water transport 



iiiiiiiiiiiiiiiiiiiuiiiiiiinuuiuii iiihiuiiiiiniihiiiuMmiiimiiiiii iiliiuiiiiiuiiiniiiiiiu iiiuiiiiiiiiiiiiii[iiiiiiiiii]HiMiiiiiilj|iiliiiiliiiiiiiiiimiHIMiuMiiimii»iii»i»uiiMiii 



shell movement 



JLi_LJuLJLJLJL_J L < .i , 1 U 



water transport 



1 



iiiiiiiii( i i i i«(i(i(iiwmmi(ii(((finmiii i i((mini(i!iii«i(i' 



stiell movement 



« 



lUJJoU^fjOiLkKJ-JJ-J- 



X 



_L 



_L 



Figure 179. — Kymograph tracings of water transport and shell movements of two adult oysters. Each vertical stroke 

 of the first and third line represents the discharge of 57 ml. of water (the capacity of the dumping vessel). Temp- 

 erature 19.5° C. Salinity 31.2 °/oo. Oyster dimensions: 10.6 x 6.5 cm. (upper); 10.5 x 6.6 cm. (lower). Time interval, 



1 hour. 



different, although both were kept under identical 

 conditions; water was delivered at a uniform rate 

 from a common supply tank, and the temperature was 

 kept constant at 19.5° ±0.1° C. The upper 

 record shows that during the S hours of obser- 

 vations the rate of water transport of one oyster 

 was fairly uniform, varying between 19 and 21 l./hr. 

 Occasional contractions of the adductor muscle 

 were followed by an immediate return to the 

 former tonus level. In the second oyster (lower 

 part of figure 179) the rate of water transport 

 decreased slightly from 14 1. per hour at the start 

 to 11 l./hr. at the end of the record; its shell 

 movements were more frequent and less regular 

 than in the first oyster. It is apparent from these 

 and other observations of the same type that 

 imder identical conditions of environment the 

 rate of water transport may be different, depend- 

 ing on the intrinsic state of the organism. 



Water transport by the gills is not carried on 

 with a machine-like performance controlled en- 

 tirely by such environmental factors as tem- 

 perature, salinity, chemical composition of water, 

 etc.; it is adjusted to or governed by the needs of 

 the mollusk. It may be assumed that the var- 

 iable daily requirements for food and o.xygen 

 and the necessity of eliminating the metabolites 

 determine both the duration of the activity and 

 the rate of water transport. In addition, the 



rate of water transport is affected by irregularity 

 of gill activity shortly after spawning during the 

 summer. Oysters often maintain a nearly con- 

 stant rate of water propulsion for 2 or 3 days. 

 Invariably these periods are followed by periods 

 of partial or total inactivity (closed valves) 

 manifested in greatly reduced rates of water 

 transport or its complete cessation. It appears 

 reasonable to deduce that the needs for food, 

 elimination of products of metabolism, and require- 

 ments for oxygen determine both the number of 

 hours per day the oysters stay open and the rate 

 at which water is being transported through 

 the gills. 



REDUCTION OF WATER TRANSPORT 



Increased and irregular shell movements are 

 usually associated with a decreased rate of water 

 transport. This relationship is apparent in trac- 

 ings which were made shortly before the closing 

 of the shells or immediately after their opening. 

 The records of three oysters kept at a temperature 

 of about 20° to 22° C. (fig. 180) show that in all 

 of them the rate of water transport slowed down 

 before the valves began to close. Complete clo- 

 sure of the valves took place in 72, 15, and 18 

 minutes after cessation of the current. In the 

 majority of cases examined the time intervals 

 between the resumption of tlie cloacal current to 



196 



FISH AND WILDLIFE SERVICE 



