Biotic resistance on muddy shores? Native epibiotic barnacles impose costs on non-native snails

Organisms that settle and grow on other organisms (i.e., epibionts) are often costly to their hosts in terms of locomotion, growth, and/or reproduction. Such costs can potentially result in biotic resistance against invasion when native epibionts colonise non-native hosts, but examples are rare. Here, we examine the extent to which native acorn barnacles Balanus glandula that grow on the non-native Japanese mud snail, Batillaria attramentaria, may offer biotic resistance against this invader. We conducted population surveys, mark-recapture experiments, and behavioural observations in British Columbia, Canada, to measure the effects of barnacles on the movement, growth, and recapture rate, a potential proxy of short-term survival, of mud snails. One-third of mud snails carried barnacles, the weight of which sometimes exceeded the weight of the mud snail carrying them. Barnacle presence, weight, and the ratio of barnacle to snail weight all decreased the probability that mud snails would move, which could have implications for the foraging success of mud snails carrying barnacles. The slopes of the relationship between shell weight and length were similar for mud snails with and without barnacles, suggesting a lack of effect on growth allometry. The probability of recapture was 36% higher for mud snails without barnacles in one of two trials (i.e., 68% probability of recapture for snails without barnacles vs 50% for snails with barnacles), which reflects weak evidence for a small detrimental effect on mud snails carrying barnacles. Overall, native barnacles appear to offer some, though weak, biotic resistance to invasive mud snails, providing new insight into the potential for ecosystems to resist invasions through epibiotic interactions.