Human activity selectively affects a dynamic defensive mutualism

Human presence and urbanization alter many species' vulnerability and perceived risk, but little research has investigated how anthropogenic impacts affect behaviour in dynamic defensive mutualisms. As human activities continue to expand in marine communities, it is important to understand how they may affect risk assessment in behaviourally dependent symbionts. Shrimpgobies ( Ctenogobiops spp.) and snapping shrimp ( Alpheus spp.) in Mo'orea, French Polynesia participate in an obligate, symbiotic relationship in areas where humans recreate. We quantified hiding time, flight initiation distance and time allocated to different behaviours to first describe this defensive mutualism, then determined whether human activity directly impacted it. We found that goby behaviour significantly explained variation in shrimp behaviour. Specifically, shrimp varied in how long they remained in their burrow, how long they remained in their burrow after their goby partner(s) emerged, the rate at which they excavated their burrows and the time spent outside their burrows as a function of goby behaviour. Our findings suggest this dynamic mutualism was selectively affected by humans. Human activity, measured by both presence and abundance, explained some variation in multiple goby behaviours that directly influence variation in shrimp behaviour. • Fierce shrimpgobies and Djeddah snapping shrimp engage in a dynamic defensive mutualism. • Goby behaviour modified a suite of shrimp antipredator behaviours. • Human recreational activity had selective effects on goby behaviour. • Human activity may modify highly evolved and adaptive defensive mutualisms.