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A Unique Respiratory Adaptation to Emersion in the Introduced Asian Freshwater Clam corbicula Fluminea (Müller) (Lamellibranchia: Corbiculacea)

When exposed to air, the Asiatic clam, Corbicula fluminea, periodically consumes oxygen across exposed mantle-edge tissues sealed together by hardened mucus. This behavior alternates with longer periods of valve closure (mean mantle-edge exposure time = 49 min; mean valve closure time =426 min). At 20 C, aerial oxygen uptake rates were very reduced during valve closure periods, averaging 0.03 μl O₂/(mg dry flesh ¸ h), whereas during mantle-edge exposure they averaged 0.88 μl O₂/(mg dry flesh · h). Total aerial oxygen consumption was approximately 21% that of normal aquatic values in this species. This aerial respiratory behavior appears to be an adaptation to prolonged emersion, allowing both maintenance of substantial levels of aerobic metabolism and minimal evaporative water loss. Corbiculafluminea has a relatively short fossil history in fresh water, and many of its physiological adaptations appear to be intermediate between those of estuarine bivalves and the more highly evolved freshwater species. Its mode of aerial oxygen consumption is unique among bivalve mollusks and appears to be an adaptation that is similarly intermediate between the continual aerial gaping behavior of estuarine and intertidal species and the direct exchange of respiratory gases across continually closed valves, characteristic of certain freshwater unionid and sphaeriid bivalves.