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Learned Orientation in the Predator Avoidance Behavior of Mosquitofish, Gambusia Affinis

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Mosquitofish, Gambusia af finis, were trained to seek refuge from predatory fishes (principally Micropterus salmoides) by orienting to prepared areas of shallow water located either in circular pools or adjacent to artificial shores of rectangular shaped tanks. Fish were trained both outdoors in view of the sun and indoors away from the sun. Most tests were conducted outdoors in a testing facility which uniformly obscured the landscape but left an unobstructed view of the sky. These tests were conducted by releasing the fish in the center of a circular test facility and observing their direction of movement. Such tests were conducted with and without landmarks present, after different durations of training, with fish of different age groups, and in the presence and absence of other mosquitofish. Mosquitofish from a population previously exposed to predation and others from a population devoid of larger predatory fishes were included in this study. Optomotor responses were examined by placing individual mosquitofish in an aquarium and moving a black and white striped structure alongside the aquarium wall. The mosquitofish learned to orient toward safety using both local landmarks and a sun compass. When using celestial cues, trained mosquitofish swam on a compass course which would have returned them to the safety of shallow water in their respective training arenas. In these experiments orientation by inspection of local landmarks was less important than by sun compass. Neonatal mosquitofish oriented to shallow water adjacent to shore without predation and formed long term memory associating the direction to shore with their sun compass. However, the presence of a larger predatory fish was necessary to cause adult mosquitofish to develop shoreward orientation. Laboratory trained adults formed only short-term memory, but long-term memory fixation in the field was not discounted. Mosquitofish from the population which had previously been exposed to predation by larger fishes learned new shoreward directions within 2-3 days and subsequently oriented toward shore regardless of the temperature of the water in the test facility. Fish from the population which had not been previously exposed to predation took longer to associate the direction to safety with their sun compass. These fish also responded to changes in the temperature of the water in the test facility by orienting toward shore when the temperature was below 30 C. and toward deep water when the temperature exceeded 35 C. Mosquitofish which were highly conditioned by the training procedure were attracted to other individuals restricted within a glass vessel which was submerged within the test facility. When tested as a group, the mosquitofish oriented more precisely than when tested alone. Some mosquitofish were displaced in a clear glass vessel in full view of their surroundings and wcrc subsequently tested. Highly conditioned fish did not respond to the displacement but swam in the trained direction. Fish which had been in training for a shorter period of time compensated for the displacement if tested immediately but no longer compensated for the displacement if held 4 hr before being tested. The optomotor responses were pronounced in the highly conditioned fish and nearly absent in fish not previously exposed to predation. The intensification of the response which results from the training situation indicates that the optomotor reflex is part of the overall behavioral complex associated with the avoidance of predators. In a pond such a response would enable a displaced mosquitofish to remain as inconspicuous as possible while evaluating local environmental information to determine the best possible direction of travel to safety. Observations of marked and unmarked mosquitofish in a pond containing numerous larger predatory fishes indicate that the laboratory findings are essentially similar to the behavior of wild mosquitofisli. All of the mosquitofish within the pond remained in shallow water adjacent to the pond shore. The normal movements of these fish were very restricted and when displaced they would swim on a compass course perpendicular to their home shore. Fish displaced to the opposite side of the pond began to swim across the pond in the direction of their home shore (via sun cues). Upon encountering predatory fishes a short distance from their point of release the mosquitofish became motionless (using optomotor reflexes) and then returned to the safety of the shallow water adjacent to the shorc at their release point (via inspection of local landmarks).

Affiliations: 1: Savannah River Ecology Laboratory, Aiken, South Carolina, USA


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