The Egg Packing Pressure Index of Calanoid Copepod as a Novel Eco-Indicator in Diverse Geographical Ecosystems

Egg-carrying ectothermic planktonic organisms in stressed conditions adapt diverse reproductive trade-off strategies, which are denoted by markers of stress, either in their physiology, morphology or reproductive characteristics. This is the first study documenting the fact that egg packing constraints can provide a novel marker of stress through experimental and field data, attuned by a remarkable bioindicator egg-bearing copepod species, Eurytemora affinis, in diverse physical conditions and transboundary sites through a multigenerational assay. This study propounds a packing pressure index (PP index), which is an efficient tool for demarcating reproductive efforts along with reference lines of packing constraints in bioindicator organisms. The packing pressure index for E. affinis varied across geographical locations, showing distinct north–south variations, along with a strong packing constraint in higher-temperature and -salinity conditions. The novel relationships between morphological and reproductive traits and packing constraints could be standardized. Ensuring the presence of the egg sac membrane and its relationship with varied physicochemical parameters can aid in developing a comprehensive understanding of reproductive strategies in keystone species like E. affinis.