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



699 



Foregut fullness 



The total number of blue, lady, and rock crab foreguts 

 examined was 1286. Foregut fullness varied by month 

 in blue and rock crabs. The average fullness of blue 

 crabs was 1% by volume from January through April, 

 and 34% for the rest of the year. Ovigerous blue crabs 

 (/? =27) averaged 40% full. Lady crabs' average full- 

 ness was 41% during the months when they were pres- 

 ent. The average fullness of rock crabs was 30% in all 

 months when they were present; a minimum occurred in 

 January when fullness was 7%. Of 419 rock crabs exam- 

 ined, intermolt crabs (rc=293) were 33% full, premolt 

 crabs (n = 9) were empty, soft crabs (n=22) were empty, 

 and postmolt crabs 1/2 = 95) were 20% full. Some rock 

 crabs in the late postmolt stage were full even though 

 their chelae were not completely calcined. 



Diet composition 



The number of crabs containing food was 975, and they 

 consumed 44 identifiable taxa (Table 2). Most of the 

 mollusks preyed upon were <15 mm in shell length. 

 The crabs consumed were mud crabs (Xanthidae) and 

 juvenile stages of other Anomura and Brachyura. When 

 foreguts were only partially full, well-digested remains 

 of prey frequently could be identified by pieces of shell 

 or opercula, mandibles (for shrimp), or chela tips and 

 carapace fragments (for crabs) (Elner et al., 1985). Rec- 

 ognizable prey taxa were grouped into 12 mutually 

 exclusive categories (Table 3), which contributed 80.1% 

 of the volume of all prey. The prey category "CRABS" 

 represented pooled fragments of all crabs except Pagu- 



120 

 100 

 80 

 60- 

 40 

 20 

 



8 9 10 11 12 1 



3 4 5 6 



1991 



1992 



Figure 4 



Number of rock crabs at each molt stage by month. 

 1991-92. 



Figure 5 



Catch per unit of effort of (Al blue, (B) lady, and (Cl rock crabs, sexes combined 

 January 1992-December 1994, mapped by the midpoint of each tow. 



ridae and Xanthidae. Crabs containing prey in one or 

 more of the 12 categories numbered 713. 



Differences in diet by predator species, sex, and size 



Although the three predator species shared most prey 

 taxa, there were differences in the proportions of the 

 taxa consumed (Fig. 7). Mann-Whitney tests comparing 

 diets of sexes within each species showed only two sig- 

 nificant differences out of 36 comparisons. After cluster 

 analysis upon the 12 prey types by species, sex, and 

 size class (immature and mature), the resulting den- 

 drogram showed that diets were most similar between 

 size classes within a species (Fig. 8). 

 Female rock crabs were not included 

 because of their small sample size. 

 When the diets of the three species 

 were compared by analysis of simi- 

 larity (ANOSIM) they were found 

 to be different (P=0.067>, but the 

 data were extremely variable and 

 not normally distributed. No signifi- 

 cant differences were found between 

 sexes within species and we there- 

 fore pooled sexes within species. 



Pairwise comparisons of the spe- 

 cies were performed by analysis of 

 dissimilarity (SIMPER). Four taxa 

 contributed significantly to the dif- 

 ference in diets of the first pair: the 

 bivalves M. edulis and M. lateralis 

 were more important in the diets 

 of lady crabs, and Xanthidae and 

 CRABS, were more important in 

 the diets of blue crabs. The diets 

 of blue crabs and rock crabs were 

 significantly different in four taxa: 

 CRABS and M. lateralis were more 

 important for blue crabs, and M. 

 edulis and Xanthidae for rock crabs. 



