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Although the lobate ctenophore Mnemiopsis leidyi is known to be an important copepod predator, the mechanistic basis for its predatory success is not well understood. We directly observed and recorded predation by free-swimming M. leidyi ctenophores on Acartia tonsa and Oithona colcarva copepods (n = 349 encounters, 25 different ctenophores) in the laboratory using videographic methods. Overall capture success was relatively high (74 % ) following copepod contact with M. leidyi. However, average retention of copepods initiaLly contacting the interior surfaces of the oral lobes (the major capture sites) was considerably lower (34 %). Average retention was low because copepods most frequently collided head-first with the oral lobes and bounced away without being captured. Escape success for copepods declined rapidly during ensuing contacts, and most captures (57 "0) involved multiple contacts within an encounter. Our most novel finding was that almost half (49 %) of the encounters involved alterations of oral lobe positions which preceded, or anticipated, actual contact with a copepod. Anticipatory responses were cued to fluid disturbances created by swimming copepods. Anticipation of prey contact allowed ctenophores to shift oral lobe positions and reduce copepod escape avenues, thereby increasing the number of contacts per encounter and significantly increasing capture efficiency.

Using digital holographic cinematography, we quantify and compare the feeding behavior of free-swimming copepods, Acartia tonsa, on nutritional prey (Storeatula major) to that occurring during exposure to toxic and non-toxic strains of Karenia brevis and Karlodinium veneficum. These two harmful algal species produce polyketide toxins with different modes of action and potency. We distinguish between two different beating modes of the copepod’s feeding appendages–a “sampling beating” that has short durations (<100 ms) and involves little fluid entrainment and a longer duration “grazing beating” that persists up to 1200 ms and generates feeding currents. The durations of both beating modes have log-normal distributions. Without prey, A. tonsa only samples the environment at low frequency. Upon introduction of non-toxic food, it increases its sampling time moderately and the grazing period substantially. On mono algal diets for either of the toxic dinoflagellates, sampling time fraction is high but the grazing is very limited. A. tonsa demonstrates aversion to both toxic algal species. In mixtures of S. major and the neurotoxin producing K. brevis, sampling and grazing diminish rapidly, presumably due to neurological effects of consuming brevetoxins while trying to feed on S. major. In contrast, on mixtures of cytotoxin producing K. veneficum, both behavioral modes persist, indicating that intake of karlotoxins does not immediately inhibit the copepod’s grazing behavior. These findings add critical insight into how these algal toxins may influence the copepod’s feeding behavior, and suggest how some harmful algal species may alter top-down control exerted by grazers like copepods.

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