RESPONSES OF A FRESH-WATER SHRIMP TO CHEMICAL AND TACTILE STIMULI FROM A LARGE DECAPOD PREDATOR

Montane, tropical streams in Puerto Rico are dominated by decapod crustaceans. The most ubiquitous freshwater shrimp, with a distribution ranging from mangrove swamps to the highest-elevation pools, is Atya lanipes, a scraper/filterer. A large predatory shrimp, Macrobrachium carcinus, is much less abundant but dominates the top of the stream foodweb. As part of our longterm ecological research, we have observed a general negative relationship between Atya abundance and the presence of the large predatory shrimp. In a series of laboratory and field experiments, we showed that this relationship was primarily due to avoidance responses by Atya to the predator species. Furthermore, our experiments showed that both chemical and mechanical/tactile cues of predator presence determined the among-pool distribution of Atya within streams. Although chemical cues alone elicited a relatively weak response, the combination of chemical and tactile cues resulted in very strong movement responses. If the prey were confined to pools with predatory shrimp, the prey significantly decreased their overall movement and spent substantially more time in crevices. If given the opportunity, prey shrimp emigrated out of areas that had predatory decapods.