\alvc (lower) than on the right one (upper), 

 the difference varying from 2.2 to 6.2 times. 



The rate of deposition of calcified material by 

 the surface of the mantle may also be studied by 

 inserting between the mantle and shell small 

 pieces of plastic or other nontoxic material of 

 known area and weight. Results obtained with 

 this method vary greatly. Observations made 

 on 16 adult oysters at Woods Hole during the 

 period of August 9 to 20, 1953, show that in 15 

 oysters the daily rate of shell deposition per square 

 centimeter varied from 0.4 to 2.1 mg. One 

 oyster deposited 14.2 mg. in 2 days or 7.1 mg. per 

 day. The amounts of shell material deposited 

 by 20 Narragansett Bay oysters kept in laboratory 

 tanks for 68 days during the period of April to 

 June varied from 0.1 to 0.79 mg. of shell substance 

 per day cm.^ In some of these oysters the 

 presence of the plastic material induced patholog- 

 ical conditions which resulted in the formation 

 of leathery capsules similar to the blisters fre- 

 quently found on the inside of shells near the 

 adductor muscle. The formation of such blisters 

 was accompanied by deposition of calcite greatly 

 in excess of the rate of calcification under normal 

 conditions. 



Seasonal variation in rate of shell deposition 

 over the inner surface of the valves was also 

 studied, using 20 adult oysters for each set of 

 determinations. Observations were continuous 

 from June 1954 until the end of February 1956. 

 To avoid possible injury to the mantle while in- 

 troducing pieces of plastic, the oysters were fully 

 narcotized in magnesium sulfate solution and 

 insertions made when the mantle was completely 

 relaxed and did not respond to touch. Thin sheets 

 of plastic were cut into rectangular pieces 0.5 cm.' 

 in area and weighed before inserting them under 

 the mantle, theu- weight varying from 5.5 to 6.0 

 mg. Some of the pieces introduced were ejected 

 by the oysters, but losses were minimized when the 

 insertion was made under full narcosis. The 

 treated oyster was then marked and placed on its 

 left valve in a large tray supplied with running 

 sea water. The temperature of the water was 

 recorded twice a day. Each set of 20 oysters was 

 kept in water as long as the seasonal rise or fall of 

 water temperature did not exceed 2.5° C. 



To obtain measurable quantities of shell de- 

 posits the pieces of plastic were left inside the 

 oysters for a longer time in winter and in August, 

 after the completion of spawning, than during the 



rest of the year. The number of days the oysters 

 with inserted pieces were left undisturbed varied 

 as follows: from 10 to 16 days in April to July; 

 from 25 to 30 days in August; from 13 to 18 days 

 in vSeptember to November; for 30 days in De- 

 cember; and 70 days in January to March. Ob- 

 servations were continued for 14 months. No 

 shell was formed in January to March except in a 

 few oysters in which the mantle was injured during 

 insertion. These samples were not included in 

 the data plotted in fig. 102. Laboratory observa- 

 tions showed that shell opening and feeding of the 

 oysters at Woods Hole are as a rule temporarily 

 reduced after the discharge of sex products which 

 takes place late in July and early in August. 

 Unequal time intervals in observing shell deposi- 

 tion do not affect the validity of the results since 

 the rates of shell formation shown in figure 102 

 are expressed as weights of shell deposited per 

 cm.- in 1 day. 



At the end of each period the oysters were re- 

 moved, the pieces of plastic recovered, rinsed in 

 distilled water, dried at 55° C, and weighed. The 

 results sunmiarized in figure 102 are shown as medians 

 (Md.) of the rate of shell deposition per cm.^ per 

 day, and as lower (Qi) and upper (Q3) quartiles. 



The curves show two periods of accelerated shell 

 growth in Woods Hole water, one in May to June 

 and another in October, and no shell growth dur- 

 ing winter from December to the end of April 

 when the temperature of the water varied between 

 1° and 2° C. These observations are in agreement 

 with many field data and with the experiences of 

 practical oyster growers of the North Atlantic 

 states, who found that oysters grow more rapidly 

 in the spring and in the autumn and cease to grow 

 when the water temperature drops to about 5° C. 

 The relatively low rate of shell deposition during 

 the summer is attributable to the inhibitory effect 

 of fully developed gonads. Observations fre- 

 quently made in the Woods Hole laboratory show 

 that shell growth in the winter will begin within 

 24 hours after the transfer of oysters from the 

 harbor to much warmer sea water in the labora- 

 tory. 



Under normal conditions no shell is deposited 

 in winter. In several instances, however, large 

 amounts of shell material were secreted over an 

 area of the mantle which was apparently injured 

 by the insertion of plastic. One of these cases is 

 shown in figure 103. In this oyster a heavy pocket 

 of shell material was deposited on the valve over 



THE MANTLE 



105 



