Exclusively subterranean bivalves—the group of molluscs comprising clams, oysters, mussels, scallops—are considered a rarity. Prior to the present study, there had only been three such species confirmed in the world: all belonging to a small-sized mussel genus known from southeastern Europe. Furthermore, bivalves are not your typical ‘underworld’ dweller, since they are almost immobile and do not tolerate environments low in oxygen.
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Source: Phys.org