SALINITY 



The feeding of green crabs was limited less by 

 low salinities tlian by low temperatures in Plum 

 Island Sound. Green crabs maintain a homoi- 

 osmotic condition when subjected to diluted sea 

 water by actively taking up ions through the gills, 

 increasing their oxygen consumption, and expend- 

 ing energy (Scheer, 1948; Waterman, 1960). 

 Duval (1925) found that green crabs lived penna- 

 nently in salinities as low as 11 p.p.t. (parts per 

 thousand), but lower salinities killed them within 

 24 to 48 hours. Green crabs sometimes seek tem- 

 poraiy shelter in moist caves, crevices, and sea- 

 weeds when the water in the estuary is drastically 

 diluted (Broekhuysen, 1936; Nicol, 1935). 



Salinities fell below 11 p.p.t. only at Plum 

 Island River and only on three occasions (table 

 8) . The proportions of crabs containing food were 

 similar during periods of high and low- salinity. 

 Only slightly fewer crabs were caught per drag 

 and fewer food items were counted in the stomachs 

 of crabs caught when the salinity was low. The 

 influence of low salinity on feeding was not ap- 

 parent, even though the values were Ijelow the 

 reported tolerance limits for green crabs. 



Table 8. — Low salinity and the percentage frequency of 

 occurrence of food in green crabs at Plum Island River, 

 1955-56 



Date 



Salinity 



Crabs Crabs 

 with recently 

 food fed 



Crabs 



with 



nearly 



empty 



stomachs 



Total 

 crabs 



p.p.t. 



Apr. 18, 1956 9.0 



Nov. 7, 1955 7.4 



May 11, 1956 7.6 



Percent Percent 



75 87 



90 61 



50 



Percent Number 



13 20 



39 49 



2 



SEX 



Female green crabs were sometimes predomi- 

 nant in the dredge catches. An analysis of the sex 

 ratio and frequency of occurrence of food by sex 

 indicated that feeding differed with sex at Plum 

 Island River. 



Many more female than male crabs from Plum 

 Island River contained mollusks (fig. 8) . Six other 

 food items also occurred slightly more often in 

 female than male cra;bs. Female crabs from Plum 

 Island River were only slightly more numerous 

 than males in the total catch and were relatively 

 more abundant than males for a shorter period of 

 time than at Lufkins Flat (fig. 9). The total 



predation on mollusks by female crabs at Plum 

 Island River was greater than by males, not only 

 because more females were present to feed at cer- 

 tain times of the year, but because they ate more 

 mollusks than did the males. 



Differences were not appreciable between the 

 stomach contents of male and female crabs caught 

 from Lufkins Flat (fig. 8), but more female than 

 mule crabs were caught in nearly every collection 

 (tig. 9). The total predation by females was 

 greater than that by males only because females 

 were more abundant. 



EGG BEARING 



In both sample areas fewer ovigerous crabs con- 

 tained food (table 9) than nonovigerous, even 

 though the kinds of food eaten by the two types 

 of crabs were the same. The reduction in food 

 intake may be accounted for by Dearborn's ■* ob- 

 servations that although ovigerous crabs readily 

 accepted food in the laboratory, they are generally 

 less active. The dredge catches indicated that fewer 

 ovigerous than nonovigerous crabs were feeding; 

 only 5.5 percent of the females from Plum Island 

 River and 4.6 percent from Lufkins Flat were 

 ovigerous. 



Table 9. — Percentage frequency of occurrence of ovigerous 

 and nonovigerous female green crabs that contained food 

 at Plum Island River and Lufkins Flat, 1955-56 



As with other crustaceans, the green crab must 

 molt its hard exoskeleton to grow. While prepar- 

 ing for ecdysis and hardening the exoskeleton 

 after ecdysis, the crab would be likely to change 

 its feeding habits because its claws and mouth- 

 parts are too soft to handle some of the usual 

 foods. To determine the degree of change in food 

 during molting, green crabs caught during 1956 

 were separated into five groups, based on molting 

 condition. One group, the premolt crabs, was 



* Footnote 1. 



196 



U.S. FISH AND WILDLIFE SERVICE 



