Why are there no fully terrestrial bivalves?

Bivalves are exclusively aquatic organisms, and there are no fully terrestrial species within the class Bivalvia. Their absence from land is primarily linked to their specialized mode of life and respiratory physiology.12

Most bivalves are sedentary or slow-moving filter feeders that rely on ciliated gills (ctenidia) to extract both oxygen and food particles from the surrounding water. This respiratory system is highly dependent on a constant flow of water, which is moved by the action of microscopic cilia. Because their gills and mantle surfaces require a moist environment to function, bivalves are physiologically restricted to aquatic habitats.31

While some intertidal and freshwater species have evolved limited mechanisms to withstand temporary drought—such as closing their shells to prevent dehydration or shifting to anaerobic metabolism—they cannot sustain a fully terrestrial existence. Unlike gastropods, which have successfully adapted to land by evolving specialized mantle cavities that function as lungs, bivalves have not developed the necessary anatomical adaptations to breathe air or thrive outside of water.32456