 C. maenas is a predator , feeding on many organisms , particularly bivalve molluscs ( such as clams , oysters , and mussels ) , polychaetes and small crustaceans . They are primarily nocturnal , although activity also depends on the tide , and crabs can be active at any time of day . In California , preferential predation of C. maenas on native clams ( Nutricola spp . ) resulted in the decline of the native clams and an increase of a previously introduced clam ( the amethyst gem clam , Gemma gemma ) . C. maenas has been implicated in the destruction of the soft @-@ shell clam ( Mya arenaria ) fisheries on the east coast of the United States and Canada , and the reduction of populations of other commercially important bivalves ( such as scallops , Argopecten irradians , and northern quahogs , Mercenaria mercenaria ) . The prey of C. maenas includes the young of bivalves and fish , although the effect of its predation on winter flounder , Pseudopleuronectes americanus is minimal . C. maenas can , however , have substantial negative impacts on local commercial and recreational fisheries , by preying on the young of species , such as oysters and the Dungeness crab , or competing with them for resources . 
