The genus Mercenaria includes the quahogs, Mercenaria mercenaria, the northern quahog or hard clam, and M. campechiensis, the southern quahog. These two species commonly hybridise where their ranges overlap.
Other species within the genus include the venus clam M. stimpsoni found in north Pacific waters.
All these species were formerly placed in the related genus Venus. Several other species are known only from fossils.