ORIENTATION OF APLYSIA-CALIFORNICA TO DISTANT FOOD SOURCES

The behavior of the marine mollusc Aplysia was examined under differen t experimental conditions designed to determine the food searching str ategy of the animals. In a small, open field tank with still water, th e animals took an average of 42 min to find a piece of seaweed, even t hough the stimulus was never located more than 30 cm away from the ani mal. Observations of the animals indicated that their search was not d irected, without a clear tendency towards the food, and during the cou rse of a search, they often crawled through most of the area of the ta nk. The search time, the distance travelled, and the strategy of the s earch of the animals was similar for different types of seaweed. If an imals were aroused into activity by the presence of seaweed extract, t he time for them to contact a piece of odorless glass fiber paper in t he open field was not significantly different than that for a piece of seaweed. The probability at which the animals contacted the seaweed, as a function of the distance travelled, resembled the detection proba bility determined according to a theory of random search. We thus prop ose that the aroused animals move in a random pattern until they are v ery close to the food. This strategy can be advantageous in still wate r since chemicals do not provide distinct gradients that can serve as cues for chemotactic orientation from distances greater than a few cen timeters from the source. In a Y-maze in still water, Aplysia did not perform above chance in selecting the arm that contained the seaweed. In streaming water, the animals entered the arm that contained seaweed significantly above chance only if a large piece of seaweed was used, so as to provide a very strong stimulus. Similarly, presentation of a high concentration of glutamate or a high concentration of seaweed ex tract to one side of the animal could evoke turning towards the stimul us. In the Y-maze, animals that repeatedly entered and exited both arm s before making a choice, performed better than those that went direct ly into one arm. We conclude that chemical food stimuli can evoke dire cted orientation only when the animal is very close to the stimulus (w ithin a few centimeters, at most). For Aplysia, the role of distant ch emical stimuli may not be to guide their orientation, but rather to ev oke the food-induced arousal state. In this behavioral state, several behaviors, such as locomotion are altered, and these changes may provi de the basis for locating food from a distance.