At each site, all M. mercenaria in these ex- 

 periments >20 mm in initial length came from one 

 of two different sources. One enclosure at each site 

 held clams planted at the constant density ^f 80 

 m"^ and collected initially from a single source, a 

 seagrass bed along the southern shore of Back 

 Sound (described in Peterson et al. 1984). One or two 

 additional enclosures held clams collected locally at 

 the specific study site, but again kept at the same 

 80 m"^ density. Low availability of clams in the 

 10-20 mm size class from the Back Sound source 

 forced us to add hatchery-reared clams (spawned 

 from Back Sound individuals) and wild-caught clams 

 from Middle Marsh also in Back Sound to represent 

 the 10-20 mm sizes in the "common source" enclosure 

 at each sita This smallest size class was available 

 locally at each site to complete the "local-origin" 

 enclosures. This design was chosen to enable us to 

 test whether there were any effects of clam origin 

 (the single Back Sound source vs. local clams) at each 

 of the five study sites. 



Upon termination of the experiment almost exact- 

 ly 2 yr after initial planting of the marked clams 

 (Tkble 2), subsets of those clams that had survived 

 the complete 2-yr period were selected from each 

 origin treatment at each study site in as wide a range 

 of sizes as possible These clams were returned to 

 the laboratory and killed by steaming. One valve 

 from each of these 2-yr survivors was then section- 

 ed, using a diamond blade on a slow-speed Buehler 

 ISOMET saw, from umbo to ventral margin along 

 the axis of greatest growth. The shell sections were 

 sanded and polished when necessary to enhance the 

 clarity of natural banding patterns. Because the ini- 



tial size in August- September 1980 and the inter- 

 mediate size 1 yr later were known for each of the 

 marked individuals and could be marked on the shell 

 surface, and because the marking and measuring 

 process itself almost invaribly induces deposition of 

 a disturbance check which serves as a very precise 

 internal shell marker (Peterson et al. 1983), we were 

 then able to count with the unaided eye the number 

 of additional growth bands deposited in the internal 

 shell matrix of each clam during its final 2 yr of life 

 We also observed where these bands were deposited 

 relative to the known sizes at the initial, interme- 

 diate, and final measuring dates. These observations 

 permit a test of whether the reliability of using sum- 

 mer growth bands to age North Carolina's M. 

 mercenaria varies with site (habitat) or clam origin 

 in the vicinity of Cape Lookout. 



Results 



We sectioned shells from a total of 89 M. 

 mercenaria collected alive in August-September 

 1982 and known by their paint codes to have been 

 present in the field since the experiment's initiation 

 24 mo before (Table 2). Of these 89 individuals, 17 

 either exhibited insufficient growth to permit an ac- 

 curate determination of the precise shell size at the 

 experiment's initiation or else lacked a disturbance 

 check to mark the precise size at initiation. Of the 

 remaining 72 individuals, all but 2 deposited exact- 

 ly 2 additional dark growth bands in the final 24 mo 

 of life (Tkble 3). This pattern was consistent across 

 all five study sites and did not change as a function 

 of clam origin (Ikble 3). The appearance of the dark 



Table 3.— For each of five new study sites: 1) numbers of hard clams cut for growth analysis from each origin 

 treatment, 2) numbers of those with insufficient growth to assess band deposition accurately, 3) range of ini- 

 tial clam lengths for those clams with sufficient growth, and 4) average number of bands deposited in the 

 2-yr experimental period. 



Clam origin 



Back Sound 



Local site 



Range in Av. no. of 



No. with initial annual bands No. with 



insufficient length added in 2 yr No. insufficient 



growth (mm) (±1 SE) cut growth 



Range in Av. no. of 



initial annual bands 



length added in 2 yr 



(mm) (±1 SE) 



674 



