been destroyed. Recently the organism has been 

 found in oyster populations on the coasts of 

 New York and North Carolina — well outside 

 previous areas of high mortality. 



Studies of these serious pathogens of oysters 

 — D. marinum and M. nelsoni — have revealed 

 the very important role of a "salinity barrier" 

 to certain diseases. The fungus D. marinum 

 exerts severe efl'ects on oyster populations in 

 high-salinity waters of the Gulf of Mexico but 

 does not flourish in low-salinity areas. M. nel- 

 soni. which has seriously affected oyster stocks 

 of the Middle Atlantic States, also occurs in 

 higher salinities. Both pathogens seem confined 

 to salinities above 15 o/oo; this fact has made 

 possible the continuation of production in parts 

 of coastal areas afiected by these epizootics. 



Inhibitory effects of low temperature have 

 been well illustrated for these serious oyster 

 pathogens. D. ma)-i)ium causes warm-weather 

 mortalities in American oysters; in fact, the 

 plantings of seed oysters are timed to take ad- 

 vantage of the relative quiescence of the disease 

 in cooler seasons. Surveys of Dermoajstidiiim 

 have indicated marked decline in winter. M. 

 nelsoni, of Chesapeake Bay and Delaware Bay, 

 is similarly cjuie.scent in winter. New infections 

 are not apparent, prevalence of disease declines, 

 existing infections seem less active, and mortal- 

 ities are reduced. 



Mackin (1961) attempted, from a review of 

 the literature, to itemize characteristics of mor- 

 talities of oysters due to various causes. As one 

 who has published extensively on the role of 

 disease in oyster populations, he naturally 

 turned his attention toward mortalities caused 

 by infectious agents. Among many interesting 

 comments in his paper, Mackin stated that "all 

 oyster producing bays are endemic areas for 

 one or more diseases" and that "not only are 

 bivalve mollusks frequent ho.sts for pathogens, 

 but they are regularly parasitized by a unique 

 group of low fungi." Mackin further stated his 

 belief that "of all causes of mortality, disease 

 ranks first." Disease, then, can cause signifi- 

 cant, if temporary, reductions in population 

 abundance of marine invertebrates. Such re- 

 ductions may exceed 95 percent of existing 

 .stocks. Additionally, there is every indication 

 that serious but undescribed diseases exist 

 among marine invertebrates. Mackin (1962), 



for example, mentioned a number of pathologi- 

 cal conditions in oysters that were not associ- 

 ated with known pathogens. Rust disease of 

 Pacific king crabs, which we have described on 

 the basis of an unpublished report, is a com- 

 monly recognized condition in the fishery, but 

 has not been described in the published scien- 

 tific literature. 



Destruction of most of a [jopulation by epi- 

 zootics and mass mortalities, of course, also re- 

 duces pathogen numbers, because the possibility 

 of finding a new susceptible host at a critical 

 point in the life cycle is reduced. 



Less spectacular mortalities, which al.so have 

 severe continuing depressive effects on host 

 population size, are probably more common 

 than large-scale or mass mortalities. Minor fluc- 

 tuations in abundance may be attributable to 

 such "background" mortalities. Also, those par- 

 asites and diseases that do not kill the host may 

 act as indirect agents of mortality. Abnormal 

 individuals are rendei'ed more vulnerable to 

 predatioii in many ways: their body muscles 

 may be partially destroyed, covering or erosion 

 of their gills may interfere with respiration, 

 or their normal protective coloration may be 

 modified or obscured. For example, Hopkins 

 (1957a) has ob.served that blue crabs prey more 

 freciuently on oysters which cannot close their 

 shells as quickly or as tightly as normal oysters. 

 Any increase in parasite burden must reduce 

 the probability of survival in an environment 

 where death, early and sudden, is the rule 

 rather than the exception. For parasites with 

 complex life cycles involving two or more ho.sts, 

 consumption of an earlier host in the cycle — 

 one weakened by the parasite — by the right 

 predator may be critical to the completion of 

 the cycle. 



Another prominent effect of parasitization of 

 marine mollusks and crustaceans is steriliza- 

 tion of the host. Larval trematodes are notable 

 for destroying the gonads of gastropods and 

 bivalves, and parasitic barnacles and certain 

 isopods produce similar effects in crustaceans. 

 In areas where levels of parasitization are high, 

 the reproductive capacity of the host popula- 

 tion may be seriously impaired. In a study of 

 the ecological relation of the marine snail, Lit- 

 torina littorea, and its trematode parasite, 

 Cryptoctyle lingua, Sindermann and Farrin 



564 



U.S. FISH AND WILDLIFE SERVICE 



