These datasets comprise data obtained during a 42 week-long exposure experiment of Mytilus spp. to two types of microplastics, namely spherical polystyrene (PS) particles and irregular polyvinylchloride (PVC) particles. This data was obtained from January 2018 to January 2019 from juvenile blue mussels with an initial size of 10mm. We recorded byssus thread production in 24 hours. Byssus thread production was measured every six weeks only during microplastic exposure. Mussels were exposed to three concentrations of PS (15, 1500, 15 000 particles/individual/week) and five concentrations of PVC (15, 1500, 15 000, 150 000 and 1 500 000 particles/individual/week) and one group to no particles at all as control.