Biogenic Shell Damage in the Small Gastropod Odostomia Impressa (Say)

Abstract

Nearly every shell and fragment of the small pyramidellid snail Odostomia impressa, occurring in very large numbers in and around modern intertidal oyster banks in coastal Georgia, shows evidence of biogenic shell damage. Almost all of the shells have repaired or unrepaired fractures caused by crabs; most of the shell fragments also appear to be the result of attack by crabs, rather than physical breakage by waves or compaction. Most of the specimens are infested by an algal microborer, which gives shells a chalky, excoriated appearance. Gastropod borings in the Georgia odostomes were very rare. In a comparison with O. impressa shells from subtidal oyster banks in Virginia, it was found that crab damage in the Georgia samples was much more extensive, but that many of the Virginia shells had been bored by the bryozoan Electra. A conservative estimate of the average frequency of fatal attacks on Georgia odestomes from intertidal oyster banks is about 25 percent, using samples from time averaged shelly deposits. The predator on O. impressa in this setting is probably the xanthid crab Panopeus herbstii, which is in turn preyed upon by raccoons, birds, and fish. The algal microborer responsible for microscopic holes and striations in adostome shells appears to be a cyanophyte typical of lower intertidal zones.