Predators can induce a suite of evolutionary responses, such as changes in behavior, life-history traits or morphology, from their prey. Our work adds to the growing number of studies of kairomone-mediated aquatic predator—prey interactions that significantly affect prey morphology. Individuals of the amphidromous shrimp Xiphocaris elongata have elongated rostrums below geomorphic barriers where predatory fishes are present and short rostrums in stream reaches above geomorphic barriers where predatory fishes are absent. Our objective was to test whether the elongated rostrum in X. elongata is a phenotypic modification induced by kairomones from predatory fish or alarm cues from conspecifics. We cut the rostrums of juvenile, longrostrum adult, and short-rostrum adult X. elongata and exposed the shrimp to predatory fish fed flakes, predatory fish fed X. elongata, predatory fish fed Atya, the nonpredatory fish S. plumieri, no fish with filtered water, and no fish with stream water. We measured the carapace length and the post-orbital carapace length (to obtain the rostrum length) of every shrimp every 2 wk for a total of 10 times/shrimp. Rostrums of X. elongata exposed to predatory fish fed flakes, predatory fish fed X. elongata, and predatory fish fed Atya shrimp grew longer than X. elongata exposed to the nonpredatory fish, filtered water, or stream water. Rostrums of juveniles grew faster than those of adults independent of treatment. Rostrum growth was not affected by the treatment × phenotype interaction. These results demonstrate that the elongated rostrum in the amphidromous X. elongata is a phenotypic response induced by kairomones from predatory fish.
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Vol. 33 • No. 2