4.0 RELATIONSHIP OF SHELL DISEASE TO INCREASED MORTALITY 



Mortality due to shell disease in natural populations of 

 crustaceans has not been determined with any degree of certainty. 

 In contrast to marine fish kills where floating fish may be seen 

 on the surface, dead crustaceans generally remain on the sea 

 bottom or are consumed by scavengers, only occasionally showing 

 up in traps or trawls. Evidence for disease or stress leading to 

 mortality has been noted; deaths often occur soon after newly 

 caught diseased specimens are placed in holding tanks. In other 

 instances, it has been noted that blackened spots or shell 

 lesions may increase in size and severity during captivity. 

 Periods of high water temperature and low oxygen tension (anoxia) 

 have been associated with mortalitites noted by divers and 

 commercial fishermen. It is likely that stressed crabs and 

 lobsters are less motile and defensive than healthy ones and are 

 slow to move out of anoxic zones. Divers have noted that 

 weakened animals are less aggressive than healthy ones, and are 

 slow to assume a defensive posture or seek shelter when 

 threatened. Blue crabs, Callinectes sapidus . known to be 

 infected by parasitic amoebae, may die in less than one hour 

 after being taken out of water, and have been observed to fail to 

 turn over when placed on their backs. Weakened or diseased 

 crustaceans may remain alive when bathed by oxygenated water, but 

 suffer respiratory stress and die when oxygen levels are low. 

 Evidence for stress-related conditions affecting rock crabs. 

 Cancer irroratus , has been observed visually among transient 

 subpopulations caught in Sandy Hook and Lower Bays, New York and 

 New Jersey. Large numbers of male crabs entering the bays to 

 molt during winter months begin their return to the ocean in late 

 winter or early spring. Crabs captured near the end of the 

 annual migration are often afflicted by severe gill blackening, 

 shell discoloration, or shell lesions, and have been referred to 

 as the "stragglers", i.e., that portion of the population 

 suffering from physiological stress. Cancer crabs, red crabs, 

 and lobsters "walk" rather than "swim" and are known to move 

 considerable distances during periods of migration and molting. 

 Disease or stress, whether biological, nutritional, or chemical, 

 all are known to weaken affected animals and contribute to 

 morbidity or mortality. 



Evidence for degraded sea bottoms affecting crustacean 

 health has been obtained from studies on "black gill disease" in 

 rock crabs captured in the New York Bight apex (Sawyer, 1982); 

 Bodammer and Sawyer, 1981, Sawyer et al, 1979, 1983, 1984). High 

 prevalences were associated with sewage sludge and/or 

 contaminated dredge material disposal sites where environmental 

 degradation was severe. Prevalences of gill disease ranged up to 

 30% in intermolt crabs. The etiology of the disease is complex, 

 but it involves accumulations of black silt between gill 

 lamellae, and the presence of fouling organisms on gill surfaces. 

 Localized "black spots" associated with melanized gill tissue 

 were often noted in stained sections of gill tissue (Sawyer, 

 1983). An association of gill blackening with shell erosion was 



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