nature are available for the oyster, but determina- 

 tions by Chipman (1955) and Chipman and Hop- 

 kins (1954) of the rate of propulsion of water by 

 the bay scallop (Pecten (Aequipecten) irradians 

 Lamark) clearly indicate this relationship. The 

 data plotted in figure 178 were taken from the 

 table of observations bj' these authors, who used 

 a suspension of radioactive Nitzschia closterium 

 and Chlamydomonas. A similar relationship was 

 reported to exist in the California mussel in which 

 the absolute rate of water transport through the 

 gill was found to be a function of the weight of the 

 soft parts of the mollusk (Rao, 1953). The 

 relationship was well defined at temperatures of 

 20° and 16°, but was indefinite at 9° C. Rao also 

 referred to the activity of mussels from various 

 geographical regions. He stated that, regardless 

 of temperature, mussels from higher latitudes 

 transport water at a greater rate than mussels 

 taken from the lower latitudes. The observations 

 were of brief duration, lasting from 1 to 3 hours and 

 therefore can not be considered as representative 

 of typical behavior of bivalves over longer periods 

 of time. 



In the Bureau's shellfish laboratory at Woods 

 Hole records taken continuously for several weeks 

 show considerable variability among oysters of 



16 

 15- 

 J4 

 13 

 12- 

 II 

 a:'° 



09 

 K e 



UJ 



U) 

 UJ ^ 



4 

 3 

 2 



41 43 45 47 49 51 53 55 57 59 61 63 65 67 

 AVERAGE SHELL HEIGHT IN MM 



Figure 178. — Mean rate of water transport of scallops 

 (Aequipecten irradians) of different shell length kept in 

 sea water at room temperature ranging from 21.9° to 

 25.8° C. The plotted values represent the averages of 

 6 to 11 speOimens. (From the data of Chipman and 

 Hopkins, 1954). 



equal size and origin that are kept under identical 

 conditions. These changes in the rate of water 

 transport can not be correlated with changes in the 

 environment. In the tests only large oysters (10 

 to 12 cm. in height and 6 to 8 cm. in length) were 

 used. The}' were healthy, free of boring sponge, 

 Polydora, and other commensals. Daily fluctua- 

 tions of temperature did not exceed 1° to 2° C. 

 and salinity changes were less than 0.1 %o. The 

 range of daily fluctuations in the rate of ventila- 

 tion by a single specimen varied from 9.9 to 24.3 

 l./hr. Ln 1 day, and from 1.1 to 24.3 l./hr. 2 days 

 later. The total quantity of water transported 

 daily by this oyster in the 2 consecutive days of 

 recording was 77.5 and 457 1. In the other 

 oyster tested within the same month of July the 

 range of daily fluctuations in the rate of water 

 transport varied from 0.2S to 3.31 l./hr. in one 

 day to 5.0 to 13.0 l./hr. the following day. The 

 total quantity of water transported in these 2 days 

 was 8.6 and 239 l./day respectively. 



The more than 2,000 records of daily activities 

 of oysters accumulated in the course of many 

 years of my studies confirm this great variation. 

 Some of the records were made continuously for 

 33 days, others were interrupted after 2 to 3 days 

 of observations. All the records were obtained 

 using the technique shown in figures 169 and 170. 



STEADY STATE 



Ventilation of the gills may continue for hours 

 without interruption or significant changes in the 

 rate of water transport. This condition, which 

 may be called a steady state, occurs when temp- 

 erature, salinity, and food content of the water 

 remain constant and the oysters are not disturbed 

 by sudden changes in illumination, vibrations, or 

 other mechanical stimuli. The heart rhythm 

 during the steady state remains constant. Judg- 

 ing by the rate of formation of fecal ribbons, the 

 ingestion of food during these periods continues 

 without interruption, provided the water does not 

 contain excessive amounts of detritus, clay, or 

 plankton which may stimulate the formation of 

 pseudofeces and cause frequent snapping of the 

 valves. The temperature at which the steady 

 state was observed ranged from 15° to 25° C. 

 It is conceivable, however, that it takes place at 

 other temperatures. 



An example of the steady state in oysters is 

 shown in figure 179. In this experiment two 

 oysters of approximately the same size were ob- 

 served simultaneously. Their activities were slightly 



TRANSPORT OF WATER BY THE GILLS AND RESPIRATION 



195 



