Many organisms rely on their sense of smell to explore and interact with their environment. Meeting another organism, infochemicals help them decide whether to eat it, flee, fight or mate with it. The infochemical 2-phenylethylamine (PEA) has previously been associated with predator avoidance in mammals and feeding deterrence in algae. This thesis explores the hypothesis that PEA also induces a comparable avoidance behaviour in crustaceans. In behaviour assays with shore crabs and hermit crabs, movement patterns and behavioural displays were explored in current average marine pH conditions and end-of-the-century average pH, as associated with climate change. Furthermore, using liquid chromatography with tandem mass-spectrometry, shore crab urine was analysed for PEA. Results indicate that PEA attracts both species in reduced pH conditions, rather than deterring them as expected for a predator cue. Agonistic displays in response to PEA and evidence for PEA in shore crab urine suggest that PEA mediates agonistic interactions in crustaceans. Excreted with urine, PEA elicits the full behavioural response in crustaceans only in decreased pH conditions. Therefore, this thesis tested the hypothesis that the pH-dependent response hinges on pH-dependent changes to the infochemical itself. Quantum chemical computations validated by nuclear magnetic resonance (NMR) spectroscopy allowed calculating PEA's different protonation states, and its interaction with a model receptor in different protonation states. Results confirm that the pH-dependent response to PEA could be attributed to changes in charge distribution in the infochemical, potentially leading to altered receptor-ligand affinity. A comparison with the neurotransmitter dopamine reveals that the exact chemical structure and charge distribution of PEA matters for its biological function. Although dopamine and PEA are broadly chemically similar, their biological role for crustaceans appears different. In summary, PEA is an infochemical mediating agonistic interactions in crustaceans in a pH-dependent manner
