SIX THOUSAND LITTLE PIECES: SHELL FRAGMENTS AS AN INDICATOR OF CRUSHING PREDATION INTENSITY

Traces of crushing attacks, such as repair scars and shell fragments, are important methods of assessing predation. Unlike field monitoring (logistically challenging) and laboratory experiments (limited application to natural systems), fragments can be collected quickly and easily. A single sample typically represents numerous attacks and can average the noise of spatial and seasonal variation. However, distinguishing fragments produced by crushing predation versus waves or currents remains a challenge.

We tested whether the nature of fragments reflects predation frequency in modern mollusk assemblages from three high- and three low-predation intertidal localities in Bamfield, British Columbia. Predation intensity at each locality was established independently by live-gastropod repairs and crab abundance. We categorized the edges of shell fragments >1/8" (N = 5800) as: intact (original, unabraded margin); rounded (original or broken edge is eroded smooth); or sharp (broken, uneroded edge). Each fragment was assigned to one of four edge categories: AR (All edges Rounded by abiotic abrasion), R&S (both Rounded and Sharp edges, fragmentation occurred after taphonomic abrasion), I&S (both Intact and Sharp edges, no evidence of taphonomy), and AS (All edges Sharp, no evidence of taphonomy). We hypothesized that AR and R&S, indicative of taphonomy, should be more frequent in low-predation localities. I&S and AS, lacking evidence of taphonomy, may result from predation and should be more common in high-predation environments.

I&S and AS fragments comprise 21-51% of fragments at high-predation localities, and only 3-15% at low-predation localities. High- and low-predation localities were distinguishable based on the abundances of the categories (Chi-square tests, p << 0.001); this is true for both gastropod and bivalve shell fragments. This supports our hypothesis that I&S and AS result from predation, while AR and R&S are taphonomic. Thus, the nature of the shell fragments independently corroborates crushing predation frequencies at our study localities. Shell fragmentation could be a valuable tool in assessing crushing predation in marine communities, and additional studies confirming the relationship between fragmentation and predation are needed, particularly targeting fossil data.