Behavioral responses of copepod Calanus sinicus to bloom-forming algae Prorocentrum donghaiense and Skeletonema costatum

Harmful algal blooms (HABs) severely threaten estuarine and coastal ecosystems in recent decades. The adverse impacts of HABs on zooplankton have been extensively studied, while the strategies employed by zooplankton to cope with HABs remain unclear. The copepod Calanus sinicus is the most dominant zooplankton species in the North Pacific Ocean during spring and early summer, coinciding with frequent blooms of the diatom Skeletonema costatum and the dinoflagellate Prorocentrum donghaiense. To investigate the behavioral responses and energy expenditures of C. sinicus under HAB conditions, we conducted both bottle incubations and high-speed video observations over 24 h. Incubation experiments revealed that the carbon intake rate of C. sinicus when feeding on these harmful algae was significantly lower (1.7 and 0.9 μg C Cop.⁻¹ day⁻¹ from P. donghaiense and S. costatum, respectively) compared to feeding on the healthy prey Platymonas helgolandica, with rates 5–10 times higher. This reduced intake barely met the daily basic metabolic requirements of the copepods. When exposed to P. donghaiense alone, copepods exhibited a pronounced escape-like jumping behavior characterized by high frequency, velocity and straight-line trajectory. In contrast, their swimming behavior differed when exposed to S. costatum alone, with a higher incidence of short, straight swim bouts likely related to the reorientation of diatom chains before ingestion. These specific behaviors were mitigated when alternative food sources were available alongside the harmful algae. We suggest that C. sinicus has evolved adaptive strategies to cope with blooms of P. donghaiense and S. costatum, including selective feeding on other phytoplankton and microzooplankton and either conserving energy by minimizing movement or rapidly escaping from bloom patches when food resources are severely depleted. These adaptive strategies of C. sinicus in HABs, highlighting the potential resilience mechanisms of zooplankton in fluctuating marine ecosystems, which could inform future conservation and management efforts in coastal waters.