FEEDING OF OYSTERS IN RELATION TO TIDAL STAGES AND TO PERIODS OF LIGHT AND DARKNESS

¹ Fish and Wildlife Service Marine Laboratory, Milford, Connecticut

1. Examination of approximately 1400 oysters collected during the different tidal stages in Long Island Sound and Milford Harbor failed to show any definite period when the stomachs of these mollusks displayed absence of food.

2. During all hours of the flood and ebb, including the low water period, the predominating majority of the oysters contained large quantities of food, whereas individuals with empty stomachs were found only occasionally.

3. The relative quantities of food found in the oyster stomachs during the ebb period were at least equal to or sometimes even exceeded those recorded during the flood.

4. Analysis of the kymograph records of the rate of water pumping by the oysters showed that they fed very actively at all stages of the tide, and that the rate of feeding during ebb was at least equal to or sometimes even more rapid than during the flood stage.

5. During the ebb some of the oysters pumped on an average of 25,000 to 27,000 cc. of water per hour, while the maximum rate of pumping in some instances ranged from 31,000 to 34,000 cc. per hour.

6. The efficiency of the feeding mechanism of an oyster may be well attested by the fact that the volume of water passed during one hour through the oyster gills may be more than 1500 times greater than the volume of the oyster's body.

7. Within the temperature range of 17.0 to 28.0° C. the oysters remained open on an average of 22 hours and 39 minutes, or 94.3 per cent, and were closed 1 hour and 21 minutes, or 5.7 per cent of a 24-hour period.

8. During the periods of flood the shells of the oysters remained open on an average of 93.4 per cent of the time, whereas during the ebb periods the shells were open 95.2 per cent.

9. During the periods of darkness the percentage of oysters with full stomachs was comparable to that of the individuals examined during the day time.

10. During darkness the oysters were found feeding very actively. The average rate of pumping at night was not lower than during the daytime.

11. The shells of the oysters remained open 94.4 per cent of the total time during daylight, and 93.8 per cent during the period of darkness. No correlation was found between the periods of closure of the shells and darkness.

12. Under favorable conditions neither tidal changes nor changes in the time of day affect the rate of feeding of oysters of Milford Harbor. These mollusks were found to be feeding all or most of the time when their shells remained open.

13. The results of this investigation do not lend any support to the generally accepted theory that the American oyster does not feed late at night and in the early morning, and is relatively inactive on the ebb tide.