Nonconsumptive predator effects on prey: dogwhelk cues limit mussel recruitment and trigger mussel shell plasticity that hinders predation

Background/Question/Methods. Prey organisms detecting waterborne cues from predators often exhibit avoidance behavior or morphological plasticity to minimize predation risk. However, the influence of avoidance behavior on prey demography has rarely been studied. Similarly, not many studies have evaluated the influence of prey morphological plasticity on actual predation rates. To investigate these knowledge gaps, we conducted field and laboratory experiments using dogwhelks (predatory snails) and mussels (prey) from Atlantic Canada. A field experiment tested the hypothesis that dogwhelks have negative nonconsumptive effects on mussel recruitment, a key demographic rate for population persistence. This experiment was done between May–July 2016 and used replicate cages to manipulate the presence of dogwhelks (simulating natural density) in rocky intertidal habitats. Each cage had a mussel recruit collector on its top that resembled preferred settlement substrate for mussels. A concurrent field experiment tested the hypothesis that dogwhelks have nonconsumptive effects on mussel shell morphology. This experiment also used cages to manipulate dogwhelk presence and absence. Each cage had an inner compartment that held two young mussels, one for the morphological analysis and one for the laboratory experiment done afterwards. During these two field experiments, to avoid dogwhelk starvation, the caged dogwhelks were exchanged at regular intervals with mussel-fed dogwhelks. The second field experiment was followed by a laboratory experiment that tested the hypothesis that dogwhelks require more time to handle a mussel to full consumption when the mussel was raised in the presence of dogwhelk cues in the field. Results/Conclusions. Dogwhelk presence limited mussel recruitment, likely through waterborne chemical cues released by the dogwhelks. This result suggests that pelagic mussel larvae avoid settlement on areas suggestive of future predaton risk (once settled prey organisms develop into adults). Dogwhelk cues also influenced mussel shell morphology, triggering a reallocation of calcification towards shell thickening at the expense of elongation. The laboratory experiment showed that dogwhelks were able to identify relatively thin shell areas when drilling to consume a mussel. Nonetheless, handling time to full consumption was considerably longer when consuming mussels that grew exposed to dogwhelk cues, seemingly because such mussels had thicker shells. Overall, then, our experiments indicate that a demographic rate of prey (recruitment) is negatively affected by predator cues and that predator-induced morphological plasticity in prey hinders predation. Therefore, this study demonstrates that the same predator species can have nonconsumptive effects on different life-history stages of the same prey species.