Table 3. — Molt interval in the blue crab, by lO-mni.-width 

 groups, St. Johns River, Fla., April 1 to November 15, 1964 

 and 1965 



[Salt- and (resh-water data combined; ranges in parentlieses] 



before the final molt was 38 days (range, 18-77 

 days); in fresh water (crabs averaged 117 mm.), 

 it was 40 days (range, 18-61 days). 



WINTER 



Data on molt intervals (from animals that 

 molted more than once in confinement) were 

 obtained on 112 crabs in salt water and 96 in 

 fresh water during winter (November 20 to 

 March 31). The time between molts was three to 

 four times as long in winter as in the rest of the 

 year. The mean interval was 46 days for crabs 20 

 to 29 mm. \vide and rose to 124 days for an individ- 

 ual 92 mm. wide (table 4). Most juveniles 20 to 59 



T.\BLE 4. — Molt interval in the blue crab, by 10-mm. -width 

 groups, St. Johns River, Fla., November 20 to March 01, 

 1964-65 and 1965-66 



[Salt- and fresh-water data combined] 



mm. wide molted two or three times, but many 

 of those larger than 99 mm. did not molt (table 5). 

 Temperature has a major influence on the molt 

 interval. In Chesapeake Bay, growth of blue 

 crabs usually ceases during the colder months from 

 November to early April (Van Engel, 1958). In 

 the St. Johns River, low temperatures delay but do 

 not prevent molting. I observed crabs shedding 

 their old shells at water temperatures as low as 

 3.8° C. Molting in winter was less frequent at the 

 lower water temperatures. The numbers of molts 

 were about the same in November and March 

 (mean water temperature 16° C.) as during the 

 3 months, December-February (mean water tem- 

 perature 12.5° C). Extended observations in 

 fresh water indicated that almost all of the 

 crabs that did not grow in winter molted during the 

 first 3 weeks of April (mean water temperature 

 had risen to 18.2° C). 



ESTIMATE OF ABSOLUTE GROWTH 



Mean percentage growth per molt (25.3) and 

 mean molt interval per size group (for summer and 

 winter) were used with relevant data from 

 laboratory experiments on blue crabs less than 

 20 mm. to obtain gross estimates of increase in 

 body size with age. Although the floats would 

 not hold crabs smaller than 20 mm. wide, I have 

 data on two individuals that reached this size 

 wlien hatched and reared in the laboratory. After 

 metamorphosis to the first crab stage, one of 

 these crabs went through' 10 molts in 68 days to 

 reach 23 mm.; the other took nine molts and 



T.VBLF, .5. — Frequency of molting of blue crabs, by 10-mm. - 

 width groups, St. Johns River, Fla., November 20 to 

 March 31, 1964-65 and 1965-66 



[Salt- and fresh-water data combined; no crabs molted more than three times] 



J Crabs wider than 99 mm. did not molt more than once. 

 286 



' Not included are 10 crabs that died before molting once. Deaths after one 

 molt: 1(20-29), 1(30-39), 1(40-49), and 1(60-69); after two molts, 1(40-49). 



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