C-l 



i^jj^^r..-^ ^jVUiloULXu^ 



C-2 



J_JU- 



FiGCEE 155. — Shell movements of type C, preceding the closing of valves (left side) and following their opening (right 

 side). Note staircase movement in C-2. Time interval: C-l and C-2, 1 hour each. 



to the water. A "staircase" in reverse direction 

 sometimes tai^es place dming the opening of the 

 valves (C-2, right half). This behavior was pro- 

 voked by small doses of oyster sperm, vitamins, 

 and sugars injected between the valves into the 

 pallia! cavity. The "reversed staircase" may be 

 interpreted as a testing reaction of the oyster, 

 which adjusts the opening of the valves to a 

 needed rate of ventilation. 



TypeD 



Shell movement of type D (fig. 156) was ob- 

 served in oysters affected by various poisons which 

 caused increased excitability of the adductor 

 muscle (D-1). In case of a prolonged action of 

 poison the periods of greater excitability (D-2 and 

 D-3) are interrupted by gradually increasing du- 

 rations of periodical closure (D-4 and D~5). This 

 type of shell movement is a symptom of a highly 

 advanced pathological condition resulting from 

 poisoning, disease, or exposure to adverse physical 

 conditions. It is typical for dying mollusks (D-5). 



Type E 



The E type of shell movement associated with 

 the spawning of the female oyster is cliaracterized 

 by great regularity, rapidity, and rhythmic up 

 and down strokes (fig. 157). At the beginning of 

 the reaction the time needed to reach the maxi- 

 mum relaxation level is very brief, almost ef^ual 

 to the time of the contraction (see: E-2). Dur- 

 ing the relaxation phase (downward stroke) there 

 is a brief period of slowing down in the decrease 

 of muscular tension. On the curve this period is 



represented by a small plateau. This moment 

 coincides with the passage of eggs tlarough the 

 gills into the pallial cavity. The eggs in the pallial 

 cavity are dispersed into tlie surrounding water 

 by rapid contractions of the adductor. 



Shell movements that take place during the 

 spawning of a female do not occur at any other 

 time and cannot be induced by drugs. They cease 

 with the cessation of spawning. The factor that 

 induces female spawning (temperature or chemical 

 stimulation by sperm) has no efTect on tlie type of 

 shell movement of a male and is ineffective on non- 

 spawning females. It is probable that this type of 

 muscular activity is associated with the discharge 

 of eggs from the gonads. 



DURATION OF PERIODS OF OPENING 

 AND CLOSING 



The length of time the shells remain open or 

 closed and the conditions that affect this behaxior 

 are of importance to oyster biology. Obviously 

 the normal functions of the organism, such as 

 respiration, feeding, and elimination of waste pro- 

 ducts, can be performed only when tlie valves of 

 tlie mollusk are open. It does not follow, however, 

 that tlie opening of the shell indicates that the 

 mollusk is feeding or is ventilating its gills. Under 

 certain conditions water may be sliut off from the 

 pallial caxity by the pallial curtain or by the ces- 

 sation of ciliary motion wiiile the valves remain 

 open. However, in the majority of laboratory 

 observations of tlie behavior of oysters in unadul- 

 terated sea water tlie opening of the valves coin- 



172 



FISH AND WILDLIFE SERVICE 



