LIFE TABLES FOR TWO FIELD POPULATIONS OF 

 SOFT-SHELL CLAM, MYA ARENARIA, (MOLLUSCA : PELECYPODA) 



FROM LONG ISLAND SOUND 



Diane J. Brousseau' and Jenny A. Baglivo^ 



ABSTRACT 



Life tables were constructed for two populations oiMya arenaria from Long Island Sound, USA, based 

 on schedules of age-specific fecundity and mortality determined under natural conditions. Mya arenaria 

 shows a basic conservatism in general life history pattern. In both populations, fecundity increases with 

 increasing female size; sexual maturity is attained at 1 year of age; a single annual breeding season occurs 

 and survivorship curves approximate the type III of Deevey, which is characterized by extremely heavy 

 mortality early in life followed by relatively constant mortality thereafter. Differences in the age-specific 

 parameters for the two populations exist, however; for clams greater than 1 year of age, both age-specific 

 fecundity and survivorship are significantly higher in the Stonington population. These differences in 

 the structure and dynamics of the two populations may be due to environmental heterogeneity. Reduced 

 body size due to slower growth in coarse substrate, as well as the increased maintenance demands resulting 

 from burrowing and valve activity in large-grained sediment, may account for the lower egg production 

 and lower survival rates found in the Westport population. 



The life history pattern of a species has been defined 

 as the way in which that species partitions the 

 limited resources of time or energy among the three 

 basic biological processes of growth, maintenance, 

 and reproduction. Efforts to generate empirical 

 values for life history parameters, age-specific fecun- 

 dity and survivorship, have only recently allowed the 

 construction of life tables for field populations. 

 Determining life history parameters for a commer- 

 cially important species such as the soft-shell clam, 

 Mya arenaria, is particularly useful since they can 

 be used in theoretical models which are designed to 

 analyze the effect of changing survival and fecun- 

 dity values on the growth rate of the population. 

 Life tables now exist for a number of benthic 

 marine invertebrates: the ha,rna,c\e—Chathamulus 

 stellatus (Connell 1961); the prosobranchs— 

 Dicathais orbita (Phillips and Campbell 1974), 

 Nucella (= Thais) lapillus (Frank 1969), Nodilit- 

 torina tuber culata (Doran 1968), and Conns pen- 

 naceus (Perron 1983); the coelenterates— Mwr-icea 

 califomica (Grigg 1977), M.fruticosa (Grigg 1977), 

 and Balanophyllia elegans (Fadlallah 1983); the 

 bivalves— Mi/a arenaria (Brousseau 1978), Tapes 

 phillipinarum (Yap 1977), and Gemma gemma 



'Department of Biology, Fairfield University, Fairfield, CT 

 06430. 



^Department of Mathematics and Computer Science, Fairfield, 

 University, Fairfield, CT 06430; present address: Mathematics 

 Department, Boston College, Chestnut Hill, MA 02167. 



(Weinberg 1985). These studies, however, examine 

 life history parameters for a single species popula- 

 tion or for successive cohorts within a population. 

 The present study examines age-specific fecundity 

 and survivorship in two geographically separated 

 populations of the soft-shell clam, Mya arenaria, 

 using identical methodology. Since differences in 

 methodology can influence estimates of demo- 

 graphic parameters (Fadlallah 1983) uniformity of 

 approach is necessary for interpopulation com- 

 parisons. This study is the first reported examina- 

 tion of species- specific traits in two naturally occur- 

 ring populations and was carried out as part of a 

 broader study of the population dynamics of this 

 species along the Connecticut shore of Long Island 

 Sound. 



MATERIALS AND METHODS 



Field Study Areas 



Field studies were conducted at two intertidal 

 sites in Long Island Sound, one located at Barn 

 Island in Stonington, CT Oat. 41°20'N; long. 

 71°53'W) and the other in the Saugatuck River in 

 Westport, CT Gat. 41°06'N; long. 73°23'W)(Fig. 1). 

 The Stonington site is a narrow intertidal sandflat 

 which extends approximately 10 m shoreward to a 

 coarse sand beach. At low tide the Westport site ex- 

 tends 30 m (at its widest point) shoreward to a Spar- 



Manuscript accepted April 1988. 



FISHERY BULLETIN: VOL. 86, NO. 3, 1988. 



567 



