Does prey elemental stoichiometry influence copepod movement over ontogeny

Many planktonic consumers alter their behavior depending on the concentration of food in the environment, but responses to changes in food quality, as characterized by its elemental stoichiometry, are less well understood. Because of different nutritional demands across life history stages, consumer's responses to prey quality may vary across ontogeny. We build on previous observations of consumer selectivity and responses to prey presence by examining responses of displacement and movement patterns to prey stoichiometry. We used high-speed videography to quantify displacement and movement patterns of the marine copepod, Acartia tonsa, as they varied with elemental content of microalgal food offered during preconditioning and during imaging trials. Life stages were sensitive to different nutrient elements in prey, with movement in copepodites generally varying with nitrogen content, and in adults with both nitrogen and phosphorus content. Net displacement was lower when adults and copepodites were offered fully replete and nitrogen-replete food, respectively. Displacement of adults was more sensitive to the quality of food offered during preconditioning, and displacement of copepodites was affected by food quality during both preconditioning and trials. Naupliar displacement and movement patterns were generally insensitive to food quality. Only adults significantly altered movement patterns associated with feeding in response to food quality; adult helical swimming significantly increased in the treatments in which stoichiometrically replete food was offered in both preconditioning and behavioral trials. Older stages of A. tonsa alter movement in response to food quality in ways that may allow the selective use of patches of high-quality prey.