Swimming behavior and energy metabolism of the calanoid copepod invader Sinodiaptomus sarsi

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.

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.

Escape performance in the cyclopoid copepod Oithona davisae

Escaping a predator is one of the keys to success for any living creature. The performance of adults (males, females, and ovigerous females) of the cyclopoid copepod Oithona davisae exposed to an electrical stimulus is analysed as a function of temperature by measuring characteristic parameters associated with the escape movement (distance covered, duration of the appendage movement, mean and maximum escape speeds, Reynolds number). In addition, as a proxy for the efficiency of the motion, the Strouhal number was calculated. The escape performance showed temperature-dependent relationships within each adult state, as well as differences between sexes; additionally, changes owing to the presence of the egg sac were recorded in females. In a broader perspective, the results collected reveal the occurrence of different behavioural adaptations in males and females, adding to the comprehension of the mechanisms by which O. davisae interacts with its environment and shedding new light on the in situ population dynamics of this species.