Estimating crushing predation through shell fragmentation frequency in intertidal gastropods

Background/Question/Methods

Crushing predation exerts a major influence on the ecology of gastropod communities. While evidence of drilling predation is easily identifiable, estimating crushing predation is problematic because crushing tends to destroy evidence of the event. Crushing predators, such as crabs, may exhibit stereotypy in their attack (such as chipping or peeling), but the fragmentation incurred can be confused with taphonomic damage due to transportation. Vermeij (1982) reasoned that because complete drill holes represent successful kills, drilled shells cannot have been killed by crushing predators as well. Thus, the fragmentation frequency of drilled shells represents taphonomic damage, so any difference in fragmentation frequency of undrilled shells is attributable to crushing predation.

Dead shells of the intertidal gastropods Olivella biplicata and Nassarius perpinguis were collected at Torrey Pines State Beach, La Jolla, California. One hundred drilled and 100 undrilled shells of each taxon were selected randomly. The frequencies of several types of damage were recorded for each group, representing variation in location, extent, and nature of damage, e.g. a missing apex, chips into the shell aperture margin, cracks in the body whorl, and non-predatory boring. Fragmentation frequencies were compared between drilled and undrilled specimens to estimate the frequency due to crushing predation.

Results/Conclusions

Undrilled O. biplicata exhibited greater frequencies of minor damage to the aperture margin (chi-square test, p < 0.001), chips into the posterior aperture (p < 0.01), and cracks or holes in the body whorl (p < 0.01) than drilled specimens. Undrilled N. perpinguis also had greater frequencies of chips into the posterior aperture (p < 0.001) and holes in the body whorl (p < 0.01) than drilled specimens. For both taxa, this difference in fragmentation frequency between drilled and undrilled shells indicates that crushing predation is a cause of those particular types of fragmentation.

This method holds promise not only for estimating crushing predation frequency in populations of gastropods, but also for identifying what types of damage are caused by crushing predation. For example, the high frequency of margin chips matches direct observations of crab predation on gastropods. Because O. biplicata and N. perpinguis have different morphologies, they are susceptible to different types of damage. This method may be used to compare the relative vulnerability of given taxa to crushing predation. The method may also be applied between localities, revealing differences in predators or environment.