Non-predatory shell damage in Neogene Western Atlantic deep-burrowing bivalves

Few shell-crushing predators can excavate prey living deep within the sediment. Despite this infaunal refuge from predators, many deep-burrowing bivalves display a strikingly high incidence of shell damage when compared with species living at shallower depths. A non-predatory origin is necessary to explain this high incidence of injury. To evaluate the generality of this hypothesis, repair frequency (number of repairs per shell), along with position and geometry, were determined for 68 Neogene bivalve species from the US Atlantic and Gulf Coastal Plain, seven of which are classified as deep burrowers (>20 cm depth of sediment penetration). Panopea bitruncata, Panopea floridana, Panopea reflexa, and Panopea americana, which are among the deepest burrowers (up to 100 cm), have repair frequencies (2.8– 6.2) that are higher than any other species, while the value for Cyrtopleura costata (0.76) is exceeded only by semi-infaunal Atrina species (1.12). However, not all deep-burrowing species had such high incidences of damage. Two moderately deep-burrowing species (Anodontia alba and Tagelus plebeius; 20–50 cm) showed repair frequencies (0.04–0.30) that overlapped the range (0.001–0.52) for shallow-burrowing, semi-infaunal, and epifaunal species. Therefore, any generalizations about the relationship between depth of sediment penetrated and frequency of shell damage are inadvisable. Most scars on deep burrowers were not attributable to the direct activity of shell-crushing predators because: (1) injury occurred late in life when individuals were positioned in the safety of a deep burrow; (2) scar types consistent with a predatory origin were rare; and (3) scars frequently occurred on the anterior of the shell, which is least accessible to digging predators.