ABIOTIC DAMAGE ON GASTROPOD SHELLS AND IMPLICATIONS FOR PREDATION RESEARCH

One impediment to studying crushing predation on gastropod shells in the fossil record is the possibility that predatory damage (particularly damage to the outer apertural lip due to shell peeling) may be confused with abiotic (post mortem) damage. Predatory damage occurs when a predator breaks the shell to access the edible tissue. Peeling the shell from the aperture is a characteristic strategy of crustaceans. Abiotic damage occurs when the shell comes in contact with hard particles (sediment) in moving water. Few studies have characterized abiotic damage to gastropod shells to assess the plausibility that such damage could be mistakenly ascribed to predation.

To examine abiotic damage, we placed empty gastropod shells in a rock tumbler with two different grain sizes of sediment to simulate the conditions of relatively high-energy environments, such as a fast-moving stream or the surf zone of a beach. Five to six clean, empty shells of five gastropod taxa each were placed in a rock tumbler with sediment and water. This protocol was repeated for two grain sizes: finer-grained (1 – 3 mm) and coarser-grained (3 – 5 mm). The shells were checked periodically during the tumbling period, then removed after ten days and examined for damage incurred during tumbling.

Despite morphological differences among the five taxa, the damage was generally consistent. The most common type of damage, observed among all taxa, was abrasion to the shell surface. In the smaller-grain trial, shell condition ranged from undamaged to minor holes due to abrasion. One cerithiid shell lost the apertural lip, likely as a result of a widening abrasion hole. The larger-grain trial resulted in considerably more damage. All taxa had holes worn into the body whorl and many also had significant damage to the spire. Most ornament was completely abraded. Apertural lip damage, as a result of growing abrasion holes, was observed among three taxa. Although apertural damage did occur in a few specimens, the damage is unlikely to be confused with predatory fragmentation. The abraded margins of the damaged aperture, along with other signs of extensive abrasion, indicate abiotic origins. However, further experiments are needed, approximating a wider range of environmental conditions, to rule out the possibility that abiotic forces can cause predation-like damage.