PRINCIPAL DISEASES OF COMMERCIALLY IMPORTANT MARINE 

 BIVALVE MOLLUSCA AND CRUSTACEA 



By Carl J. Sindermann and Aaron Rosenfield, Fishery Biologists 

 Bureau of Commercial Fisheries Biological Laboratory, Oxford, Md. 21654 



ABSTRACT 



Diseases of commercially important marine bivalve 

 mollusks and crustaceans can cause mortalities in natural 

 and captive populations. Oysters have suffered frequent 

 and extensive mass mortalities. Epizootic disease has 

 sometimes been indicated, as for example in "dermo- 

 cystidium disease." caused by a fungus, in the Gulf of 

 Mexico, and "Delaware Bay disease," caused by a proto- 

 zoan. In the Middle ."Vtlantic States. Fungus organisms 

 have also been Implicated In "shell disease" of European 

 oysters and a fatal disease of bivalve larvae in hatcheries. 

 Several species of haplosporidan Protozoa cause serious 

 mortalities of oysters and mussels. Larger animal para- 

 sites, such as larval trematodes and parasitic copepods, 

 can affect reproduction, growth, and survival of bivalve 

 mollusks. 



Diseases of crustaceans are usually less well known. 

 Effects of pathogens are most apparent In captive popu- 

 lations, where mortalities may result from outbreaks of 

 microbial agents^ Lobsters have two known bacterial 

 diseases: the often very lethal "gaffkaemla" which oc- 

 curs In wild populations, but severely affects Impounded 

 stocks; and "shell disease," caused by ehitln-destroying 

 bacteria, which is Important In captive populations, prob- 

 ably by interfering with respiration of Infected Individ- 

 uals. Mortalities of captive blue crabs have also been 

 attributed to possible respiratory impairment caused by 

 ciliate gill associates. Microsporidan Protozoa can be im- 



portant parasites of crabs and shrimps; depending on 

 their habitat within the host, they may destroy muscle 

 tissue or gonads and have been postulated as causes of 

 mortality. Among the larger parasites of crustaceans, rhi- 

 zocephalan barnacles have long been known to cause de- 

 generation of gonads In crabs of many species. 



The described pathogens of commercial invertebrate 

 species probably constitute only a small percentage of the 

 many disease agents that affect marine populations. Many 

 mass mortalities have been described in the literature; 

 disease has been implicated In some of them, but the etio- 

 logic agent rarely has been determined precisely, and the 

 relative importance of other environmental factors has 

 been assessed Infrequently. Diseases and parasites can 

 also exert significant "background" effects such as contin- 

 ued low-level mortality, depressed reproductive capacity, 

 and increased susceptibility to predatlon. 



Disease control measures are possible and have been 

 applied to a few populations of sedentary inshore marine 

 invertebrates. Possible methods of control include quar- 

 antine, selective breeding of disease-resistant strains of 

 shellfish, environmental manipulation (dredging of grow- 

 ing areas, chemical treatments, control of density of 

 planting, and scheduled harvest), and more extensive use 

 of artificial environments such as hatcheries and artificial 

 ponds. 



As understanding of factors that influence 

 the numbers of animals in the sea has ex- 

 panded, it has become evident that disease, 

 among other environmental variables, can dras- 

 tically affect abundance. This fact has been 

 clearly demonstrated in populations of seden- 

 tary inshore invertebrates. Many marine in- 

 vertebrate species, harvested in great numbers, 

 constitute marine crops of high value. Some, 

 such as mussels, oysters, clams, crabs, lobsters, 

 and shrimps, occur in inshore or estuarine 

 waters, and have been cultivated in varying de- 



grees in different parts of the world. Under 

 natural conditions or under cultivation, mass 

 mortalities occasionally occur; here disease 

 can be an important contributing factor. 



The word "disease," as used in this paper, 

 includes abnormalities resulting from microbial 

 pathogens or parasite invasion, and tumors. 

 Not included are genetically or environment- 

 ally induced abnormalities, or physiological dis- 

 turbances not related to an infectious agent or 

 parasite. For each ho.st group considered in 

 this review a sequence of diseases, beginning 



FISHERY BULLETIN: VOL. 66, NO. 2 



Published December 1967. 



335 



