| Summary: | The effects of patchy distribution of phytoplankton on the swimming behavior of marine copepods was studied using a video-computer system for motion analysis. The roles of chemoreception and mechanoreception in the identification of food was investigated by exposing the copepod Pseudocalanus minutus to conditions designed to simulate only the chemosensory or mechanosensory stimulation associated with phytoplankton in the environment. The observed behavioral changes were then compared with those found for copepods in the presence of phytoplankton. The results suggest that both chemoreception and mechanoreception contribute to the recognition of food in P. minutus. The ability of the copepods Pseudocalanus minutus and Acartia hudsonica to locate small scale patches of phytoplankton was examined by recording changes in spatial distribution of groups of copepods exposed to horizontal gradients of food and food odor in a multi-channeled laminar flow choice chamber. Only P. minutus showed a limited positive response to the food gradients and neither species showed a clear response to the food odor gradients. The ability of these copepods to remain within a patch once they encountered it was investigated by quantifying changes in swimming speed and turning behavior of copepods exposed to different food concentrations. While A. hudsonica shows no immediate change in swimming pattern with changes in food concentration, P. minutus swims slower and turns more when initially exposed to an increased food concentration. This behavior may allow P. minutus to spend more time in areas of high food concentration in nature. Under most circumstances, dense patches of phytoplankton should provide the best grazing areas for marine copepods. Defense mechanisms such as toxicity and bioluminescence may have evolved in some marine phytoplankton species to reduce the impact of zooplankton grazing. To test the hypothesis that bioluminescence in dinoflagellates is an adaptation for repelling nocturnal grazers, the swimming patterns of the copepod Acartia hudsonica were compared in the presence of bioluminescent and nonbioluminescent clones of the dinoflagellate Protogonyaulax tamarensis. Bioluminescent dinoflagellates increased the number of high speed bursts by the copepods and thus decreased the amount of slow speed swimming characteristic of their grazing behavior.
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