Interspecific interactions in an ecosystem dominated by physical forces

The mole crab Emerita talpoida and the clam Donax variabilis inhabit Atlantic swash zones, where waves break and recede on sandy beaches. High water velocities and unpredictable flow patterns make swash zones challenging for the navigation of macroscopic animals. Both species ride waves and quickly burrow into the substrate, where they avoid wave action. Any factor that slows burrowing increases the likelihood that an animal will be swept away, vulnerable to predators and unable to feed. Though hydrodynamics is therefore a primary concern in the swash zone, we hypothesized that interspecific interaction may also affect burrowing time, and thus the distribution, of E. talpoida and D. variabilis. Specimens of both species were collected from Cape Henlopen, DE, from May to August 2005. Using sieved sand in laboratory conditions, we determined that E. talpoida take significantly longer to burrow in substrate containing naturally-occurring levels of D. variabilis. In addition, D. variabilis were ejected from the substrate when they encountered a burrowing crab. These results may explain patterns of negative correlation between the distributions of these animals in the field, and interspecific interactions may therefore play a role in the distribution of animals in an ecosystem dominated by physical forces.