GROWTH OF JUVENILE BLUE CRABS, Callinectes sapidus RATHBUN, 

 IN THE ST. JOHNS RIVER, FLORIDA 



BY Marlin E. Tagatz, Fishery Biologist 



Bureau of Commercial Fisheries Biological Laboratory 

 Beaufort, N.C. 28516 



ABSTRACT 



Relative growth was studied by holding juveniles 

 (20-139 mm. wide) in anchored floats. The floats were 

 at two locations — one in salt water and one in fresh 

 water. Molt intervals were similar at both sites, but 

 the average growth was generally more per tnolt in 

 salt water. From April to mid-November, the mean 

 molt interval was 11 days for crabs 20 to 29 mm. wide; 

 it increased to 41 days for crabs of 130 to 139 mm. 



Frequency of molting decreased in winter, but most 

 juveniles 20 to 59 mm. wide molted two or three times. 

 Growth increments per molt varied from 7.8 to 50 per- 

 cent. Mean increment, by 10-nim. width groups, was 

 20.9 to 34.2 percent. Estimates of increase in width 

 with age indicate that most blue crabs in the St. Johns 

 River reach harvestable size (width of 120 mm.) within 

 1 year after hatching. 



This report presents the results of studies from 

 April 1964 to March 1966 on growth increments, 

 molt intervals, and the effects of salinity and 

 tem])erature on growth of juvenile blue crabs, 

 Callinectes sapidus Rathbun, in the St. Johns 

 River, Fla. This information on growth is needed to 

 estimate the time required for crabs of any 

 particular size to reach harvestable size. 



Because the juvenile (postlarval) crab has a rigid 

 shell, it can grow only when it molts. The shell 

 is composed of chitin strengthened by the 

 deposition of calcium salts (Rees, 1963). Before 

 molting, the crab forms a new exoskeleton inside 

 the old shell, which then loosens. During molting, 

 the carapace separates from the abdomen along 

 the sides of the undersurface, and in a few minutes, 

 the crab backs out of the old shell (fig. 1). Initially, 

 the new shell is soft and wrinkled. Water, absorbed 

 after molting, stretches the thin and elastic new 

 cuticle and increases the size of the crab. Crabs 

 grow from the first postlarval crab stages, about 

 2.5 mm. wide, to full size after 18 to 23 molts 

 (Van Engel, 1958). In the St. Johns River, the 

 width of fully grown blue crabs ranges from less 

 than 100 mm. to more than 240 mm. 



Few studies have been made on the rate of 

 growth of juvenile blue crabs. C'luu-chiU (1919) 

 used the limited number of molting records avail- 

 able at that time for Chesapeake Bay crabs and 



Published November 1968. 



FISHERY BULLETIN: VOL. 67, NO. 2 



attempted to determine size at various instars 

 and the frequency of molting from hatching to 

 maturity. He found that time between molts was 

 progressively longer with increase in size. The 

 first few molts were a few days apart and the last 

 at intervals of a month or more. He reported that 

 growth ceases from October until late April or 

 early May and that the animals probably do not 

 molt at tempeiatm'es less than 60° F. (15.5° C). 

 Studies with blue crabs over 29 mm. wide by Gray 

 and Newcombe (1938) and on smaller crabs by 

 Newcombe, Sandoz, and Rogers-T albert (1949) 

 provide the best published data on growtli. The 

 mean increases in width of the various instars 

 ranged from 15 to 48 percent; the mean was 25 

 percent for males and 27 percent for females. No 

 information was obtained on molt intervals be- 

 cause the animals were held for only one molt. 



Niunerous reports of unusually large blue crabs 

 in low-salinity waters and very small adult crabs 

 in high-salinity waters suggest a possible negative 

 correlation of size with salinity of tlie water in 

 wliicli growth occurs (Newcombe, 1945; Porter, 

 1956; Cargo, 1958; Fischler, 1959; Tagatz, 1965). 

 ^'an Engel (1958) believed that crabs in low salin- 

 ity absorb more water at the time of the molt and, 

 thus, increase more in size than crabs that molt 

 in water of higher salinity- 



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