Predation by the black-clawed mud crab, Panopeus herbstii, in Mid-Atlantic salt marshes:: Further evidence for top-down control of marsh grass production

Although top-down control of plant growth has been shown in a variety of marine systems, it is widely thought to be unimportant in salt marshes. Recent caging experiments in Virginia and Georgia have challenged this notion and shown that the dominant marsh grazer (the periwinkle, Littoraria irrorata) not only suppresses plant growth, but can denude marsh substrate at high densities. In these same marshes, our field observations suggest that the black-clawed mud crab, Panopeus herbstii, is an abundant and potentially important top-down determinant of periwinkle density. No studies have quantitatively examined Panopeus distribution or trophic interactions in marsh systems, and its potential impacts on community structure remained unexplored. We investigated distribution and feeding habits of Panopeus in eight salt marshes along the Mid-Atlantic seashore (Delaware-North Carolina). We found that mud crabs were abundant in tall (4-82 ind m(-2)), intermediate (0-15 ind m(-2)), and short-form (0-5 ind m(-2)) Spartina alterniflora zones in all marshes and that crab densities were negatively correlated with tidal height and positively correlated with bivalve density. Excavation of crab lairs routinely produced shells of plant-grazing snails (up to 36 lair(-1)) and bivalves. Lab experiments confirmed that mud crabs readily consume these abundant marsh molluscs. To experimentally examine potential community effects of observed predation patterns, we manipulated crab and periwinkle densities in a 1-mo field experiment. Results showed that Panopeus can suppress gastropod abundance and that predation rates increase with increasing snail density. In turn, crab suppression of snail density reduces grazing intensity on salt marsh cordgrass, suggesting presence of a trophic cascade. These results indicate that this previously under-appreciated consumer is an important and indirect determinant of community structure and contribute to a growing body of evidence challenging the long-standing notion that consumers play a minor role in regulating marsh plant growth.