Behavior is a major determinant of predation risk in zooplankton

Abstract

Zooplankton exhibit different small-scale motile behaviors related to feeding and mating activ-ities. These different motile behaviors may result in different levels of predation risk, which may partiallydetermine the structure of planktonic communities. Here, we experimentally determined predation mortal-ity associated with (1) feeding activity (ambush feeders vs. feeding-current vs. cruising feeders) and (2)mate-finding behavior (males vs. females). The copepods Oithona nana, O. davisae (ambush feeders), Temoralongicornis (feeding-current feeder), and Centropages hamatus (cruising feeder) were used as prey for differ-ent predatory copepods. Copepods with “active” feeding behaviors (feeding-current and cruising feeders)showed significantly higher mortality from predation (~2–8 times) than similarly sized copepods with lowmotility feeding behavior (ambush feeders). Copepod males, which have a more active motile behaviorthan females (mate-seeking behavior), suffered a higher predation mortality than females in most of theexperiments. However, the predation risk for mate-searching behavior in copepods varied depending onfeeding behavior with ambush feeders consistently having the greatest difference in predation mortalitybetween genders (~4 times higher for males than for females). This gender-specific predation pressuremay partially explain field observations of female-biased sex ratios in ambush feeding copepods (e.g.,Oithonidae). Overall, our results demonstrate that small-scale motile behavior is a key trait in zooplanktonthat significantly affects predation risk and therefore is a main determinant of distribution and compositionof zooplankton communities in the ocean.