Crab-resistant features of shells of burrowing bivalves: Decreasing vulnerability by increasing handling time

Abstract

The vulnerability of burrowing bivalves to shell-breaking predation by crabs was found to be influenced strongly by shell features: size, shell thickness, degree of inflation, and the presence or absence of a gape. The relationship between the critical size of a bivalve (maximum size of vulnerability) and crab size was determined for four different morphotypes of bivalves. For the three bivalves where a “size refuge” was present, critical size increased with crab size. Nevertheless, when offered a choice crabs preferred clams well below the critical size and ate them in the order predicted by the critical-size experiment. Examination of the mechanics of shell crushing revealed how these shell features decreased vulnerability. Larger crabs could efficiently handle larger clams because both chela strength and degree of chela gape increased with crab size. Strain gauges attached to crab chelae showed that thick-shelled clams resisted a greater total number of force pulses than did thin-shelled clams of the same body weight. This may be related to the ability of thick-shelled clams to withstand greater loads than thin-shelled clams when loaded only once. This suggests that the reason for the increased resistance to crabs is prolongation of the shell-breaking time. Even though a large thick-shelled, tightly-closing, clam could eventually be opened, it will probably be rejected in favour of prey with shorter handling times.