Hidden diversity and distinct sensitivities in the Eurytemora affinis cryptic complex: Implications for environmental risk assessment?

Several aquatic invertebrates commonly used in standardized ecotoxicological bioassays exhibit cryptic diversity, a factor often overlooked in environmental risk assessment. Cryptic diversity raises concerns about the reproducibility and comparability of bioassay results. The calanoid copepod Eurytemora affinis is a cryptic species complex broadly distributed across the Northern Hemisphere in estuarine waters (0.5-20 PSU, Practical Salinity Unit). Thus, it represents a promising bioindicator species for assessing environmental risk in estuarine ecosystems. This study aims to assess the extent to which the cryptic diversity and the ecotoxicological responses observed within the E. affinis cryptic complex may compromise its usefulness in environmental risk assessment (ERA). The ecotoxicological responses of two E. affinis cryptic species (clades), the European (E) and the North Atlantic (NA) clades, originating from the Seine (France) and the St. Lawrence (Canada) estuaries, respectively, were assessed. Both clades were exposed to benzo[a]pyrene (BaP) using a standardized semi-chronic larval bioassay for 96 hr. Endpoints assessed included naupliar survival, growth and development. Median lethal concentrations (LC50) were calculated. The results showed that the E clade was 2.5-fold more sensitive than the NA clade with LC50 values of 8.41 and 22.24 µg L-1, respectively. Benzo[a]pyrene also had a greater effect on growth and development of the nauplius stages from the E clade compared to the NA clade. Such differential sensitivity is likely due to genetic divergence, phenotypic plasticity or environmental factors during sampling. Despite the observed inter-clade variation, the magnitude remains within the acceptable range of environmental variability considered in ERA. This study provides valuable insights into the implications of cryptic diversity for the use of E. affinis in ecotoxicological bioassays and supports its application within the ERA framework.