Having two shells or valves that are equal in size and shape, as in certain bivalve mollusks.
From 'equi-' (equal) plus 'valve' from Latin 'valva' (leaf of a door, shell). Used primarily in malacology (the study of mollusks) and zoology.
Clams and mussels are equivalve, but scallops and oysters are inequivalve—meaning their shells are different sizes—a detail that actually helps marine biologists quickly identify species and understand how they evolved different hunting or anchoring strategies.
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