the accustomed are called "dilutions," those above, 

 "concentrations." The literature shows that the 

 effects of dilution upon Mya arenaria are most 

 noticeable when reduced to the stress point. The 

 stress point for Massachusetts clams is ap- 

 proximately 15°/oo (Matthiessen 1960), 22-24°/oo 

 for Medomac River, Maine, clams (Welch and 

 Lewis 3 ) and 5%o for Chesapeake Bay clams 

 (Schubel 4 ). 



Pumping activity and associated feeding and 

 ciliary movements of many bivalves are also 

 known to be directly affected by temperature 

 changes (Nelson 1923; Gray 1924; Galtsoff 1928; 

 Hopkins 1931, 1933; Elsey 1936; Loosanoff 1939, 

 1950, 1958; Harrigan 1956; Goggins et al. 1964; 

 Feng 5 ). 



To our knowledge, only Loosanoff (see footnote 

 2) and Welch and Lewis (see footnote 3) have 

 attempted to relate changes in bivalve behavior to 

 changes in both salinity and temperature. 



This investigation was undertaken to establish 

 the relationship of temperature to acclimation 

 time when Mya is immersed into dilutions and 

 concentrations of seawater. The results are 

 applicable to many real situations where Mya are 

 harvested from an area with one set of en- 

 vironmental conditions and subjected to accli- 

 mation and depuration in an area of another. 



Materials and Methods 



Salinity Control Apparatus 



The constant flow apparatus used in the follow- 

 ing experiments was similar in principle to that 

 used by Loosanoff and Smith (1950). The complete 

 system consists of freshwater and saltwater 

 constant head reservoirs and nine adjustable head 

 units, four regulating the freshwater flow and five 

 the seawater flow. Water from each adjustable 

 head or pair of heads flowed through plastic tubing 

 into the bottom of a large mixing tube and then 

 into the test tank. In this manner, ambient salin- 

 ity and four dilutions could be maintained 

 simultaneously. Temperature differences be- 



3 Welch, W. R., and R. D. Lewis. 1965. Shell movements ofMya 

 arenaria. Unpubl. manuscr., [U.S.] Bur. Commer. Fish. Biol. 

 Lab., West Boothbay Harbor, Maine. 



"Schubel, J. 1973. Report on the Maryland State Department 

 of Health and Mental Hygiene cooperative study to determine 

 cause and extent of high bacteria counts found in Mya arenaria 

 in 1973. Md. Dep. Health Ment. Hyg., 57 p. 



5 Feng, S. Y. 1963. Activity of the hard clam Mercenaria mer- 

 cenaria. Talk at Rutgers, the State University of New Jersey and 

 NAS Meeting July (Furfari 1966). 



tween the freshwater and saltwater constant head 

 reservoirs were eliminated by the installation of a 

 temperature equalizer functioning on the heat 

 exchanger principle. 



Experimental Design 



Clams were dug by commercial clam diggers 

 (under Department of Marine Resources super- 

 vision) from moderately polluted clam flats at 

 Parker Head, Maine, and transported to the 

 laboratory shortly thereafter. Broken clams and 

 clams under 50 mm were discarded, and the 

 remaining clams were thoroughly washed and 

 held in flowing control salinities until shell liquor 

 salinities were the same as control salinities. The 

 experimental temperatures desired were obtained 

 over a 10-mo period using the natural range of 

 ambient seawater temperature available. Ap- 

 proximately 1 bushel of clams was used in each set 

 of dilution and concentration experiments testing 

 salinity acclimation rates at ambient water 

 temperature. Clams were acclimated to control 

 salinities of 30.54-31.80%o (dilution experiments) 

 and 16. 26-17. 14%o (concentration experiments) 

 and then roughly divided into five groups; one 

 group remained in the control salinity and the 

 other four groups were immersed into tanks set at 

 other dilutions and concentrations of seawater. 



Changes in shell liquor salinity were chosen as 

 the criteria for the measurement of acclimation 

 because shell liquor was easily obtained from each 

 group of six clams by inserting a knife into the 

 region of the foot opening and draining the con- 

 tents into a paper cup. Five milliliters of this total 

 and a sample of tank water were analyzed for 

 salinity by the Knudsen Method. Acclimation had 

 occurred when shell liquor salinities were the 

 same as tank salinities. The oxygen content of the 

 water flowing into and out of each test tank was 

 measured by the Azide Modification of the 

 Iodometric Method (American Public Health 

 Association 1967). We attempted to regulate the 

 flow rate in each tank at approximately 1,000- 

 1,100 ml/min. All temperature measurements 

 were made with a calibrated glass thermometer. 

 Measurements of salinity, temperature, and flow 

 rate were recorded as the mean±l SE. Appropri- 

 ate curves were fit where necessary. 



Results 

 The dissolved oxygen content of the water used 



226 



