Meeting Abstract
Top-down control by consumers helps regulate prey populations and thereby structure communities. Small shore crabs (genus Hemigrapsus), usually not considered to be significant predators, may contribute to very high field mortality of newly settled clam spat before the baby bivalves achieve a size refuge from crushing. We used a Materials Testing System (MTS) to compare the peak load (N) required for crushing (shell failure) of small clams of different species, including Manila (Venerupis), softshell (Mya), and the invasive Varnish (Nuttallia) clams, across an ontogenetic range of sizes (2 to 19 mm length). For each individual clam, we measured shell length, width, and thickness and recorded the peak load (N) at which the shell failed. We also compared the effects on crushing force required for clams grown for a year in the field with and without oyster shell pieces added to the sediment to locally buffer pH. We found that the peak load vs. length scaled allometrically for Varnish and Manila clams, and isometrically for softshell clams. The force required for crushing (indicated by the intercept of the allometric line) also varied among the clam species. We also looked at morphological scaling of shell thickness vs. length and found the scaling to be similar among species. We did not find a difference in the scaling relationship for clams grown with and without added shell but we did find a difference in force to fail, with control clams grown in plain sediment withstanding more force, contrary to what would be expected if there was a pH buffering effect of shell addition. These results, when combined with Hemigrapsus feeding trials, will provide a better understanding of size refugia for very small clams and of predator-prey interactions across ontogeny.
