INTRASPECIFIC AND CROSS-SUPERORDER RESPONSES TO CHEMICAL ALARM SIGNALS BY BROOK STICKLEBACK

Fathead minnows (Pimephales promelas) and brook stickleback (Culaea inconstans) are often sympatric, occupy similar microhabitats, and share common predators. Therefore, individuals that detect alarm signals of both conspecifics and heterospecifics should gain antipredator benefits. We conducted a series of experiments to determine the extent to which these species respond to chemical alarm signals from conspecifics and heterospecifics. In laboratory experiments, brook stickleback responded to chemical stimuli from injured conspecifics with increased shoaling, but did not increase shoaling following exposure to chemical stimuli from injured swordtails (Xiphophorus helleri), an unfamiliar tropical fish that is not closely related to either stickleback or fathead minnows. These data are the first to document a chemical alarm signal for fishes in the order Gasterosteiformes. Brook stickleback also exhibited fright responses to extracts from injured fathead minnows in both laboratory and field tests. Stickleback did not exhibit a fright reaction following exposure to chemical stimuli from fathead minnows that had been treated with testosterone to decrease the concentration of alarm substance cells. This result suggests that the minnow alarm pheromone is the active component rather than some other constituent of the minnow skin extracts. In contrast to the response of the stickleback, fathead minnows did not respond to extracts from brook stickleback with a fright response. These data suggest that brook stickleback may benefit from close proximity with fathead minnows by gaining early warning of danger.