BROUSSEAU and BAGLIVO: FIELD POPULATIONS OF MYA ARENARIA 



The equilibrium settlement rate (r,) for the West- 

 port population is 3.7453 x 10'^ and that for the 

 Stonington population is 3.4318 x 10"^. The equi- 

 librium first year survival rates (i.e., the probability 

 of surviving from egg to year 1 necessary to main- 

 tain a population at equilibrium) then, become 

 1.0599 X 10-^ and 2.014 x lO'^ for the Westport 

 and Stonington populations, respectively. 



Under the assumption of equal rates of larval im- 

 port and export, it is possible to calculate empirical 

 settlement rates by estimating the total number of 

 eggs released per unit area by spawning females in 

 1985 and dividing this value into the densities of 

 2-20 mm spat settled per unit area 2 months later. 

 Size-frequency data for the populations was used to 

 determine the number of clams in each 10 mm size 

 class (Fig. 3). The number of animals in each class 

 (assuming that one-half the population is female) was 

 multiplied by the mean size-specific annual fecun- 

 dity value for that size class. Summing over all size 

 classes of reproducing females gives an estimate of 

 the 1985 oocyte production per unit area. Empirical 

 settlement rates of 8.4589 x 10"*^ and 7.1953 x 

 10"^ were calculated for the Westport and Stoning- 

 ton populations. These represent values of 2.2585 

 and 2.0967 times the estimated equilibrium settle- 

 ment rate of these populations. Multiplying the em- 

 pirical settlement rate by the probability of surviv- 

 ing the remainder of the year give actual first year 

 actual first year survival rates of 2.3939 x 10"^ 



and 4.2229 x 10"^ for Westport and Stonington, 

 respectively. 



The heavy mortality evident by the low juvenile 

 survivorship rates {l{) for M. arenaria take into ac- 

 count the losses incurred during fertilization, meta- 

 morphosis and recruitment, and subsequent survival 

 through the first year of life. Table 8 compares the 

 first year survival rates of representative marine 

 invertebrate species with planktonic and non- 

 planktonic modes of development. Early survival in 

 species lacking planktonic larvae is on average, 

 three orders of magnitude higher than that of 

 species which pass through a planktonic larval stage. 



DISCUSSION 



General comparisons of life history traits are use- 

 ful but quantitative comparisons are possible only 

 from the more detailed information found in life 

 tables (age-specific fecundity and survivorship). The 

 difficulty in generating such information, especial- 

 ly for marine bivalves, many of which have plank- 

 tonic larval stages during their life cycles, has 

 resulted in the construction of few complete life 

 tables. Moreover, no reported field study has ex- 

 amined life history traits for more than one popu- 

 lation of a species simultaneously. Consequently, the 

 extent to which quantitative differences in life 

 history parameters are characteristic of the life 

 history of a single species is unknown. 



Table 8.— Empirical estimates of first year survival rates of marine invertebrate species with and 

 without planktonic larvae (adapted from Perron 1983). 



'Does not include prehatching mortality. 

 ^Survival through 30-d planktonic period only. 



573 



