Zooplankton motile behavior: traits and trade-offs in planktonic copepods

Research on planktonic copepod ecology is vital to understand the factors controlling marine food web dynamics since copepods are the major components of zooplankton communities and the main link between trophic levels in marine environments. Despite their taxonomic diversity, copepods share certain phenotypic characteristics, or ´traits´, that are essential in determining trophic interactions and fitness. One important characteristic
that decisively influences organism interactions is behavior. Copepods display two distinct behavioral strategies in terms of motility: ´active´ (feeding-current and cruising feeding) or ´passive´ (ambush feeding). Differences in motile behavior between strategies imply different consequences for encounter rates with prey, sex partners, and potential predators. This thesis aims to provide a mechanistic understanding of how the different behavioral strategies in zooplankton result in different trade-offs between efficient feeding, mate
finding, and predation risk. We experimentally quantified i) swimming behavior, ii) feeding rates on different prey, iii) escape capability and iv) predation risk in various copepod life stages and genders with different motile and feeding behavior. We found that an active feeding behavior resulted in high feeding efficiency on both motile and immotile prey, but also high predation mortality from rheotactic predators. In contrast, ambush feeding was inefficient for non-motile prey (one of order of magnitude lower than for active feeders), but least risky in term of predation risk (up to eight times lower than for active feeders). Strict ambush feeders, which depend on the fluid disturbance generated to perceive the prey, are therefore more constrained in terms of prey availability than other feeding strategies. Hence, a high feeding efficiency is traded of
against a high predation risk in zooplankton. We show that differences in predation rates and feeding efficiency between copepod genders are dependent on the feeding behavior. In ambush feeders (Oithona spp), where feeding and mate finding are conflicting activities, males have to sacrifice feeding time
to search for females and display high-velocity mate-searching behavior. Consequently, males showed a lower feeding efficiency and significantly higher predation mortality than females. On contrast, in active feeders both sexes moved more during feeding and showed small differences between genders in feeding efficiency and predation risk. Finally, we also found that foraging activity decreased with increasing food availability, especially in active feeding strategies, resulting in a decrease in predation risk. Therefore, changes in behavior depending on food availability have implications on zooplankton
predation risk (“bottom-up behavioral cascades”) in marine plankton food webs. Our overall conclusion is that behavior is a key trait in copepods that plays a decisive role in the trade-off between feeding, mate finding, and survival. The optimality of each behavioral strategy is determined by the environmental conditions particularly by prey availability, prey type, and predation pressure. Copepod behavior is therefore a determining factor of grazing impact, distribution and composition of zooplankton in the marine environment