Summary
We investigated habituation in killifish (Fundulus diaphanus) to repeated exposure of predator models of varying shades in two positions (above and below) against backgrounds of varying shades. Killifish that had habituated to a model of a given shade recommenced responding to a model of a different shade when the contrast between the second model and its background was greater than that of the first. They showed no increase in response to a model-background combination of lower contrast (Experiment 1). After a single presentation of one model-background combination (i.e. minimal habituation and sensory adaptation) an increase in contrast again resulted in an increase in response, and a decrease in contrast resulted in a lower response (Experiment 2). The killifish reacted to models which were presented above the shoal following presentations below the shoal, but not if the order was reversed. This difference was found with models darker or lighter than the background (Experiment 3). These results were interpreted in relation to the predators present in the environment in which the killifish live, and the visual mechanisms with which the killifish perceive these predators.
Methodology
About 150 killifish were seined at Queen’s University Biological Station on Lake Opinicon, 45 km northeast of Kingston, in early June 1987, and about 200 were caught in June 1988. The fish ranged in size from 4 to 7 cm, and their ages varied between 1 and 2 years. They were transported to the Biology Department of Queen’s University at Kingston, where they were kept in 86 L holding tanks on a 12:12 light:dark cycle with approximately 50 fish per tank. The fish were treated with antibiotic to prevent tail-rot, but were otherwise left untreated for parasites. Each killifish had one or two visible parasitic trematodes. Because this condition is so extensive in the population of killifish at Lake Opinicon, and because none of the fish used in these experiments appeared to become sick or behave abnormally, no attempt was made to treat the fish for this condition. The water was kept at room temperature (22°C).