FISHERY BULLETIN: VOL. 70, NO. 2 



day; the water was changed after each feeding. 

 Preliminary studies were undertaken to deter- 

 mine the amount of HgCl2, an inorganic mercury 

 compound, that could be added to the water with- 

 out killing the crabs. A concentration of 9 X 

 10-^ M HgCl2 was found to be sublethal for crabs 

 which were kept under optimal conditions of 

 temperature and salinity. Under these condi- 

 tions crabs survived for a 2-month period with 

 only slight mortality. The experiment was 

 terminated at this point. The initial concen- 

 tration of mercury, 9 X 10 "'^ M HgCl2 was 

 0.18 ppm Hg (or 0.18 mg/liter seawater). 



Tissues of crabs were analyzed for mercury 

 following exposure to 0.18 ppm mercury in 30^c 

 seawater at a temperature of 25°C for 1, 3, 7, 14, 

 and 28 days. Tissues of crabs maintained under 

 the same conditions but without added mercury 

 were also analyzed. Tissues were removed from 

 10 crabs for each assay and frozen immediately. 

 The concentration of mercury in each tissue was 

 then determined on a Perkin-Elmer Mercury 

 Analyzer System-50.' The techniques were 

 based on the Environmental Protection Agency 

 method developed by the Analytical Quality 

 Control Laboratory, using dilute nitric acid to 

 digest the samples. Determinations were made 

 by South Carolina State Board of Health person- 

 nel. Five tissues were assayed: gill, hepatopan- 

 creas, green gland, abdominal muscle, and car- 

 apace. None of the tissues were kept frozen 

 for more than 1 week. Tissues from 20 males 

 and females (two determinations each) were 

 assayed for each of the five experimental time 

 exposures. Since the amount of mercury proved 

 to be essentially the same in tissues of both males 

 and females, all data were pooled. 



To determine the synergistic effects of the 

 normally sublethal concentrations of mercury 

 and stressful environmental factors, crabs accli- 

 mated to 25°C, 30^^( seawater were placed in 

 seawater with a salinity of 5'/!, containing 0.18 

 ppm mercury or in 5'/ic seawater without mercury 

 and maintained at either 5°C or 35°C. At each 

 experimental temperature, four groups of ani- 



' Reference to trade names in this publication does 

 not imply endorsement of commercial products by the 

 National Marine Fisheries Service. 



mals were used. Thus at 5°C, 5^f , one group of 30 

 males and a second group of 30 females were used 

 as controls; in the experimental group 30 males 

 and 30 females were maintained under the same 

 conditions except the water contained 0.18 ppm 

 mercury. The same procedure was followed at 

 35°C and in a salinity of 5%c. Survival of both 

 experimental and control crabs was followed for 

 28 days or until 50 9f of any one group had died. 

 The temperatures of 5°C and 35 °C were selected 

 since they represent low and high temperature 

 extremes which fiddler crabs experience season- 

 ally in South Carolina marshes. A salinity of 

 5%f is also encountered by them in the field. 



Oxygen consumption of control and experi- 

 mental animals was determined by means of a 

 Gilson respirometer using respiration flasks with 

 a volume of approximately 140 cc. Base-line 

 oxygen consumption measurements were made 

 on 10 males and 10 females in untreated sea- 

 water (30;^c) at 25°C. These same crabs were 

 then maintained under the same conditions but 

 with 0.18 ppm Hg added to the water, and me- 

 tabolic determinations made on days 1, 3, 7, 14, 

 21, and 28. Only medium-sized crabs in the in- 

 termolt stage were used to avoid any variation 

 due to molting or metabolic size relationships. 



Oxygen consumption rates were also deter- 

 mined on crabs exposed to mercury in combina- 

 tion with temperature and salinity stress. Me- 

 tabolic measurements were made on crabs main- 

 tained in 5/^r'f seawater at 5°C (control crabs) 

 and crabs kept in 5%c seawater at 5°C with 

 0.18 ppm Hg added to the water (experimental 

 crabs) . Oxygen consumption rates were then 

 determined after 1 and 3 days exposure for ex- 

 perimental crabs and 1, 3, and 7 days for control 

 crabs. These conditions proved too stressful for 

 most of the crabs to survive longer periods of 

 time. The same experimental procedures were 

 followed for crabs kept in 5^f at 35°C with and 

 without added mercury. Since these conditions 

 were less stressful than the combination of low 

 salinity and low temperature, it was possible to 

 measure the metabolic rate of these crabs on 

 days 1, 3, 7, 14, and 21 for experimental animals 

 and to day 28 for control crabs. All results are 

 expressed as />tl iters of oxygen consumed per 

 hour per gram live weight. 



416 



