Green crab (Carcinus maenas (Linnaeus, 1758)) foraging on soft-shell clams (Mya arenaria Linnaeus, 1758) across seagrass complexity: Behavioural mechanisms and a new habitat complexity index

• I examined crab foraging on clams across seagrass complexity.
• I used a new index, behaviours, and above- and belowground complexity gradients.
• The index IBG was interstitial area between rhizomes divided by claw area.
• Foraging was sigmoid across increasing IBG; aboveground structure had little effect.
• Encounter rate, handling time & probability of capture upon encounter were important.

Although many studies have examined foraging success across seagrass complexity, few have identified underlying behavioural mechanisms or examined effects of belowground complexity. Here, I used a new habitat complexity index (IBG), behavioural data, and laboratory manipulations of artificial above- and belowground structures and predator sizes to understand foraging of invasive green crabs (Carcinus maenas) on soft-shell clams (Mya arenaria) in seagrass (Zostera marina) beds. IBG was calculated as interstitial area between rhizomes (i.e., belowground complexity) divided by claw area; belowground complexity was varied while claw area was held constant. This meant that as IBG increased, belowground complexity itself decreased. Belowground complexity strongly affected crab foraging, and a sigmoid function described predation rate across increasing IBG. This relationship was not evident when aboveground complexity was high (i.e., interstitial area between shoots was small), and no patterns in predation across a gradient of aboveground complexity were observed. Important behavioural mechanisms included encounter rate with prey and the probability of capture upon encounter, both of which increased hyperbolically as IBG increased, and handling time per prey, which decreased exponentially as IBG increased. Most handling time was spent excavating prey from the sediments. Predator size did not change these relationships, although larger crabs had more difficulty capturing prey at low IBG than smaller crabs. Clearly, success of crabs feeding on infauna in seagrass beds is limited most by claw size relative to opening size from which prey are extracted. IBG incorporates this limitation, and can be used to predict effects of habitat complexity on foraging success of epibenthic predators in various habitat types.