mg/liter, mercury was not toxic and did not have 

 an effect on the growth of limb buds. At 1.0 mg/ 

 liter, mercury caused almost total inhibition of 

 limb growth, but also proved lethal to 60% of the 

 crabs. Therefore, the inhibition of regeneration 

 may not be a specific effect of the mercury, but just 

 an indication of the toxicity of the metal to the 

 crabs. In this light, Uca is seen to be much more 

 resistant to mercury than the porcelain crab, Pet- 

 rolisthes armatus, in which the 96 h LC50 (.mean 

 lethal concentration) was 0.050-0.064 ppm 

 (Roesijadi et al. 1974). With long-term exposure to 

 mercury, however, Uca can tolerate only 0.18 ppm 

 (Vernberg and O'Hara 1972). In the present study, 

 cadmium might have shown a greater effect than 

 mercury at 0. 1 mg/liter because it was absorbed to 

 a much greater extent than the mercury. It is pos- 

 sible that exposure to mercury at levels between 

 0.1 and 1.0 mg/liter could inhibit regeneration 

 without causing mortality. Despite the high 

 amounts absorbed, lead had no effect on regenera- 

 tion rate. 



At both dose levels of cadmium and the higher 

 concentration of mercury, the retarding effects 

 were greater the first time the experiment was 

 performed (July) than the second (August). Since 

 these crabs normally molt in August, it is probable 

 that they have higher titers of ecdysone at that 

 time, and their progress toward ecdysis cannot be 

 inhibited to the same extent. A similar seasonal 

 difference in sensitivity to cadmium was seen in 

 the shrimp Paratya tasmaniensis, which showed a 

 threefold higher LC50 value in mid-October than 

 in early July (Thorp and Lake 1974). 



Thurberg et al. (1973) have found that cadmium 

 reduced the level of oxygen consumption in the 

 crabs Carcinus maenas and Cancer irroratus. A 

 reduction of oxygen consumption of the gills of the 

 mud crab, Eurypanopeus depressus , exposed to 

 cadmium was found by Collier et al. (1973). Re- 

 duced metabolism may be responsible for the re- 

 tardation of regeneration of the crabs in cadmium. 

 Cadmium has been found to inhibit oxygen con- 

 sumption and metabolism of fishes (Thurberg and 

 Dawson 1974; Jackim et al. 1970) and has simi- 

 larly been found to retard fin regeneration in 

 fishes (Weis and Weis in press). 



In this sort of study it is difficult to extrapolate 

 laboratory findings to the field. In nature, metals 

 would tend to be concentrated in the sediments 

 more than the water, and it would be primarily 

 from the sediments that these estuarine intertidal 

 crabs would pick up the metals. Crabs would not 



normally be subjected to the loss of many ap- 

 pendages. Loss of a single limb is not particu- 

 larly debilitating to a decapod. Should many 

 limbs be lost, however, the crab's locomotion 

 would be impaired, and it would be at a disadvan- 

 tage. It would therefore be advantageous to re- 

 generate the lost limbs as quickly as possible. 

 Crabs which could not regenerate as quickly 

 could be more subject to predation, and the toxic 

 heavy metal pollutant would then be passed on to 

 higher trophic levels. 



Acknowledgments 



Thanks are extended to John C. Baiardi, Direc- 

 tor of the New York Ocean Science Laboratory, for 

 making the facilities of the laboratory available 

 for this study. Appreciation is also extended to 

 Jennifer and Eric Weis for assistance in collecting 

 the crabs, and to Linda Mantel and Rita Levin for 

 their help. This research was supported by a 

 N.I.H. Biomedical Grant #RR 7059. 



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