populations in this century, however, have 

 forced us to look closely at disease as a cause 

 of mass mortalities of epic proportions, and of 

 subsequent major declines in abundance of 

 commercial species. 



Molluscan and crustacean species of economic 

 value as food have been affected by diseases, 

 some of which have produced epizootics with 

 resultant mass mortalities. Much attention has 

 been directed toward oyster diseases, possibly 

 because oysters have been cultivated more in- 

 tensively than most other inshore or estuarine 

 species. Microbial di.sease.s — including tho.se of 

 bacterial, fungal, and protozoal etiology — have 

 affected oyster stocks in many parts of the 

 world. Bacterial diseases have had .serious ef- 

 fects on lobsters, and a number of Protozoa, 

 particularly Microsporida, affect crab and 

 shrimp populations. 



It is often difficult to establish the precise 

 cause of death of marine invertebrates to de- 

 termine whether a suspected pathogen is a pri- 

 mary or secondary invader. Environmental and 

 physiological factors can be inextricably asso- 

 ciated with apparent disease: their relative 

 efl^ects are often not easy to assess. Thus, the 

 literature on mass mortalities contains meas- 

 urements of many environmental variables, de- 

 scriptions of physiological conditions of host 

 animals, and reports of suspected disease agents 

 — but too frequently the studies have been un- 

 able to point to a single cause of death. The 

 search for a single cause may have been an 

 oversimplified approach to a complex prob- 

 lem. In other situations, epizootics of specific 

 pathogens, possibly influenced to some extent 

 by environmental factors, can be directly' re- 

 lated to the state of resistance of the host poJ)u- 

 lation, the virulence and infectivity of the path- 

 ogen, and infection pressure. 



Mass mortalities, many of undetermined 

 cau.ses but some definitely the result of disease, 

 have occurred in commercial invertebrate pop- 

 ulations. These mortalities are a natural method 

 of regulating population size; they have re- 

 ceived increasing scrutiny in recent years. The 

 development of methods of cultivation and of 

 limited manipulation of the inshore environ- 

 ment should make it possible to reduce or elimi- 

 nate the serious threat of disease to populations 

 of commercial shellfish. 



ACKNOWLEDGMENTS 



We received suggestions and comments from 

 Glenn Hoffman, Eastern Fish Disease Labora- 

 tory, U.S. Fish and Wildlife Service, Bureau of 

 Sport Fisheries and Wildlife, Leetown, W. Va. ; 

 Harvey Rabin, Department of Pathobiology, 

 The Johns Hopkins School of Public Health, 

 Baltimore, Md. ; Joseph Uzmann, BCF Biologi- 

 cal Laboratory, Boothbay Harbor, Maine; and 

 John A. Couch, C. Austin Farley, and Robert 

 W. Hanks, BCF Biological Laboratory, Oxford, 

 Md. C. Austin Farley and John Couch exam- 

 ined oyster samples as part of a general study 

 of oyster diseases, and other staff members of 

 the Shellfish Mortality Program at Oxford 

 helped collect and prepare many samples. Re- 

 sponsibility for statements, conclusions, or 

 omissions in this paper rests, however, with us 

 and not with these reviewers. 



Helen Lang, Librarian, BCF Biological Lab- 

 oratory, Oxford, Md.. helped as.semble biblio- 

 graphic material from many sources and pre- 

 pared the literature citations. Photographs used 

 in figures were provided by C. Austin Farley 

 (figs. 1, 2, 5, and 6) ; John A. Couch (figs. 3 

 and 7) ; and Gareth Coffin (figs. 8 and 9). 



LITERATURE CITED 



Aldrich, David V. 



1965. Observations on the ecology and life cycle 

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Allen, J. A. 



1966. Notes on the relationship of the bopyrid 

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Andreu, Buenventura. 



1960. Dispersion de MytiUcola intestinalis Steuer 

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1961. Un parasito del mejillon. Propagacion del 

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Andrews, Jay D. 



1955. Notes on fungus parasites of bivalve niol- 

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DISEASES OF THE MARINE BIVALVE MOLLUSCA AND CRUSTACEA 



367 



