increase in the pumping rate occurs, and it 

 is within this range that the maxinnum average 

 pumping rate of about 3-1/2 gallons per hour 

 per oyster was recorded. Beyond 93° F., 

 however, oysters begin to show a marked 

 decrease in pumping rate, and their shell 

 movements become abnormal. 



The maximum rate of pumping for an indi- 

 vidual oyster was registered at a little less 

 than 10 gallons per hour. For shorter periods 

 of 5-15 minutes the rate of pumping of the 

 sarne oyster exceeded 10-1/2 gallons per 

 hour. This oyster was only about 4 inches 

 long; it is probable that larger oysters can 

 pump even larger quantities of water per 

 given unit of time. 



As mentioned before, oysters feed most 

 efficiently when the surrounding water is 

 relatively warm. Under favorable conditions 

 the oyster keeps its shell open about 11 hours 

 during a 24-hour period. There is no corre- 

 lation between opening and closing of the shell 

 and the time of day. 



The pumping rate of oysters kept at tem- 

 peratures below 40° F. and then quickly changed 

 to a temperature of about 650-68° F. was 

 virtually the sanne as the control ' oysters, 

 thus indicating that the oysters respond and 

 adjust to such radical changes. Oysters, there- 

 fore, are physiologically well adapted to rapid 

 changes which they sometimes encounter in 

 nature, for example, when living on tidal flats, 

 where at autumn low tides the night air may 

 cool the water of the small pools containing 

 the oysters almost to freezing, while during 

 the day the incoming tide may cover the same 

 oysters with much warmer water. 



Turbidity caused by various substances, in- 

 cluding natural silt, also may affect the rate 

 of pumping. Although very small quantities 

 of silt sometimes stimulate the normal pump- 

 ing activities of oysters, heavier concentra- 

 tions significantly reduce the rate of water 

 pumping and strongly affect the shell move- 

 ments. Long Island Sound oysters living in 

 relatively clear water reduced the pumping 

 rate to about 68 percent of normal after being 

 exposed to a concentration of silty water 

 (one-fiftieth of an ounce of silt per quart of 

 water). Greater concentrations of silt more 

 strongly affected shell movements and rate of 

 water pumping. In turbid waters the oysters 

 discharged large quantities of pseudofeces 

 containing silt. Shell movements of oysters 

 kept in turbid waters were clearly associated 

 with frequent ejection of large quantities of silt 

 and mucus accumulated on the gills and palps. 

 The size and shape of turbidity-producing par- 

 ticles are important. Different turbidity-creat- 

 ing substances, when present in the water in 

 the same concentrations, affect experimental 

 animals in different degrees. 



The oyster eats plankton, which consists of 

 nnicroscopic plants and animals living in the 

 water. According to some authorities, organic 



detritus, the product of disintegrating plants 

 and animals, may also contribute to the oyster 

 diet. Many kinds of marine bacteria are also 

 ingested and possibly some nnay be digested. 

 No definite experimental proof exists that 

 oysters can absorb nutriments directly from 

 sea water. Oysters gather food with their gills, 

 and in filtering the water through the gills 

 the oyster retains many microscopic orga- 

 nisms, although some small, elongated forms 

 without appendages may escape. Only 10-50 

 percent of the bacteria present in sea water 

 are detained by the gills. 



The food particles caught on the gill sur- 

 faces are embedded in the mucus and are 

 pushed along the upper or lower edges of the 

 gills to the palps, which either may direct 

 food into the nnouth or reject it. Unwanted 

 material is expelled by a sudden closing of 

 the valves. 



The mouth of the oyster lies between the 

 palps and opens into the esophagus, which 

 leads to the stomach. The stomach opens into 

 the intestine, a long, coiled tube ending in the 

 vent or anus. The stomach is surrounded by 

 a brown digestive gland, which in fat oysters 

 IS obscured by the mantle and by gonad nnate- 

 rial, but in poor oysters, especially after the 

 spawning period, shows clearly through the 

 surrounding tissue. This dark-colored mass 

 of tissue is sometimes misnamed the liver, 

 but usually biologists call it the digestive 

 diverticula. A series of ducts unites this organ 

 with the stomach. 



As has been described by many authors and 

 so well summarized by Yonge in his book, the 

 digestive system of the oyster possesses a 

 most remarkable structure. Called the crys- 

 talline style, it is a gelatinous rod and occurs 

 only in other bivalves and in certain snails. 

 The head of the style projects from the elon- 

 gated style sac where it is formed and extends 

 across the stomach cavity, pressing against 

 an area known as a gastric shield, which is 

 the only area in the stomach not covered by 

 cilia. The crystalline style rotates continu- 

 ously. This rotation assists in mixing the 

 food, aids digestion, and also brings par- 

 ticles of food in closer contact with the 

 stomach walls. The crystalline style, inci- 

 dentally, is the only known rotating part in 

 any animal. 



The style, along with several other organs 

 of the oyster, including the digestive diverticula 

 and even blood cells, or leucocytes, is con- 

 cerned with digestion. It is made of protein 

 produced in a long style-sac which is lined 

 with numerous cilia that rotate the style. 

 Normally, under healthy conditions the style 

 is added to continuously by formation of new 

 material. As a result, the style is pushed 

 forward while it is rotated, and its head, 

 directed against the gastric shield of the 

 stomach, dissolves continuously. The disso- 

 lution of the style releases digestive enzymes 



