(full paper is archived in the Miller Library)
Title: Fatigue and fracture at the muscle scar of four intertidal limpet species: C. limatula, C. pelta, C. digitalis, and A. mitra
Student Author(s): Bagdadi, Daniel
Location: Final Papers Biology 175H
Date: June 1988
Keywords: biological studies on gastropods
Abstract: This study examined and compared the forces required to fracture limpet shells. Shells of recently-killed Collisells pelta, C. limatula, C. digitalis, and Acmaea mitra were fractured in an apparatus mimicking the crushing behavior of crabs. In addition, shells of C. limatula and C. pelta were tested for fatigue fracture by repeatedly loading the shells. All species save A. mitra showed increased force needed for fracture as shell thickness increased. No significant differences existed in force as a function of shell thickness among the species nor within each species for forces applied at the length and width of the scar and margin. However, shell size at the width of the scar was significantly smaller than at the other orientations for each species, suggesting that crabs could attack more shells at the scar witdth than at other places. Also, shells of C. limatula had significantly smaller widths at the scar than the other species, suggesting that perhaps more C. limatula shells are found fractured at the muscle scar because these smaller scar width shells are within the grasping rand of more crabs. Fatigue fracture proved significant (causing fracture below 200 cycles) at force loads above 60% of the predicted breaking force. Above 70-75% of predicted strengths, fracture required less than 50 cycles, indicating that within short time spans crabs couldf racture shells without ever exerting a maximum force. Scanning electron microscope observations revealed that the crossed-lamellar layers along the horseshoe-shaped scar layer form almost continuous sheets that allow cracks to fracture the upper portion of the shell cleanly at the muscle.