BROUSSEAU and BAGLIVO: FIELD POPULATIONS OF MYA ARENARIA 



adult survival is not altered by salinities of 2.5°/oo 

 (Chanley 1957; Pfitzenmeyer and Drobeck 1963; 

 Castagna and Chanley 1973) to 35"/oo (Castagna 

 and Chanley 1973). Annual water temperature and 

 salinity patterns in Long Island Sound fall well 

 within the range of conditions tolerated by M. 

 arenaria (see Materials and Methods). If tempera- 

 ture and salinity differences did occur, their effects 

 would be minimal. 



Substrate differences at the Stonington and West- 

 port sites appear to be the most immediate cause 

 of the observed differences in age-specific fecundity 

 and survivorship. Several investigators have found 

 that sediment type is important in controlling 

 growth rate and shell allometry in M. arenaria 

 (Belding 1930; Swan 1952; Newell and Hidu 1982). 

 Clams grown on coarse sediments (gravel, cobble), 

 such as that at the Westport site are slow-growing 

 and more globose in shape than clams from sand or 

 mud environments. Although differences in growth 

 rate could be due to differences in food availability 

 at the two sites, the allometric variations (Brous- 

 seau and Baglivo 1987) strongly support the hypoth- 

 esis that substrate effect is the factor controlling 

 growth. Reduced body size in Westport clams may 

 have an indirect effect on fecundity by restricting 

 egg production. In addition, both Glude (1954) and 

 Pfitzenmeyer and Drobeck (1967) demonstrated that 

 M. arenaria burrows fastest in fine-grained sedi- 

 ments since physical resistance to burrowing in- 

 creases with increasing particle size (Trueman 1954). 

 Hence, the high daily maintenance requirements of 

 Westport clams may result in less energy available 

 for reproduction, long-term maintenance, and sur- 

 vival. The Westport population may be an example 

 of a population inhabiting a marginal environment. 



The underlying pattern of life history (reproduc- 

 tive effort, patterns of recruitment, survival profile, 

 and growth schedule) for both populations of M. 

 arenaria is very similar. This is not surprising since 

 population parameters are viewed as evolved, 

 species-specific traits. Nevertheless, our evidence 

 for local differentiation in different habitats demon- 

 strates the degree to which individual populations 

 of a widespread species can respond in different 

 ways to their immediate environments; environmen- 

 tal heterogeneity can be reflected in the structures 

 and dynamics of local populations. 



ACKNOWLEDGMENTS 



We wish to thank K. Schellinkhout, J. Smeriglio, 

 J. Trautman, and J. Wachter for technical assist- 

 ance in the field. Working space and running sea- 



water raceways for marking and holding animals 

 were made available at the National Marine Fish- 

 eries Laboratory, Milford, CT (J. E. Hanks and A. 

 Calabrese, Directors). Temperature and salinity data 

 for Long Island Sound was provided by W. Blogos- 

 lawski. Financial support for this study was provided 

 under grant number NA82AA-D-00018 of the Con- 

 necticut Sea Grant Program. 



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