Microplastic pollution has become a pervasive environmental challenge due to their global distribution and putatively harmful effects on organisms at different ecotoxicological endpoints. However, in some cases, the effects of microplastics are similar to, or even less harmful than those of naturally occurring particles. Bioplastics, developed as a more sustainable alternative to traditional plastics, still have unclear effects compared with oil-based microplastics. This study uses a laboratory-controlled experiment to compare the physical effects of oil-based high-density polyethylene (HDPE) and bio-based biodegradable polylactic acid (PLA) on the consumption rate, isotopic composition, respiration rate, stoichiometry, and growth rate of model freshwater snail Radix balthica. The experiment included a positive control using silica sand (Si) to simulate natural particles and a control with no experimental additions. It was hypothesised that exposure to microplastics affects the snails solely through the physical influence of the particles, and that the effects of microplastics depend on their composition. The results show that the effects of HDPE were attributable solely due to the physical presence of the particles, but this was not the case for PLA. Exposure to PLA led to increased growth (as final mass). Consumption, respiration, and growth rates were not significantly affected when exposed to the experimental additions. However, the isotopic composition and C:N of snails was altered after exposure to HDPE and Si compared with control. The isotopic composition of snails exposed to PLA did not show significant changes, but they did have higher phosphorous content in their tissues. In this study, HDPE microplastics were not more harmful than naturally occurring particles, and PLA microplastics appeared to influence growth patterns differently than expected. Further research is needed to confirm these observations and fully assess the environmental and biological implications of these contaminants.