A larpe percentage of stomach contents con- 

 sisted of organic material in advanced stages of 

 digestion which could not be identified. Sand, 

 mud, wood, charcoal, and similar debris (not 

 recorded) were often taken by crabs incidentally 

 in feeding. Sand, ranging from a smgle grain to 

 several hundred grains, occurred in all gastric 

 mills. A greater proportion of sand, i-elative to 

 food content, was present in smaller crabs (less 

 than 50 nnn. wide) than in larger ones. 



Generally all sizes of blue crabs ate the same 

 types of foods (table 9) at all four stations 

 sampled (table 10). Most of the moUusks eaten 

 were mussels and clams. No mussels were found 

 in crabs less than 21 mm. wide. Mussels eaten in 

 the lower river stations (Clapboard Creek, Dunn 

 Creek, and Trout Kiver) were almost all Modiolus 

 demissiis but included a few Mti^sculus niger. The 

 only sjDecies present and eaten at Picolata was 

 Myfilopsis leucophcata. Mussels were strongly 

 predominant in crabs from Picolata where clams 

 played a relatively minor role in the diet. Small 

 numbers of M. hucopheafa were present in Trout 

 River, but none appeared in stomach samples. 

 Mussels selected were less than an estimated 15 

 mm. long, and usually the shell in the stomach was 

 fragmented. Very small mussels, 1.5 to 5.0 mm., 

 were preferred; the gastric mill of adults often 

 contained 25 to 50. Of the clams, Sphaerridae, 

 Rangia cuneata^ and Mtdinia lateralis were con- 

 sumed, and probably other clams common to the 

 lower river — TelUna cdtemata, T. syharitica, 



Tagelus gihhus, and Mactra fragilis. Crabs less 

 than 21 mm. wide selected clams less than 2 mm., 

 and even adults usually took clams under 10 mm. 

 The stomachs of small crabs most often contained 

 5 to 10 clams, whereas those of larger crabs usually 

 had 20 to 35 individuals. A few of the mollusks 

 eaten were oyster spat, Cra-ssostrea virginica, and 

 snails, Nassarms oisoletus and possibly Nassarius 

 vihex and Neritina reclivata. 



Amphipods and crabs were the dominant Crus- 

 tacea eaten. Gammarus fasciatus was taken at all 

 stations, and CoropMum sp. was a particularly 

 important food of all sizes of blue crabs at 

 Picolata. Carinogamimarus mucronatus and Tal- 

 orchesti-a longicomis, present in the lower river, 

 probably were among other amphipods eaten. 

 Crabs less than 31 mm. wide commonly had 10 to 

 15 amphipods in their stomachs. Of the crabs 

 eaten, about one-half of those that could be identi- 

 fied were blue crabs and the remainder mud 

 crabs — Eithropanopeus harrisi and probably 

 Panopeus herbstii and Ei(rypannpens depressus. 

 No more than four individuals were in any food 

 tract, and none appeared to have exceeded 20 mm. 

 The mysid, Neomysis amencana (listed as a 

 shrimplike form), was taken fairly frequently, 

 jjarticularly in Clapboard Creek. Shrimplike 

 fonns as large as an estimated 30 mm. provided 

 food but were too fragmented for positive iden- 

 tification. The most common shrimp at all stations 

 were Palaemonetes pugio and P. vidgaris., and at 

 Picolata, also Macrohrdchvum ohione. Ostracods 



Table 9. — Food of the blue crab, by crab size, in the St. Johns River as percentage frequency of occurrence and {in parentheses) 



as percentage of total volume per stomach 



[Number of stomachs with food in parentheses below designation of size group of crabs] 



1 Less than 0.6 percent. 

 BIOLOGY OF BLUE ORAB IN ST. JOHNS RIVER, FLORIDA 



29 



