How do crabs influence the evolution of mollusk shells?

Crabs influence the evolution of mollusk shells through a process of coevolutionary escalation, often described as an evolutionary arms race. As crabs develop more powerful chelae (pincerlike claws) to crush or drill into their prey, mollusks respond by evolving increasingly robust defensive features.1

This selective pressure has driven several significant adaptations in mollusk shell morphology:

• Increased Thickness: In environments where crab predation is high, such as in Lake Tanganyika, mollusks have evolved significantly thicker calcareous shells compared to related species in areas with less intense predation.1
• Elaborate Ornamentation: Many gastropods have developed complex surface sculptures, including knobs and spines, which serve as physical deterrents against predators attempting to grasp or crush the shell.2
• Structural Reinforcement: Bivalves may develop radial ribs, concentric rings, or sharp ridges that strengthen the shell against damage. Additionally, features like interlocking hinge teeth help prevent the valves from being sheared apart.34
• Behavioral Defenses: Beyond physical shell modifications, the threat of crab predation has influenced behaviors such as burrowing deep into sediment, where mollusks often adopt smoother, more compressed shell shapes to facilitate movement and escape.34

These changes demonstrate how the predatory capabilities of arthropods directly shape the defensive anatomy of their mollusk prey over generations.1