Species-specific digestive enzyme profiles reveal trophodynamic differences between blue mussels and their endoparasitic copepods but limited responses to feeding, starvation, and microplastic exposure.

Blue mussels (Mytilus edulis) from the southern North Sea are exposed to multiple natural and anthropogenic stressors, including parasite infestation and microplastic pollution. For example, the intestinal copepods Mytilicola intestinalis and Mytilicola orientalis are common endoparasites of blue mussels in the coastal areas. We investigated the prevalence and composition of Mytilicola species in mussels from the Wadden Sea. The infestation rate of M. edulis by Mytilicola spp. was 78%. One third of infested mussels harboured both parasite species. These parasites may feed on intestinal tissue, detached cells, or the gut content of the host which also may contain natural and anthropogenic microparticles that were ingested by the mussel. In a 5-day laboratory experiment, mussels either received no food, were fed with microalgae, or were exposed to microplastics. Activities of selected digestive enzymes were measured in the midgut gland of the mussels and in whole parasites. Digestive enzyme profiles revealed significant functional differences between the host and either parasite. Mussels showed high esterase and β-glucosidase activities, reflecting the utilization of algal food. In contrast, both Mytilicola species showed low β-glucosidase activities. Elevated α-glucosidase and N-acetyl-β-D-glucosaminidase activities in M. orientalis suggest the utilization of glycogen and mucus from the host. Treatments involving starvation, feeding, and microplastics had no significant effects on the enzyme profiles of the host, and there was no correlation between the enzyme activities of the host and its parasites. Enzyme profiles proved useful for interpreting trophodynamic interactions between mussels and their Mytilicola endoparasites, but there were limited responses to exposure conditions for all species.