The defensive role of scutes in juvenile fluted giant clams (Tridacna squamosa)

This study tests the hypothesis that the scaly projections (scutes) on the shells of juvenile giant fluted clams, Tridacna squamosa, are an adaptation against crushing predators such as crabs. The forces required to crush scutes and clams were measured with a universal testing machine whereas crab chela strength was measured with a digital force gauge connected to a set of lever arms. Results for shell properties and chela strength are used to create two, non-mutually exclusive, predator–defense models. In Model 1, scutes increase the overall shell size, consequently reducing the number of crab predators with chelae that are large enough to seize and crush the prey. In Model 2, the chela has to open more to grasp a prey with these projecting structures which leads to a loss of claw-closing force such that crabs fail to crush the scutes, and consequently the clam. Clam scutes may also deter crab predators by increasing the risk of claw damage and/or handling time.