Ecological, biomechanical and neurological correlates of escape behavior in calanoid copepods
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Copepods are dominant members of planktonic communities and provide an important link within oceanic food webs. They are active grazers on phytoplankton and microzooplankton. In turn, copepods are preyed upon by a wide range of predators with diverse foraging tactics from ambush-entangling predators (ctenophores and jellyfish) to cruising raptorial feeders (fish). Copepod survival depends on their ability to both remotely detect and avoid approaching predators, and they have evolved many behavioral and physiological features to increase their chance of survival. One such behavior is an extraordinary escape reaction during which copepods typically achieve velocities greater than 100 body lengths per second. Accelerations up to 30 G allow them to reach these speeds in milliseconds. A successful escape reaction is not a singular event. It results from the integrated function of several escape reaction components, including sensitivity to hydrodynamic stimuli, response latency, reactive distance, escape speed, acceleration, and jump distance. vi Copepods were exposed to various hydrodynamic stimuli and the components of their escape reactions were quantified using video-to-computer motion analysis techniques. Copepod escape performance differed among copepod species and varied with the strength of the impending stimulus. In response to a repeatable near-field hydrodynamic stimulus, minimum behavioral response latencies of 2 ms were recorded for myelinated and non-myelinated species, and larger copepods had greater kinetic performance. Most prior copepod research has been performed under still conditions; however, copepods normally experience small-scale turbulence in their natural environments. Results from this research indicate that turbulence affects several copepod escape reaction components, including reactive distance, and sensitivity, indicating that future research must be conducted under more realistic conditions if we are to accurately evaluate copepod behavior. Copepod escape behavior was also examined in response to two predators having contrasting foraging tactics. Despite their perceptual abilities and short response latencies, copepods were readily consumed by the visual predator, the spinyhead blenny, and the flow-generating predator, the ctenophore. Copepods modified their kinetic escape performance relative to the strength of the applied stimulus. Their greatest kinetic performance was in response to the blennies, which were also the more successful predators.