A scanning electron microscopy based survey of the impact of infestation by parasitic freshwater mussel glochidia upon the gill morphology of two species of salmonid fish

Unionid mussels are a distinct order possessing a parasitic life history stage known as a glochidium that generally infests the gills of fish. Upon contacting the host tissue, the glochidium 'bites' down causing minor surface trauma but leaving most structural tissue unharmed. Host tissue immediately reacts and encompasses the larval mussel in a cyst where, if able to survive, the glochidia will develop and ultimately excyst as free-living mussels; in the case of the freshwater pearl mussel (Margaritifera margaritifera) adults can live in the sediment for up to 200+ years. While many histological studies have detailed both the encystment process and larval development with a fair degree of detail, few have utilized scanning electron microscopy to add further prospective. Few have investigated the effects of juvenile mussel excystment on host tissue. The freshwater pearl mussel is the longest encysting unionid mussel, remaining on their hosts for close to a year. Here, we investigate three stages of freshwater pearl mussel glochidia development on two host salmonid species (Salmo trutta and S. salar). Our survey supports previously published results and suggests that juvenile mussel excystment causes significantly more harm to host tissue than initial encystment. We provide a large library of images as supplements to this survey for both researchers and educators to use as references, either for educational purposes or out of general interest.