Contrasting life-history strategies drive divergent microplastic responses in a cryptic marine nematode species complex

Cryptic diversity is often overlooked in ecotoxicological risk assessment, potentially leading to underestimated environmental risks. This study investigates, for the first time, the differential sensitivity to microplastics (MPs) within a marine cryptic species complex, using three cryptic species of the nematode morphospecies Litoditis marina (Pm I, Pm III, and Pm IV). We assessed both acute (72 h) and sublethal (10-day, i.e. two generations) responses to 1-μm polystyrene microspheres across environmentally relevant particle densities. Acute toxicity tests revealed a clear sensitivity gradient: Pm III was the most sensitive species, Pm IV showed intermediate tolerance, and Pm I was highly tolerant. Sublethal responses were more complex and divergent, challenging the simple hierarchy observed in acute tests. We observed three distinct patterns of population growth: (i) dose-dependent inhibition in the tolerant Pm I; (ii) a non-linear inhibition in the sensitive Pm III, where lower densities were most detrimental; and (iii) a hormetic effect in Pm IV, where low densities stimulated population growth. Fecundity was negatively impacted in all three species, highlighting reproduction as a consistently sensitive endpoint. These findings demonstrate that morphologically almost identical species can exhibit fundamentally different ecotoxicological responses, from acute mortality to complex sublethal patterns like hormesis. We conclude that assuming functional equivalence among closely related species can lead to misleading conclusions about the ecological risks of microplastic pollution. Incorporating cryptic species complexes into ecotoxicological testing frameworks could improve sensitivity resolution and strengthen environmental risk assessments of emerging pollutants, such as microplastics.