Laboratory experiments on Neverita didyma (Roding) preying on Ruditapesphilippinarum (Adams & Reeve) indicate distinct prey size preference which is a function of predator size. Prey size limits are determined by foot size, the organ used in captunng and handling prey. When presented with 2 prey patches equal in area but containing different prey sizes, the predator orients itself in the direction of the preferred prey size. N. didyma is unable to assess prey shell thickness and relies solely

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... prey size, gauged by the ease of 'handling', for selecting prey. Evidence and arguments are presented to show that optimal foraging in N. didyma, and possibly other predators which seize their prey, evolved from passive mechanical selection leading to increased predator fitness.