Predator Cues Induce Defensive Responses in Marine Prey: A Metabolomics Decoding of Blue Crab Urine Biomarkers

Prey rely on chemical cues to detect predators, especially in coastal ecosystems where danger is concealed by turbid waters. Juvenile oysters (Crassostrea virginica) respond to blue crab (Callinectus sapidus) chemical cues by growing stronger shells, while mud crabs (Panopeus herbstii) reduce foraging to avoid detection. However, the chemical composition of marine fear cues is largely unknown, partly due to methodological challenges. In this study, metabolomics was used to pinpoint predator metabolites that correlated with induced juvenile oyster defenses when exposed to the urine of blue crabs fed either oysters or mud crabs and to determine the chemical composition of these fear cues. Genetic algorithms applied to urine mass spectra coupled with bioassay data confirmed that metabolites from blue crab urine are responsible for fear responses by oysters. Sixteen compounds (of over 700) were determined to be potentially important cues used by juvenile oysters to detect blue crabs, with 3-(1-methyl-1H-imidazol-4-yl) propanoic acid identified as the most important contributor to this effect. These results indicate that oysters rely on certain metabolites that are ubiquitous in a variety of chemical blends to evaluate predation risk, and the response of juvenile oysters is not dependent on a singular metabolite but on numerous metabolites working in conjunction.