Directionality and Swimming Speeds in Predator-Prey and Male-Female Interactions of Euchaeta rimana, a Subtropical Marine Copepod

Abstract

This examination showed how the sexual dichotomy in morphology and feeding was reflected in the swimming behavior of Euchaeta rimana. Nonrandom swimming was clearly exhibited by this copepod, and the evolutionary reasons for the behaviors involve the dual requirements of encountering food and mates. Mechanoreceptive females, with their enlarged feeding appendages and elongated antennal setae, must find prey to feed. Non-feeding males, with reduced mouthparts and antennal setules, must find females to inseminate before exhausting their lipid reserves which were accumulated during juvenile stages. Directional swimming by the female predatory copepod supports the predictions of models in which encounter rate was maximized by swimming orthogonally to their mates and their prey. The female swam horizontally in a turn-and-search pattern to intersect the male which swam vertically in a swim-up-and-sink pattern. Adult female copepods (~2.5 mm prosome length) generally swam smoothly and continuously at an average swimming speed of 7 mm's-I, with their antennae oriented into the flow not disturbed by their own movements. Besides mating, females also must find and capture prey. Analysis of swimming by one potential prey, Acartia fossae, showed that these smaller copepods darted up and stopped in various directions to counteract sinking due to gravity. This resulted in a strong vertical component to their directionality which increased the likelihood of encounter with the predatory copepod. The dart-and-stop swimming pattern of Acartia fossae may be an alternate mode of escape from a mechanoreceptive copepod, such as Euchaeta which can not sense prey when they are not moving.