Behavioral adaptations of cruise-feeding copepods to harmful algal blooms: Insights from the East China Sea

Harmful algal blooms (HABs) have become a global environmental concern, significantly impacting marine life and the fishing industry. However, the tolerance and adaptive mechanisms of zooplankton to HABs remain poorly understood. This study examines the behavioral and feeding responses of the cruise-feeding copepod Centropages dorsispinatus to summer HABs in the East China Sea (ECS), focusing on interactions with the blooming diatom (Skeletonema costatum) and dinoflagellates (Prorocentrum donghaiense, Karenia mikimotoi, and Alexandrium tamarense). Using short-term incubations and high-speed filming, we compared the ingestion rates and behaviors of C. dorsispinatus fed mono-algal diets and mixed diets containing neutral distraction particles (polystyrene beads). The results revealed that C. dorsispinatus obtained limited carbon from each algal diet (1.02–7.02 μg C cop.⁻¹ day⁻¹). The presence of distraction particles reduced carbon intake from S. costatum, P. donghaiense, and A. tamarense, but significantly increased intake from the healthy control, Platymonas helgolandica. Behavioral responses varied among algal diets: compared to P. helgolandica, C. dorsispinatus exhibited more frequent but shorter swims in S. costatum diets and less frequent swims in K. mikimotoi, and A. tamarense diets. These algal-specific responses were generally mitigated when copepods simultaneously exposed to the neutral distraction particles. Copepods achieved higher carbon intake with lower mechanical energy expenditure when grazing on large dinoflagellates compared to diatoms. We suggest that cruise-feeding copepods can actively adjust their behavior to adapt to varying food conditions, including the density, morphologic characteristics, and toxicity of algae. It allows copepods to better survive and forage in dinoflagellate HABs than in diatom HABs. However, the low ingestion rates observed limit the potential for cruise-feeding copepods to exert top-down control on HABs.