THE FEEDING HABITS OF THE GREEN CRAB, CARCINUS MAENAS (L.) 



BY JOHN W. ROPES, Fishery Biologist 



BUREAU OF COMMERCIAL FISHERIES BIOLOGICAL LABORATORY 



OXFORD, MARYLAND 21654 



ABSTRACT 



Green crabs are believed to be the cause of a decline 

 during the 1940's in the commercial landings of the 

 soft-shell clam (Mya arenaria L.)- Collections of 3,979 

 green crabs from Plum Island Sound, Mass., and 

 vicinity in 1954-56 provided the stomachs for a study 

 of their feeding habits. Laboratory and field observa- 

 tions were also made. The green crab is an omnivore, 

 but usually contains pelecypods. It uses its claws for 

 both digging and feeding. Accessory organs other than 

 eyes aid in directing it to food. Feeding is influenced 

 by the abundance, size, and kind of food. Although 

 all sizes of green crabs contained essentially the same 



kinds of food, pelecypods were most frequent in crabs 

 with carapace widths of 30 to 59 mm. Most crabs ap- 

 parently feed during the night; crabs with food in 

 their stomachs were most numerous in catches made 

 just after sunrise. Water temperatures of 7° C. and 

 below reduced activity and presumably feeding as well. 

 Low salinities, however, apparently did not affect 

 feeding. Because female crabs were relatively more 

 numerous than males in the fall, the season when 

 newly settled juvenile Mya occur in abundance, they 

 probably were more destructive than males in Plum 

 Island Sound. 



The New England landings of soft-shell clams, 

 Mya arenaria L., declined from a high of 14.7 mil- 

 lion pounds in 1938 (Fiedler, 1940) to a low of 

 2.3 million pounds in 1959 (Power, 1961). The de- 

 cline seriously affected the fishery and stimulated 

 action to determine the cause. In 1948, Congress 

 authorized the Bureau of Commercial Fisheries to 

 study the resource ( Glude, 1955 ) . 



The research of the Bureau has resulted in many 

 publications on the biology of the soft-shell clam, 

 with especial attention being paid to the predators 

 of the clam. Some of the studies included observa- 

 tions on mortality (Smith, 1950), population size 

 (Spear, 1953), activity (Baptist, 1955; Smith, 

 1955) , and parasites (Uzmann, 1952 ; Uzmann and 

 Stickney, 1954; Stunkard and Uzmann, 1958; 

 Stunkai-d, 1960). The effects of man (Glud, 1951 : 

 Glude, 1954) and other predators (Smith and 

 Chin, 1953) on the fishery were also observed. Ani- 

 mal predators were eventually identified as the 

 most probable cause of the decline in clams; many 

 species are known to feed on soft-shell clams. For 

 example, soft-shell clams are sometimes included 



Published Octolier 1968. 

 FISHERY BULLETIN: 



VOL. 67, NO. 2 



in the diet of rock crabs, Cancer irroratiis (Mead 

 and Barnes, 1904) ; lady crabs, Ovalipes occeJatiis, 

 and blue crabs, Callinectes sapidits (Bel ding, 

 1930) ; horseshoe crabs, Limulus polyphcmus 

 (Belding, 1930; Turner, 1949; Smith and Chin, 

 1953) ; moon snails, Polinices dupJicatus (Sawyer, 

 1950; Hanks, 1952, 1957) ; winter flounders, Pseti- 

 (lopleur&nectes ame7%ca7ius (Medcof and Mac- 

 Phail, 1952) ; herring gulls, Lanhs argentatus 

 (Mendall, 1934; Medcof, 1949) ; and black ducks. 

 Anas ruhripes (Mendall, 1949) . None of these ani- 

 mals, however, seemed to be responsible for the low 

 abundance of clams. Neither blue crabs nor lady 

 crabs are common north of Cape Cod, and none 

 were found on the clam flats. The other predators 

 either fed infrequently on clams or were found in 

 only some of the areas where clams were not 

 abundant. 



One other possible predator, the green crab, Car- 

 cinus maeruts (L.), was widespread, abundant, and 

 apparently a species that had recently extended its 

 range north of Cape Cod. Scattergood (1952) doc- 

 umented the spread of the green crabs from south 



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