(1962), and Sindermann (1965, 1966a) found 

 prevalences of the parasite of over 50 percent 

 in certain coastal areas, indicating that the 

 reproductive potential of snail populations was 

 suppressed by that amount. An excellent review 

 of parasitic castration of Crustacea has been 

 provided by Reinhard (1956). 



Effects of disease can be generally catego- 

 rized as catastrophic, resulting in mass mortali- 

 ties, or continuing, producing a constant drain 

 on population numbers. Although disease is 

 always with us, and mortalities have undoubt- 

 edly occurred in the past, new factors have been 

 introduced by man to set the stage for the 

 spread of epizootic disea.se. For example, oys- 

 ters are transferred promiscuously from one 

 geographic area to another; populations are 

 often crowded in dense beds, sometimes in areas 

 where natural populations did not exist previ- 

 ously ; drastic physical and chemical changes 

 have been made in oyster habitats ; and new 

 predators have been introduced. A dominant 

 mortality factor — disease — has been aided by 

 human activities ; it must be controlled, if we 

 are to achieve maximum production of culti- 

 vated inshore mollusks and crustaceans. 



Jlany different environmental factors — phys- 

 ical, chemical, and biological — can kill oysters, 

 crabs, or other animals of commercial value 

 (Brongersma-Sanders, 1957). Any single fac- 

 tor may become overriding, however, at a par- 

 ticular time in the life of a species — and in this 

 paper we have described several examples of 

 how the factor of disease can reduce the abun- 

 dance of marine species. 



It is easy, of course, to overextend any point 

 of view ; we do not imply here that every de- 

 crease in abundance can be blamed on disease. 

 An excellent case could also be made from the 

 published literature for the significant role 

 of predation — particularly during population 

 peaks of particular predator species — as a 

 major cause of fluctuations in abundance of 

 commercially valuable species. Man-made 

 changes in environment can also affect abun- 

 dance. Because of industrial pollution, shellfish 

 populations have been eliminated from certain 

 localized areas within estuaries and along the 

 coast. In addition, other types of pollution have 

 made extensive areas in rivers and bays un- 

 available for the harvesting of shellfish. It is 



likely that mass mortalities are, and have al- 

 ways been, natural methods of population regu- 

 lation — but, until recently, these mortalities 

 would have been accepted with the same dazed 

 bewilderment and inaction that must have char- 

 acterized the behavior of our ancestors during 

 the plagues of the dark ages. We can now look 

 to methods of environmental control and stock 

 manipulation, particularly for sedentary shal- 

 low water species such as oysters, clams, and 

 even certain Crustacea, as part of the method- 

 ology of an increasingly complex system of cul- 

 tivation of our inshore waters. 



CONTROL OF DISEASES OF MARINE 

 INVERTEBRATES 



The original, persistent, and largely errone- 

 ous feeling about disease in marine populations 

 is that little can be done about it. This pessi- 

 mistic attitude is definitely unwarranted for 

 species that live inshore — particularly the sed- 

 entary invertebrates — where practical meas- 

 ures of disease control are possible and have 

 already been applied in some situations. Possi- 

 ble methods include the follov/ing: 



1. The transfer of susceptible animals into 

 epizootic areas, or of individuals from such 

 areas, should be prevented. Because each dis- 

 ease is discrete in terms of transmission and 

 infectivity, risks of transfer will vary as well. 

 When intermediate or alternate hosts play a 

 significant role in maintaining the disease in a 

 given geographic area, transfer of infected in- 

 dividuals to other areas where these hosts are 

 absent may be a reasonable management pro- 

 cedure. Diseases that have been demonstrated 

 experimentally to be transmitted directly, how- 

 ever, such as Dermocystidium infections of oys- 

 ters, may be maintained at epizootic levels by 

 repeated introduction of susceptible animals. 



2. Disease-resistant stocks should be devel- 

 oped by selective breeding of survivors. Epizoo- 

 tics of several oy.ster diseases have apparently 

 produced increased resistance among survivors. 

 During the outbreak of "malpeque disease" in 

 Princo Edward Island (Canada) oysters, re- 

 sistance developed to an unidentified pathogen, 

 and stocks returned to previous levels of abun- 

 dance after several decades. That the pathogen 

 is still present is indicated by deaths of suscep- 



DISEASES OF THE MARINE BIVALVE MOLLUSCA AND CRUSTACEA 



365 



