When confronted with patches of high quality food, it should be ecologically and evolutionarily advantageous for consumers to adopt behaviors that keep them within those patches. This behavioral adaptability may change with ontogeny due to differences in motility and dietary needs among life history stages. Early life history stages often have lower carbon overhead and thus a requirement for higher dietary N and P content. We studied whether copepods, lynchpin organisms in marine foodwebs, change swimming behavior with food quality over the course of their ontogeny. We used high-speed videography to track movements of Acartia tonsa, a well-studied species readily cultured which exhibits selective feeding, during exposure to food of different quality. We preconditioned animals from each stage (nauplii, copepodites, adults) on algae that was either nutritionally replete, or depleted in either N or P content. Copepods from each preconditioning treatment were then fed each type of algae in a fully factorial design. We then quantified travel speed and the amount of time spent displaying several behaviors by 15 individuals over two replicate bouts. ANOVA revealed whether behaviors depended on (1) quality of food items, (2) the quality of food used during the acclimation step, and (3) copepod stage.
Results/Conclusions
Treatments differed in the frequency of helices, a swimming pattern that reduces net distance traveled per unit time and is typically displayed during feeding. The number of helices performed per second was greater when adult copepods were fed replete food. This finding suggests copepods were displaying “epicurean” behavior rather than compensatory behavior– i.e., tending to feed more on high quality food than on poor quality food. Adults preconditioned on nutrient depleted food performed fewer helices regardless of which food was present during the feeding bout. Thus, copepod behavior responded more quickly and persistently to the presence of poor quality food than to the presence of high quality food. Nauplii, which have the highest nutrient requirements, tended to perform more helices when fed replete food, but the effect was not as large as for adults. Copepodites, which have restricted motility and intermediate N and P requirements, never responded to food quality. Our results suggest that selective use of high quality patches is likely restricted to adults, which possess the motility needed to exploit such patches. Early life history stages will find it difficult to exploit such patches despite having more restrictive dietary requirements, possibly resulting in demographic bottlenecks.