Fright reaction in Gymnocharacinus bergi (Pisces, Characidae), a relic fish from Patagonia

Fright reaction, mediated by pheromones, is an antipredatory mechanism in t he scale-less 'mojarra' Gymnocharacinus bergi (Pisces, Characidae). The aim of this study was to characterise and quantify the fright reaction of this species and to analyse the alarm response to different doses of alarm subs tance. The alarm response consisted in a general decrease in the swimming s peed and a tendency to remain at the bottom of the test aquarium. A thresho ld effect was induced by a very low dose of alarm substance, and the intens ity of the fright reaction was positively correlated to the dose concentrat ion. The main characteristics of the alarm response appear to be consistent with G. bergi's environmental conditions such as: a higher predation risk in the upper stratum, the abundant pond-weeds of the stream and the great f low and fast speed of the clear waters.