Shellfish The principal shellfish used for food are divisible into two classes: (1) mollusks, including oysters, clams, mussels, and scallops; (2) crustaceans, including lobsters, crabs, shrimps, and crawfish. Of all these the oyster is by far the most important as a factor in the general food supply. It is estimated that the oyster crop of the United States (representing about two thirds of the world's supply) approximates 25,000,000 bushels annually, valued at about $20,000,000. The total amount paid at retail would of course be much larger. The shores of Long Island and of Chesapeake Bay produce oysters abundantly. According to statistics collected by Stiles, the chief oyster-producing states are, in order of rank: New York, Virginia, Connecticut, Maryland, New Jersey, Rhode Island, Louisiana, Mississippi. A considerable proportion of the oyster crop, perhaps one fifth, is preserved by canning. The oyster-canning industry grew up around Baltimore, but is now carried on to an even larger extent in some of the more southern states, where there are areas well suited to oyster culture but not readily accessible to the large markets. Although still classified with the fisheries, the oyster industry is rapidly becoming a kind of farming. Submerged lands suitable for oyster culture are either owned or rented from the state, and many people devote themselves exclusively to the care of these oyster farms, which in some cases are natural oyster beds which have been conserved and in other cases are the result of artificial planting. The oyster reproduces by eggs which on hatching yield free-swimming larvæ, but when about two weeks old the young oysters have secreted shells of sufficient weight to cause them to sink and they then "set" on any object with which they come in contact, and thereafter are stationary. By the end of the first season the young oyster is from one to two centimeters, or about one half to three fourths of an inch in diameter and at this stage in their development are often sold as "seed oysters" (Fig. 22) to be replanted in other beds. This transplanting of oysters is a matter of growing importance and appears to make possible a great development of the industry, since there are large areas on both the Atlantic and Pacific coasts, as well as in other countries, where the conditions are unfavorable for spawning but entirely suitable for the raising of transplanted oysters. The rate of growth of the oyster is dependent upon its environment, but in general it is expected that the oysters will be marketed when from three to five years old. It is partly for the sanitary protection of the shellfish grounds of the estuaries and inland waters that the state and federal governments are now taking steps to prevent or regulate the discharge of raw sewage into rivers and harbors. The feeding habits of oysters, clams, and mussels make it probable that they will contain considerable numbers of bacteria of the types characteristic of the waters in which they live. Hence when these shellfish are taken from (or have been kept in) sewage-polluted waters, they may easily contain bacteria of the intestinal types and thus may become carriers of typhoid fever as well as less serious intestinal disorders. This was first clearly demonstrated by Conn, in the investigation of an epidemic of typhoid among the students of Wesleyan University in 1894. The epidemic was confined to members of the various college fraternities which had held banquets in their several houses, but all on the same evening. Each of these banquets had included raw oysters and all the oysters came from one dealer and had been "floated " fattened" at the mouth of a stream. This stream was found to be highly polluted, and further investigation showed that in one house near by there were two cases of typhoid the discharges of which passed through the house drain into the stream without disinfection. There "or 2 was left no room for doubt that the typhoid bacteria passed from the patients in the house near the stream to the oysters and so to the students at the banquet. In 1902 there occurred simultaneous outbreaks of typhoid fever at Winchester and at Southampton, England, which were traced to contaminated oysters from a common source. In 1912 two epidemics of typhoid and other intestinal disorders were clearly traced by Stiles to oysters obtained from a dealer who was accustomed to store his shellfish in water which on investigation was found to be contaminated. As Prescott and Winslow point out: "It should be noted that it is unfortunately not only raw shellfish which are responsible for the spread of disease. Most of the processes of cooking to which these foods are subjected are insufficient to destroy pathogenic germs." These authors quote results showing that, with steamed clams, the bacteria present could not be destroyed except by a temperature high enough and prolonged enough to ruin the clams for eating and that oyster stew, fried oysters, and fancy roast oysters may still contain active bacteria of the types indicative of sewage pollution. Clams in chowder, on the other hand, were found to be practically sterilized. When shellfish are carelessly opened and handled, they may receive additional contamination in the process. Stiles, in 1911, found enormously greater numbers of bacteria in the "shucked" than in the corresponding "shell" oysters. Gorham has found that oysters taken from the same beds show much less contamination in winter than in summer. He believes that during the cold weather the oysters assume a condition of rest or hibernation, during which the process of feeding is suspended. In such a condition no organisms would be taken in from the outside water and those within the oyster are grad 1 See references at the end of the chapter. 2 Elements of Water Bacteriology (3d ed.), page 248. |