- waiQht ramovQd 



Figure 166. — Shell movements of C. rirginica in running 

 sea water under a continuous pull of 1 kg. (312.5 g./cm.^ 

 of the cross-sectional area of the adductor muscle). 

 Temperature 13.0° to l.S.O° C. Time interval: 0.5 hour. 



If imich greater weight (4 kg. per oyster or 1 ,150 

 g./cin.^ of muscle area) is applied shell movements 

 become abnormal at the very beginning of the test. 

 This is demonstrated in the records of two C'otuit 

 (Mass.) oysters (C rirginica) and one f. gigas 

 shown in figure 167. 



The stretching of the adductor muscle by a pull- 

 ing force not exceeding 4 kg. per oyster did not 

 interfere with their feeding; a strong current was 

 maintained by the gills, and the feces were formed 

 and discharged in a normal way. However, the 

 secretion of mucus by the mantle and gills was 

 greatly increased. Vast quantities of slimy mate- 

 rial accumulated at the mantle edge and were dis- 

 carded as pseudofeces. 



The resistance of the adductor nmscle to a pull- 

 ing force exceeds by many times the force required 

 to overcome the elasticity of the ligament and 

 close the shell. This additional force is apparently 

 needed to keep the valves hermetically sealed. 

 Tlie ability to keep the valves tightly closed has 

 definite survival value. Mollusks possessing it 

 are able to protect themselves against desiccation 

 when exposed to air, or against adverse conditions 

 caused by the presence of toxic substances in the 

 water. Powerful muscular mechanism also helps 



Figure 167. — Shell movements of two Cotuit oysters, C. 

 virginica (lines A and B), and C. gigas (line C) in sea 

 water under a continuous pull of 4 kg. or about 1,150 

 g./cm.2 of cross-sectional area of the adductor muscle. 

 Temperature 14.5° to 16.5° C. Salinity 32.0 to 32.3 °/oo. 

 The exact time of tonus loss is shown by the broken line 

 and arrow. Time interval: A, B, and C, 0.5 hour each. 



them to resist attacks of starfishes, crabs, and other 

 enemies that attempt to pry open their valves. 



CYCLES OF SHELL MOVEMENTS 



There is no indication of any periodicity in 

 muscular activity in the kymograph records of 

 shell movements of oysters that were kept in 

 running sea water in the laboratory or kept out- 

 side on a suitable platform submerged from a pier 

 (LoosanofT and Xomejko, 1946). Brown and his 

 associates (Brown, 1954; Brown, Bennett, Webb, 

 and Kalph, 1956) claim, however, that C. virginica 

 possesses a persistent lunar cycle of activity with 

 the maxima occuring at about 12.5 hour intervals. 

 Oysters used for obtaining tracings of shell move- 

 ments were kept for a fortnight or longer in about 

 4 or 5 1. of sea water which was not changed but 

 was adjusted by occasional addition of distilled 

 water to compensate for evaporation. The mean 

 daily cycles were calculated for 15-day periods 

 by obtaining the average value of opening for 

 each hour of tlie day and appljang to the data a 

 very complicated method of adjustment. The 

 main conclusions reached by the authors were 

 that: (1) oysters and qualiogs display "statistical 

 rhythms of opening of shell while the overt 

 rhythms are not apparent from kymograph 

 records", (2) short periods of opening tend to 

 occur about 6:00 a.m. and more or less prolonged 

 periods of openings happen through much of the 

 remainder of the day. The observations and 

 their mathematical treatment are of interest from 

 a theoretical point of view, but the ecological 



180 



FISH AND WILDLIFE SERVICE 



