This may be due to the traditional practice in ecotoxicology to determine toxicity thresholds for “apical” endpoints (e.g., mortality, reproduction, development), for which many regulatory agencies use to justify environmental quality objectives (e.g., water, sediment, etc.).

While some studies find acute mortality thresholds in a range of species with microplastics, sub-lethal endpoints occur at exposure concentrations several orders of magnitude lower.  Mehinto et al. (2022) explored how eco-toxicity thresholds (built using species sensitivity distributions) change based on the type of endpoints considered, and found that fitness endpoints (i.e. mortality, reproduction, development) occur at concentrations ~2-5x higher than all endpoints (i.e. non-apical and apical endpoints). For mortality only, thresholds increase by ~2-3 orders of magnitude! (See table below from the supplemental information from Mehinto et al. (2022):

Table S4E: Sensitivity analysis of endpoints used to calculate thresholds

Because microplastics are persistent in the environment, many ecotoxicologists recommend applying a precautionary approach for management, including California’s Ocean Protection Council. An application of precautionary principles in this context may mean including “non-apical” biological endpoints for which adverse outcome pathways are not fully understood (e.g., biochemical changes, behavioral perturbations, etc.). This rationale was taken by the SCCWRP Expert Workshop in developing their ecotoxicological thresholds for microplastics (Mehinto et al. 2022).

When designing a toxicity experiment, one may consider using ToMEx to understand which data is already available (and of high quality) for a particular endpoint/species/particle range to either avoid unnecessary replication or inform experimental design.