Stehlik et al.: Distribution patterns of various crab species in the Hudson-Raritan Estuary 



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val release, any of which might be a reason for the use 

 of the estuaries by female lady crabs. 



The rock crabs that enter the estuary were predomi- 

 nantly males, and many females may never enter the 

 estuary. Males use the estuary to molt, and possibly to 

 avoid predators offshore. In comparison, on the north- 

 west Atlantic continental shelf, the sex ratio in winter 

 dredge collections was 1:2.2 males:females (Stehlik et 

 al., 1991). 



Feeding periodicity 



Food consumption in crabs is affected by daily and 

 seasonal cycles, temperature changes, reproductive 

 rhythms, and molt (Warner, 1977; Stevens et al, 1982; 

 Ryer, 1987; Mantelatto, 2001). In our study area, blue 

 crabs ate little when inactive during the winter months, 

 as reported above. Choy (1986) reported less feeding 

 during egg-brooding in Portunidae, but in our study we 

 found that fullness was about 40% in both egg-bearing 

 and non-egg-bearing females in summer. A lack of feed- 

 ing before and during molt, until calcification has suf- 

 ficiently progressed, is typical of crabs (Warner, 1977). 

 Empty stomachs in premolt and soft rock crabs in our 

 study supported this observation. 



Diet composition 



We found that in the Hudson-Raritan Estuary, the most 

 important prey items of blue crabs by volume were 

 Xanthidae, then the mollusks M. edulis and M. late- 

 ralis, whereas only 2% of the prey volume was from 

 cannibalism. In contrast, small blue crabs are of major 

 importance in the diets of large blue crabs in Florida 

 (Laughlin, 1982) and Maryland (Hines et al., 1990). 

 and cannibalism is the source of more than 75% of the 

 mortality of juveniles near estuarine shores (Hines and 

 Ruiz, 1995). The major targets of cannibalism, early 

 instars or molting juveniles, may be more abundant in 

 rivers adjacent to our study area (Meise and Stehlik. 

 2003). 



The diets of rock crabs in estuarine and coastal 

 Canada and Maine usually contained a larger num- 

 ber of prey categories than did the diets in the pres- 

 ent study (Scarratt and Lowe, 1972; Drummond-Davis 

 et al.. 1982; Hudon and Lamarche, 1989; Ojeda and 

 Dearborn, 1991). These northern studies were done on 

 rock, boulders, cobble, sand, and algal beds, where the 

 diversity of habitats within a study area may offer a 

 larger assortment of potential prey than the soft-bottom 

 habitat of our estuary. 



In the Hudson-Raritan Estuary, juveniles of commer- 

 cially or recreationally harvested species were rarely 

 consumed by the three species of crabs. Among mol- 

 lusks, M. arenaria and M. mercenaria were scarce in 

 crab stomachs, perhaps because other taxa such as M. 

 lateralis, N. trivittatus, and Xanthidae provided abun- 

 dant prey. The other commercially important species 

 eaten by crabs was the blue crab juvenile, but infre- 

 quently as mentioned above. 



Differences in diet among species, sexes, 

 and size classes of predators 



Our data did not support our hypotheses, based on exist- 

 ing studies, that blue, lady, and rock crabs would have 

 different diets as a consequence of their species-specific 

 body and chela structures. Blue and lady crabs (unlike 

 rock crabs) swim, allowing them a greater foraging 

 area than rock crabs. Chela structure affects the type 

 and size of prey that can be crushed (Vermeij, 1978; 

 Seed and Hughes, 1995; Behrens Yamada and Bould- 

 ing, 1998). In Portunidae. the long chelae (in relation 

 to their CW) have short muscle fibers better suited to 

 quick grabbing than to prolonged crushing (Warner 

 and Jones, 1976; Seed and Hughes, 1997). The chelae 

 of Cancridae are monomorphic (same characteristics 

 left and right sides), have relatively short, stout teeth, 

 and close relatively slowly because of their muscle fibers 

 (Warner and Jones, 1976). Chela crushing force (New- 

 tons), measured with a force transducer, is positively 

 correlated with chela height and thickness (Govind 

 and Blundon, 1985; Block and Rebach, 1998). Although 

 the chela structures of blue and rock crabs are quite 

 different, the chelae of mature rock crabs (9-13.5 cm 

 CW) generate crushing forces comparable to those of 

 cutter and crusher chelae of mature male blue crabs 

 (12-16 cm) (Govind and Blundon, 1985). 



Chela crushing force in mature blue and rock crabs is 

 likely to be more than sufficient for successful foraging 

 upon all but the largest prey (Block and Rebach, 1998) 

 and may not be a major determinant of diet. In fact, 

 crabs often prey upon small or young bivalves rather 

 than on large sizes, perhaps because the latter require 

 more handling time and may damage chelae (Juanes. 

 1992; Seed and Hughes, 19951. Because Portunidae 

 swim and have more versatile chelae, they may be ex- 

 pected to have broader trophic niches than Cancridae. 

 In our study, blue crabs had the broadest trophic niche, 

 lady crabs had an intermediate trophic niche, and rock 

 crabs had the narrowest trophic niche. 



We found no significant differences in diet by sex 

 within species. Sexual dimorphism within a crab spe- 

 cies accelerates after puberty (Hartnoll. 1978), but our 

 study included many immature crabs. Some experiment- 

 ers using force transducers found no significant differ- 

 ence in crushing force between the sexes of blue crabs 

 of a broad size range (Blundon and Kennedy, 1982; 

 Seed and Hughes, 1997), but in blue crabs >135 mm, 

 males produced significantly more force than females 

 (Eggleston, 1990). Sexual dimorphism is found in chela 

 length, but not chela height, in lady crabs (significantly- 

 different slopes of CL/CW by regression; Stehlik, un- 

 publ. data). 



Carapace width and the proportion of chela height to 

 carapace width are positively correlated with crushing 

 force, which makes it possible for larger crabs to con- 

 sume larger, harder-shelled mollusks or crustaceans 

 (Hartnoll, 1978; Block and Rebach, 1998). The larg- 

 est lady crabs do not grow to the carapace widths or 

 chela lengths of mature blue crabs; therefore the force 



