EFFECTS OF ENVIRONMENT AND HEREDITY ON GROWTH 

 OF THE SOFT CLAM (Mya arenaria) 



By Harlan S. Spear and John B. Glude, Fishery Research Biologists 



The relation of the soft, or soft-sliell, clam 

 {Mya arenaria) to its environment is sucli that 

 some flats are favorable for seed-clam production 

 and are not favorable for growth, while the reverse 

 is true of otlier flats. This situation has resulted 

 in the practice of transplanting clams from "seed" 

 areas to "growth" areas, in order to take full 

 advantage of both environments. Obviously it is 

 desirable to know the relative effects of heredity 

 and environment on the growth of the clams; if 

 heredity has the greatest influence it would be 

 desirable to select clams for transplanting from 

 fast-growing stocks, whereas if environment is 

 the dominant factor any convenient source of 

 seed may be used with equal success. The 

 relative efi"ects of stock origin and growth en- 

 vironment, on clam growth, therefore constitute 

 a subject of commercial importance as well as 

 a subject bearing on tlie biological problem of 

 heredity versus environment, or "nature versus 

 nurture." 



The growth rate of the soft clam varies along 

 the New England coast (Turner 1948); in general, 

 growth is slower in the more northerly and colder 

 areas. In addition, there are local variations in 

 growth rate not obviously caused by water 

 temperatures. The experiment described here 

 was designed to provide information on tlie relative 

 effects of heredity and environment on the growth 

 rate of soft clams. 



Assistance in the field work of this experiment 

 was provided by Richard E. Tiller, formerly of the 

 Fisli and Wildlife Service, and by Dana Wallace 

 and John Hurst, of the Maine Department of Sea 

 and Shore Fisheries, which cooperated in the 

 experiment. David W. Calhoun, formerly of 

 the Fish and Wikllife Service, assisted in tiie 

 statistical analyses. 



The Clam Investigations stafl^ of the U. S. Fish 

 and Wildlife Service has been studying the pro- 



ductivity of Sagadahoc Bay on Georgetown Island, 

 Maine, in terms of the numbers of clams that can 

 be removed annually without causing depletion. 

 The annual clam census, conducted as a part of 

 these studies, has shown that clams in the main 

 part of Sagadahoc Bay grow much faster than 

 those in Bedroom Cove, an adjacent part of the 

 bay (fig. 1). Figure 2 shows comparative growth 

 rates for the main part of Sagadahoc Bay and for 

 Bedroom Cove, as determined by interpretation 

 of rings on the shells.' 



Tlie reason for the difference in growth rates of 

 clams in the two parts of Sagadahoc Bay must be 

 known for efficient management of the resource. 

 One possible reason is heredity, that is, that the 

 clams in Bedroom Cove are a slow-growing race 

 while those in tlie center of Sagadahoc Bay are a 

 fast-growing race. Another possible reason is that 

 a combination of factors makes the environment in 

 the center of the bay conducive to rapid growth, 

 whereas the environment in Bedroom Cove permits 

 only slow growth. If growtli rates differ because 

 of heredity, a management plan to increase produc- 

 tion might include replacing the slow-growing 

 stock with fast-growing clams; if dift"erences in 

 growth rates are due to environment, the best 

 management plan miglit be to harvest the clams 

 from Bedroom Cove at a smaller size tiian those in 

 the center of the bay, or to transplant them to 

 areas where they would grow faster. 



Several researchers have discussed tiie causes of 

 variations in the growth rate of soft clams. Mead 

 (1900) observed tliat clam growth depended di- 

 rectly upon the supply of microscopic organisms in 

 the water. Kellogg (1905) indicated tliat dam 

 growth depended on the amount of available food 



I Speai. llarlnn S., iy.'i4. Rtsulls of poiJUlntlon ci'nsu.s. Sapadahoc Buy. 

 Maine. lin|)iil)lishcil report on file at V. S. Fishery Laboratory. Boothbay 

 Harbor. Maine, 



279 



