FISHERY BULLETIN: VOL. 86, NO. 3 



The two populations of M. arenaria from Long 

 Island Sound appear to show a basic conservatism 

 in general life history pattern. In both populations, 

 fecundity increases rapidly in young females with 

 individuals reaching sexual maturity in time to 

 reproduce at the beginning of their second year of 

 life. Similarly, larval and adult survivorship sched- 

 ules follow the type III survivorship curve of Deevey 

 (1947). Extremely heavy mortality early in life is 

 followed by roughly constant mortality thereafter. 

 This pattern is similar to that described for a popu- 

 lation of M. arenaria from Gloucester, MA (Brous- 

 seau 1978) except that in the latter, age of first 

 reproduction occurs at the end of the second year. 

 It is interesting to note, however, that some of the 

 major life history features of M. arenaria show con- 

 siderable latitudinal variation within the species. 

 Frequencies of spawnings during the year increases 

 (for review see Ropes and Stickney 1965; Brousseau 

 1987a) and length of life and body size show a ten- 

 dency to decrease with decreasing latitude (Belding 

 1930; Newcombe 1935). Details of the ways in which 

 such variations affect the life history parameters, 

 however, remain to be studied. 



The possibility of gene flow between populations 

 of animals with planktonic larval stages always 

 exists. Nevertheless, the amount of genetic overlap 

 is effectively reduced as the geographical distance 

 between the populations increases. The significant 

 quantitative differences in the age-specific demo- 

 graphic parameters for the two populations studied 

 here suggest that v^ithin the framework of a general 

 life history strategy, a response to the biotic and 

 abiotic components of the immediate environment 

 is possible. Evidence from this study indicates that 

 environmental conditions at the Westport site may 

 be less optimal for the growth and maintenance of 

 M. arenaria than are those at the Stonington site. 

 The higher equilibrium settlement rate calculated 

 for the Westport population indicates that on aver- 

 age, a larger annual spatfall is needed to maintain 

 that population than is required at the Stonington 

 site. 



The biotic factors most often cited as agents capa- 

 ble of altering the survival and fecundity of in- 

 dividuals are predation, competition, disease, and 

 parasitism. It is unlikely that predation was a major 

 source of adult mortality at either site for reasons 

 previously discussed. Moreover, the effects of preda- 

 tion would be limited to the small, surface-dwelling 

 clams, since 1) crabs, fish, and birds are unable to 

 capture deep burrowing adults and 2) it has been 

 demonstrated that M. arenaria exhibits a "size 

 refuge" from naticid predators (Edwards and Hueb- 



ner 1977). Hancock (1973) has suggested that com- 

 petition for food or space between the spat and 

 adults may contribute to lowered survival in newly 

 settled clams. If this were the case, one would ex- 

 pect lower juvenile survivorship rates in the Stoning- 

 ton population where population densities were 

 greater (Fig. 3). 



Trematode infestations have been demonstrated 

 to cause castration and high mortalities in a popula- 

 tion of venerid clams, Transenella tantilla, in 

 California (Obrebski 1968). Although both trematode 

 infections (Stunkard 1938; Uzmann 1951) and fungal 

 parasites have been reported in M. arenaria 

 (Andrews 1954), no evidence of either was observed 

 in the 2,826 histologically prepared clams (1,583 

 from Stonington; 1,243 from Westport) examined 

 in a study of the reproductive cycle in the two popu- 

 lations (Brousseau 1987a). Sarcomatous neoplasia, 

 a proliferative disorder characterized by increased 

 number of "leukemia-like" cells in the tissues and 

 organs (Farley 1969; Brown et al. 1977; Cooper et 

 al. 1982), has been identified in samples of clams 

 from both Stonington and Westport. Prevalence of 

 the disorder in the Stonington population ranges 

 from to 46%, whereas 0-69% of the M. arenaria 

 from Westport were neoplastic, depending on the 

 collection date (Brousseau 1987b). At present it is 

 not known if neoplasia is a source of mortality in 

 field populations of clams. It seems reasonable to 

 assume that it is, however, since neoplastic cells are 

 invasive and at times cause the destruction of organs 

 and tissues in infected animals (Yevich and Barszcz 

 1977). If sarcomatous neoplasms prove to be malig- 

 nant, this disease could be responsible to some 

 degree for the higher mortality rates reported in the 

 Westport population. 



The abiotic factors with the greatest effect on the 

 biotic potential of estuarine organisms are temper- 

 ature, salinity, substrate, and food availability. Adult 

 M. arenaria typically inhabit the intertidal zone and 

 are adapted to a vvide range of fluctuations in water 

 temperature and salinity. In addition, sediments 

 tend to buffer temperature and salinity fluctuations 

 (Sanders et al. 1965; Johnson 1965, 1967). There- 

 fore, infaunal organisms like the soft-shell clam are 

 subject to less extreme environmental fluctuations 

 than are exposed organisms living on or attached 

 to the surface. Belding (1930) noted that M. 

 arenaria withstands extreme variation in salinity, 

 being able to adjust to changing tides every six 

 hours. Shaw and Hamons (1974) found that lethal 

 conditions for burrowed clams were met only when 

 temperatures persisted in the high 20 °C range and 

 salinities were 2°/oo or lower. In the laboratory. 



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