# Potential effects of acanthocephalan and microsporidian parasites on the trophic status of the freshwater isopod Asellus aquaticus

## TL;DR

This study explores how two types of parasites affect the feeding habits of a freshwater isopod, revealing changes in their nutrient levels over time.

## Contribution

The study identifies new microsporidian species and shows how parasite infections influence isopod trophic status.

## Key findings

- Microsporidian-infected isopods showed higher nitrogen levels, indicating possible metabolic changes.
- Acanthocephalan-infected isopods had elevated nitrogen levels in specific months, possibly due to parasite development.
- Parasite infections in isopods varied seasonally, with a strong peak in microsporidian infections in November.

## Abstract

Parasites are known for their ability to induce a variety of changes in their respective hosts, including morphological characteristics and trophic interactions. For many host-parasite relationships, however, these aspects are yet to be explored. We assessed the occurrence of acanthocephalans and microsporidians in a population of the isopod Asellus aquaticus from a stream in western Germany over several months. We aimed to contrast the trophic positions of Acanthocephala-infected, Microsporidia-infected and uninfected isopods by assessing the stable isotope ratios for nitrogen (δ15N) and carbon (δ13C). We found acanthocephalans of the genus Acanthocephalus as well as five different microsporidian species, three of which are novel isolates. Prevalences were generally low among the 538 tested isopods (1.3% in September to 4.0% in January for acanthocephalans, and 0.7% in January to 12.3% in November for microsporidians), with a strong peak of microsporidian infections in November. The stable isotope analysis revealed temporal shifts in both δ13C and δ15N values, probably corresponding to dietary changes. Isopods infected with the microsporidian isolate EFB02 were enriched in 15N compared to uninfected ones, suggesting possible infection-associated physiological or metabolic changes. Acanthocephalan-infected isopods resembled uninfected ones in the two autumn samplings, but showed elevated δ15N values in September and January. This pattern may reflect active development of cystacanths in September and January, possibly linked to higher nutrient demands. Our findings emphasize the ecological importance of parasite infections in freshwater detritivores and underscore the need to consider the environmental and temporal context in host-parasite trophic studies.