Coastal environments are increasingly exposed to microplastic pollution. It is known that large microplastics impair the filtration capacity of bivalves, which provide the invaluable ecosystem service of alleviating the effect of coastal eutrophication through their biofiltration mechanism. However, the effect of smaller microplastics, and specifically microfibers, is still unknown despite the fact that they are the most common microplastics in the coastal marine habitats and similar in size to phytoplankton, the main food source of bivalves. Hence, in this study we investigate the effects of long-term exposure to microfibers (