Chemically-mediated foraging by subtidal marine predators : a field test of tritrophic signaling

Includes bibliographical references (p. 23-26).Although predators can use waterborne chemical cues to find dead or damaged prey, the usefulness of distance chemoreception for finding living prey is poorly known in marine systems. In contrast, terrestrial predators like predatory insects and parasitoids commonly use volatile plant compounds released during grazing to find herbivorous prey (i.e. tritrophic signaling). Because few studies have tested for such interactions in marine organisms, it is unclear if marine predators commonly use these fluid-borne cues to find living, herbivorous prey, especially under more realistic settings. Such interactions might occur frequently in the ocean because marine predators often use chemosensory-based foraging, and herbivoreinduced chemical responses are common in some algae (especially the Phaeophyceae). To examine the ability of predators to find herbivores using waterborne chemical cues released by grazed algae, we examined the response of marine predators to several bait types using in situ trapping and video experiments. These experiments tested the attractiveness of chemical cues released by (1) Macrocystis pyrifera and Ulva sp. actively grazed by herbivores for short- or long-term periods, (2) injured herbivores, (3) fresh carrion, (positive control), or (4) negative controls. The majority of predators caught or observed were spiny lobsters. In both trapping and video experiments, actively grazed algae with live herbivores failed to attract predators relative to controls, suggesting that under realistic field conditions, these predators do not use cues from these sources to find living herbivores. Consistent with previous studies, injured herbivores and fresh carrion were highly attractive, with similar predator species contributing to the highest catch rates, visitation rates, and time spent near these treatments. Thus, chemical cues associated with live herbivores, Macrocystis, and Ulva have little influence on predator foraging strategies, especially relative to cues released from more potent or valuable food sources