The sea lamprey (Petromyzon marinus), a devastating invasive species within the Laurentian Great Lakes, exhibits a spectacular alarm response to the odor emitted from dead conspecifics that may differ substantially in function from the well-characterized system in ostariophysan fishes. Here, we report the first field test and a series of laboratory experiments designed to characterize the behavioral responses of migratory-phase lampreys to a set of odors derived from conspecific and heterospecific tissues, determine whether sex or sexual maturation alters these responses, ascertain if the putative alarm substance derives from a particular region of the body, and evaluate how variation in dilution and previous exposure history influence the response. The field test clearly demonstrated that sea lampreys restrict their migration path by avoiding areas activated with conspecific alarm cues. A number of findings were consistent with prevailing predation-risk model for fishes in that dilute odors from conspecific skin elicited avoidance response, reactivity was a function of phylogenetic relatedness between species, and the response attenuated in females upon maturation but not in males. However, unlike alarm cues for other aquatic species, the sea lamprey alarm substance activates a larger space, is contained within multiple tissues, and is persistent over time (survives 96 h of decay). These features suggest a broader ecological function than the detection and avoidance of a predator. Given the strength and consistency of the response (>99% repellency at the highest concentration) these semiochemical(s) could provide a beneficial supplement to current sea lamprey management practices by redistributing the annual spawning run.
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