Reduced mobility is associated with compensatory feeding and increased diet breadth of marine crabs

Direct effects of predation have been widely recognized as important in affecting prey population dynamics and evolution. However, less attention has been devoted to the consequences of indirect effects of predators on prey behavior. For example, to avoid predation many animals restrict their activities to physical refugia and adopt low-mobility lifestyles, yet the consequences of these anti-predator behaviors for foraging and diet selection are relatively unknown. In this study we examine the relationships between mobility, feeding preferences, and compensatory feeding for 3 species of marine decapod crabs feeding on seaweeds in North Carolina, USA. Low mobility and high site fidelity of crabs were associated with a broad, non-selective diet and compensatory feeding. The majid Mithrax forceps exhibited the lowest mobility, highest site fidelity, and least selective diet of the 3 species, whereas another majid Libinia dubia was intermediate in both mobility and selectivity, and the xanthid Panopeus herbstii had the greatest mobility and narrowest diet. Of these 3 crabs, only M. forceps compensated for low food quality by increasing consumption rates in single food-species feeding assays. This may be because M. forceps is resistant to (or tolerant of) seaweed chemical defenses, while other crab species are not. The ability to consume, and presumably subsist on, a wide variety of potential foods including those defended from more mobile consumers may facilitate a low-mobility lifestyle, allowing the crab to minimize movement and reduce exposure to predators. Low mobility and high site-fidelity may thus facilitate the formation and use of associational refuges with sessile benthic organisms that are resistant to predators; these associations can have important community and ecosystem-wide consequences.