Quantitative and Developmental Analyses of the Alarm Reaction in the Zebra Danio, Brachydanio rerio

The response of Brachydanio rerio in aquaria to alarm substance extracted from injured conspecifics was analyzed over 180 min periods. Three indices were devised to facilitate description: 1) an index of the fish's vertical distribution, 2) an index of their horizontal distribution and 3) a clumping index. In response to alarm substance, fish moved toward the substrate and clumped, but showed no consistent change in horizontal distribution. The magnitude of the response, as measured by the time for recovery to pretesting distributions, became greater with increasing pheromone concentration. The fish's response was not affected by a preceding test, but longer term conditioning effects are possible. Experiments on the development of the response during ontogeny showed that naive fry first react to alarm substance 48-52 days after hatching, at least 17 days after the substance is contained in their skin. The method of behavioral assay described here can detect subtle changes in the alarm response, and thus may be useful in future studies of alarm behavior.