is the distance between the tips of the large lateral 

 spines, and length the distance between the median 

 anterior notch and the posterior margin. Measnre- 

 nients of crabs that died within a week after 

 molting were not nsed in estimating growth. 

 Except for Icngtli-widtli ratio, I discnss growtli in 

 terms of widtli. 



Salinity aiul temperature were measured at 

 each observation. Salinity determinations, ob- 

 tained witliin 2 liours of low tide at Cedar Point 

 Creek, probably were near mininuun values for 

 this site. Salinities at Green Cove Springs were 

 always less than 1 p.p.t. — the lowest concentra- 

 tion detectable with the hydrometer. Blue crabs 

 and other nuu'ine forms are able to live in tlie 

 up])er St. Johns River because it has a high con- 

 centration of calcium chloride and also localized 

 areas of high sodium chloride (Odum, 1953). 



I defined as summer studies those from early 

 April to mid-November and as winter studies 

 those from near the end of November through 

 March. Summer work ended earlier in salt water 

 because of damage to tlie floats from a September 

 storm. During the summer, crabs were replaced 

 (randomly collected near floats) when they died, 

 escaped, attained maturity, or surpassed 139 mm. 

 All new crabs were used at the start of winter, 

 and none were replaced. 



LENGTH-WIDTH RATIO 



Length-width ratios were obtained for blue 

 crabs 10 to 149 mm. wide captured in the St. 

 Johns River (table 1). For comparison, I have 

 included the data given by Newcombe et al. (1949) 

 for Chesa])eake Bay blue crabs. The two sets of 

 data show similar changes in body form with 

 growth. As size increases, the length-width ratio 

 decreases. For St. Jolms River crabs the ratio 

 expressed as a ])ercentage was 50 in juveniles 

 10 to 19 mm. wide and decreased to less than 42 

 in crabs over 119 nun. wide. In crabs over 69 mm. 

 wide, nuiles were ])roportionally longer tlum 

 females. 



The body proportions of blue crabs from the 

 St. Johns River differed from those of crabs from 

 the Chesapeake Bay. In every widtli class tlie 

 Chesapeake Bay crabs liad a greater mean lengtli 

 than the St. Jolms River crabs. Geographic 

 variation is to be expected in an animal as widely 

 distributed as the blue crab. 



Table 1. — Length-width ratios of blue crabs from 

 Johns River, Fla., and Chesapeake Bay 



St. 



' Data from Newcombe et al. (1949). 



RELATIVE GROWTH 



Observations on growth were made during 

 summer and winter. 



SUMMER 



I obtained data on 1,085 molts in salt water 

 (April 1 to September 15, 1964) and on 1,152 in 

 fresh water (April 24 to November 15, 1965). 



Water temperatures were somewhat similar at 

 the sites, but salinities difl'ered by as much as 

 about 26 p.p.t. The mean water temperature was 

 26.8° C. in salt water (range, 13.8°-32.1° C.) and 

 25.8° C. in fresh water (range, 16.2°-31.5° C). 

 Mean salinity (readings taken within 2 hours of 

 low tide) at the Cedar Point Creek site was 

 18.8 p.p.t. (range, 7.5-25.8 p.p.t.); at Green 

 Cove Springs, salinity was always less than 

 1 p.p.t. 



Confinement did not appear to inhibit growth 

 of blue crabs. The experimental animals ate 

 readily, remained pugnacious, and many molted 

 six or seven times. Growth increments of crabs 

 that molted more than once in confinement 

 averaged about the same as those for crabs of 

 comparable size that molted only once. 



GROWTH OF JUVENILE BLUF, CRABS IX ST. JOHNS RIVER, FLA. 



283 



