate soil quality because they are sensitive to contaminants and easy to maintain under laboratory conditions. Little attention has been given to the initial health status of the animals that may affect the biological responses measured in ecotoxicological studies. In this study, we found that many individuals of E. fetida were naturally infected by Monocystis sp., a parasite known to inhabit the male reproductive organs of earthworms. The infection caused a reduction in the number of spermatozoa, disruption of the seminal vesicle, and encapsulation of the parasite. Therefore, the parasite can lead to reproductive impairment, which could be wrongly associated with the effects of pollutants. Our findings suggest that Monocystis sp. may bias soil-toxicity results and highlight the importance of screening laboratory earthworm cultures for parasites to ensure reliable ecotoxicological results.

Eisenia fetida 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 E. fetida and report the occurrence and histological alterations associated with Monocystis 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 Lumbricus terrestris. These findings suggest species-specific differences in immune response and indicate that Monocystis sp. infection can induce reproductive impairment and activate energy-consuming immune responses. Because these parasite-induced changes closely resemble pollutant-driven ecotoxicological effects, Monocystis 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 E. fetida as a bioindicator.