Why can unrelated bivalves develop similar shell shapes?

Unrelated bivalves often develop similar shell shapes because their body form is intimately linked to their specific habitat and their need to defend against predation.1

The bivalve body plan exhibits significant plasticity, allowing different groups to repeatedly evolve similar structures—such as elongated, triangular, or circular shells—as a direct response to similar modes of life. For example, species that occupy similar environments, such as those burrowing into sediments or attaching to hard substrates, face comparable physical pressures. These environmental demands drive the evolution of convergent features, such as lateral compression for efficient burrowing or specific shell geometries that facilitate secure attachment to rocks.123

Ultimately, because shell morphology is a functional adaptation to the environment, unrelated lineages often arrive at the same structural solutions when exploiting the same ecological niches.12