HORWITH, M.J.*; MERZ, R.A.; Swarthmore College; Swarthmore College: Sediment Size and Interspecies Interaction affect Burrowing Times of Emerita talpoida in Swash

The mole crab Emerita talpoida inhabits Atlantic swash zones, where waves break and recede on sandy beaches. High water velocities and unpredictable flow patterns make swash zones challenging habitats for macroscopic animals. Mole crabs ride waves and quickly burrow into saturated sand, where they can safely feed, avoiding undesirable wave action and predators. Any factor that slows burrowing increases the likelihood that a crab will be swept away and unable to feed, or stranded on unsaturated sand and vulnerable to predators. Drag is likely the primary force affecting ease of burrowing in the swash zone, but we hypothesized that other factors also affect burrowing time. The bivalve Donax variabilis occurs seasonally in Atlantic swash zones, and we sought to determine whether interspecies interaction or sediment size limit the ability of mole crabs to burrow. Using sieved sand in laboratory conditions, we determined that mole crabs take significantly longer to burrow in coarser sand. Crabs also took significantly longer to burrow in sand inhabited by naturally-occurring levels of D. variabilis, a result confirmed in the field. In coarse sand or in the presence of bivalves, larger crabs were differentially slowed, suggesting that habitat quality within swash zones may change as crabs grow. D. variabilis were displaced at a high rate when large crabs burrowed nearby, suggesting the possibility of mutual exclusion within the swash zone. Though wave action may be the primary force affecting macroscopic animals in the swash zone, our study indicates that sediment size and interspecies interaction are also important, and may affect settlement patterns.