Lakhasly

arasites can affect their hosts in various ways, and this implies that parasites may act as additional biotic stressors in a multiple-stressor scenario, resembling conditions often found in the field if, for example, pollutants and parasites occur simultaneously.After briefly explaining their life cycles, we focus on parasite stages affecting selected ecotoxicologically relevant target species belonging to crustaceans, molluscs, and fish.

arasites can affect their hosts in various ways, and this implies that parasites may act as additional biotic stressors
in a multiple‐stressor scenario, resembling conditions often found in the field if, for example, pollutants and parasites occur
simultaneously. Therefore, parasites represent important modulators of host reactions in ecotoxicological studies when
measuring the response of organisms to stressors such as pollutants. In the present study, we introduce the most important
groups of parasites occurring in organisms commonly used in ecotoxicological studies ranging from laboratory to field
investigations. After briefly explaining their life cycles, we focus on parasite stages affecting selected ecotoxicologically
relevant target species belonging to crustaceans, molluscs, and fish. We included ecotoxicological studies that consider the
combination of effects of parasites and pollutants on the respective model organism with respect to aquatic host–parasite
systems. We show that parasites from different taxonomic groups (e.g., Microsporidia, Monogenea, Trematoda, Cestoda,
Acanthocephala, and Nematoda) clearly modulate the response to stressors in their hosts. The combined effects of environmental stressors and parasites can range from additive, antagonistic to synergistic. Our study points to potential drawbacks of ecotoxicological tests if parasite infections of test organisms, especially from the field, remain undetected and
unaddressed. If these parasites are not detected and quantified, their physiological effects on the host cannot be separated
from the ecotoxicological effects. This may render this type of ecotoxicological test erroneous. In laboratory tests, for
example to determine effect or lethal concentrations, the presence of a parasite can also have a direct effect on
the concentrations to be determined and thus on the subsequently determined security levels, such as predicted no‐effect
concentrations.