Deciphering the bioindicator potential and phenotypic plasticity of Eunicidae in coralline reef ecosystems: Global...

Eunicidae, a prominent family of polychaetes, function as reliable bioindicators of coral reef ecosystems. Global patterns revealed their consistent dominance across diverse reef systems. Indicators of both healthy and deteriorated reefs, eunicids interact symbiotically with corals and serve as both eroders and builders of coral reefs, promoting growth through aggregation but causing structural loss when bioerosion exceeds natural thresholds. The dominance of Eunicidae was further supported by our observations in the contrasting coralline reefs of the Gulf of Kachchh (GoK) and Gulf of Mannar (GoM) Marine Protected Areas (MPAs), two of India's four major coral reef ecosystems. Lysidice unicornis, abundant in these reef systems, was selected as a model organism to investigate phenotypic plasticity manifested in morphological, behavioural, and life-history traits. Morphological plasticity revealed clear regional variations. Smaller forms were prominent in GoK linked with strong currents, high sedimentation, and localised pressures, but more robust k-selected morphologies with bigger body dimensions, feeding structures, locomotory appendages, and setal features were prevalent in GoM with more stable ecosystem. Behavioural traits such as motility, feeding modes, living habitat, and sediment position also displayed plasticity, while life-history traits remained largely conserved. Trait expression in Eunicidae was primarily influenced by total organic carbon in GoK and silt content in GoM. These findings demonstrate how divergent environmental regimes and ecological stressors shape the phenotypic plasticity of Lysidice unicornis, highlighting the broader capacity of reef-associated polychaetes to modulate trait expression under differing disturbance intensities and reinforcing their value as indicators of ecosystem condition within tropical MPAs.