Crushing predation and shell fragmentation on marine mollusks

Background/Question/Methods

Assessing shell crushing predation on marine mollusks is logistically challenging. It is usually impractical to monitor the behavior of predators such as crabs. Laboratory trials have limited application in natural communities. Thus, traces of predatory attacks, including repair scars and prey shell fragments, are valuable data sources: a single sample holds information from numerous attacks and can average out spatial and seasonal variation. However, fragments created by crushing can be confused with fragmentation by abiotic forces, such as wave or current transport. To determine the feasibility of shell fragments to measure predation intensity, we tested whether (a) crushing fragments can be distinguished from abiotic fragments; and (b) the nature of shell fragments reflected the intensity of crushing predation. We collected sediment samples from three high-predation and three low-predation intertidal localities in Bamfield, British Columbia. The incidence of crushing predation at each locality was established independently through live-gastropod repair scar data and crab attack frequencies on wax model gastropods. For each sample, we sorted shell fragments >1/8" (N = 5800) and examined. Edges were categorized as: intact (original, unabraded shell margin); rounded (original or broken edge that is eroded smooth); or sharp (broken, angular edge that is not yet eroded).

Results/Conclusions

Each fragment, based on its multiple edges, was assigned to one of four categories, each with a hypothesized origin based on prior studies: All Edges Rounded (abiotic forces eroded all edges; not predatory); Rounded and Sharp (rounded edges indicate abiotic, post-mortem erosion, so adjacent sharp edges must also be post-mortem and therefore  abiotic; not predatory); Intact and Sharp (fragmentation is recent, abiotic erosion has not yet occurred; likely predatory); and All Sharp (same as Intact and Sharp; likely predatory). I&S and AS categories comprised 21-51% of fragments at high-predation localities, whereas these categories comprised only 3-15% of fragments at low-predation localities. High- and low-predation localities were distinguishable based on abundance of fragmentation categories (Chi-square tests, p << 0.001). The results hold for both gastropod and bivalve shell fragments. These results support our hypothesized fragment origins, that I&S and AS are the result of predation and that AR and R&S are caused by abiotic erosion. Thus, the nature of the shell fragments is independently indicative of crushing predation frequencies at our study localities. If this relationship between fragmentation and predation is corroborated in subsequent studies, shell fragmentation will be a valuable tool in assessing crushing predation between marine communities.