INFERRING CRUSHING PREDATION FROM FRAGMENTATION FREQUENCY IN THE MODERN GASTROPODS OLIVELLA BIPLICATA AND NASSARIUS PERPINGUIS

Crushing predation is a major source of molluscan mortality in marine ecosystems and probably has been a factor since the rise of decapod crustaceans in the Mesozoic Era. Quantification of crushing frequencies would help estimate the influence of crushing predation on the ecology and evolution of present and past prey populations. However, quantifying crushing frequency in modern and fossil mollusk populations is difficult. Unlike predatory drilling, crushing does not leave a clear signature on the shell and may be confused with taphonomic fragmentation due to transportation or compaction.

The goal of this study is to assess the origin of damage on modern gastropod shells to provide a baseline for future studies. Because complete drill holes indicate a successful kill, other damage to drilled specimens is most likely postmortem and of taphonomic origin. In undrilled specimens, crushing predation cannot be ruled out as a cause of damage. Thus, the difference in fragmentation frequency between drilled and undrilled specimens may serve as a proxy for crushing frequency (Vermeij 1982).

Dead shells of the modern gastropods Olivella biplicata and Nassarius perpinguis were collected at Torrey Pines State Beach in La Jolla, California. One hundred specimens bearing complete predatory drill holes and 100 undrilled specimens were randomly selected for each taxon and the frequencies of eleven types of shell damage were tallied for each group. In both taxa, chips or cracks on the outer lip of the aperture were significantly greater among undrilled shells (chi-square test, p < 0.01), while other types of damage showed no significant difference.

If the damage frequencies of the drilled specimens represent typical taphonomic damage to a shell, the differences in frequencies found for the undrilled specimens probably represent damage due to predatory crushing. This approach identifies types of damage, such as chips or cracks to the shell aperture, that are likely to be predatory in origin. In addition, relative differences in crushing frequency between mollusk populations may reflect the relative evolutionary or ecological influences of crushing on prey populations. The application of this method may facilitate the comparison of crushing predation between prey species, across habitats, and over time.