 The thick shell and rounded shape of bivalves make them awkward for potential predators to tackle . Nevertheless , a number of different creatures include them in their diet . Many species of demersal fish feed on them including the common carp ( Cyprinus carpio ) , which is being used in the upper Mississippi River to try to control the invasive zebra mussel ( Dreissena polymorpha ) . Birds such as the Eurasian oystercatcher ( Haematopus <unk> ) have specially adapted beaks which can pry open their shells . The herring gull ( Larus argentatus ) sometimes drops heavy shells onto rocks in order to crack them open . Sea otters feed on a variety of bivalve species and have been observed to use stones balanced on their chests as anvils on which to crack open the shells . The Pacific walrus ( Odobenus rosmarus divergens ) is one of the main predators feeding on bivalves in Arctic waters . Shellfish have formed part of the human diet since prehistoric times , a fact evidenced by the remains of mollusc shells found in ancient middens . Examinations of these deposits in Peru has provided a means of dating long past El Niño events because of the disruption these caused to bivalve shell growth . 
