Interaction between anthropogenic stressors affects antipredator defence in an intertidal crustacean

The marine environment is increasingly subject to changes driven by anthropogenic stressors which may alter species’ key behaviours and impact phenotypic plasticity. Such stressors rarely occur in isolation, yet our understanding of how simultaneous stresses affect marine organisms is limited. Here we study the combined impacts of a major global stressor, temperature increase, and a local stressor, anthropogenic noise, upon key defensive traits of the shore crab, Carcinus maenas. We tested the colour change and behavioural responses of crabs in relatively colder and warmer water, and in the presence of natural ambient or ship noise. Using image analysis and a model of predator vision, we demonstrate that crabs change colour, and improve camouflage, fastest in warmer water in the absence of anthropogenic noise. When anthropogenic noise was present it adversely impacted crab colour change and camouflage, to the extent that the accelerated change due to temperature was negated. Additionally, anthropogenic noise affected C. maenas’ behaviour, reducing the likelihood and increasing the latency of antipredator response to stimuli. This reveals a possible mitigative antagonism, with the biological response to temperature reduced by the presence of anthropogenic noise. Our study demonstrates how interactions between anthropogenic stressors may impact marine life.

Methods
Juvenile Carcinus maenas were exposed to a combination of anthropogenic noise and raised temperatures in a laboratory environment. Impacts of this stressor combination on crab luminance change and background matching ability were measured through image analysis with respect to an avian predator vision model. Crabs were also montored in behavioural trials for their likelihood and time taken to react to a simulated predatory attack under stressor combinations.