Behavior of Acartia tonsa in patchy food environments

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Abstract
Motility of individual female Acartia tonsa in response to patchiness of the diatom Thalassiosira weissflogii was investigated with a digitizing technique and standard video recordings. Motility, measured as 10‐s displacement, was reduced after 24 h of starvation but was unaffected by 4 h of starvation; it increased temporarily after transfer from food to filtered seawater. When food was present only in the upper half of the aquarium, copepods spent most of their time there and produced as many fecal pellets as in homogeneous food distributions. Detailed video analysis showed that feeding bout frequency was higher and jump frequency lower in the presence of food than in filtered seawater. Copepods performed fast vertical ascents by repetitive jumps to reach the food layer from greater distances; by such behavior they were able to remain in food layers only 30 mm thick placed in the middle of a 200‐mm column of filtered seawater. Even with such thin layers of food, copepods produced as many fecal pellets as in homogeneous food distributions over 2‐h periods. Fast response to food and strong ability to remain inside patches probably is crucial for copepods confronted with ephemeral or thin patches of food.

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