# Olfactory Cues in the Odour Plume of Predatory Fish Reduce Foraging and Elicit Anti-Predator Behaviour in the European Green Crab Carcinus maenas

## TL;DR

European green crabs avoid predator odors even without direct threat, suggesting smell could be used to deter them in aquaculture.

## Contribution

Demonstrates that green crabs become hypersensitive to predator odors over time, leading to reduced foraging and anti-predator behaviors.

## Key findings

- Exposure to predatory fish odors reduced crab foraging by 45%.
- Continuous exposure increased sensitivity, causing a 68% overall reduction in feeding behavior.
- Predator odor elicits anti-predator behaviors like hiding and escaping in green crabs.

## Abstract

The interaction between a predator and its prey is a race for survival, characterised by both sides employing specific tactics to increase their chances. Amongst the key mechanisms enabling prey and predator to detect each other is the sense of smell. Odour cues often alert either side to the presence of the other, and prey typically employ ways to hide or mask their presence, as famously observed in deer, where fawn have no scent at all. Detecting a predator by its smell should initiate hiding or flight responses in marine animals such as the European green crab, Carcinus maenas. However, the question remains whether animals living permanently in the odour plume of a predator can still detect this or whether they adapt to the smell when no physical harm occurs. This type of acclimation or learning would benefit ambush hunters such as predatory fish as prey would become desensitised. Here we tested if such desensitisation occurs in green crabs when exposed to sea bream odour and found that the opposite is the case: crabs become hypersensitized and react stronger to predator odour over time, decreasing foraging for food and initiating hiding or escape behaviour. As many crab species are globally invasive, the fact that they increasingly avoid predator odour even when no physical threat is involved can provide a starting point to explore the potential of odour as a deterrent against crabs, with implications for aquaculture and fisheries once the chemical nature of the active compounds has been elucidated.

The European green crab (Carcinus maenas) is an intertidal crustacean that has extended its invasive range globally and is regarded as a major pest species for bivalves, impacting coastal food chains and aquacultures. Crabs primarily use chemosensory cues to sense their environment for feeding to avoid predation and to locate mates and induce mating. Consequently, known attractants including food baits are frequently employed in trapping and control efforts. Here, we investigate the effects of introducing a predatory fish odour to the environment and show that it elicits anti-predator behaviour in C. maenas. A 45% reduction in crab foraging and feeding behaviour was observed when predator odour was introduced compared with food control experiments. A further 23% reduction (68% reduction overall) in feeding behaviour was observed after crabs were exposed to (though housed separately from) the same seawater as the live predator for several days. This highlights the increased awareness or continuous stress of these habituated crabs towards predator odour. This study also presents potential for application as a deterrent in integrated pest management strategies for this global invader.