NOTE Beal et al : Shell marker for juvenile, hatchery-reared Mya arenaria 



381 



attached by staples to a 40 x 2.5 x 2.5 cm wooden 

 stake at each corner. This square, fenced assembly 

 was then pushed 8 to 10 cm into the sediments leav- 

 ing a mesh wall projecting from 5 to 7 cm vertically. 



Groups of clams were assigned randomly to the 

 open enclosures on 26 May 1984. This completely 

 random design resulted in five replicates of each of 

 four evenly spaced planting densities ranging from 

 400 to leOO/m-. The fate of the 200 clams marked 

 with two dots is reported here (see Beal [1994] for 

 results of increasing intraspecific density on the fate 

 and growth of the remaining clams). To ensure that 

 seagulls (Lams spp.) did not prey on the clams be- 

 fore they had the opportunity to burrow, one of us 

 remained at the site until the tide had completely 

 covered the clams and the enclosures. During this 

 interval (ca. 2 hours) clams burrowed into the sedi- 

 ments approximately 20 minutes after planting. No 

 clam was visible at the sediment surface at the time 

 of tidal inundation. 



Live and dead clams were recovered from each of 

 the twenty open enclosures after 99 days ( 1 Septem- 

 ber 1984) by collecting the top 15 cm of sediment from 

 each enclosure and sieving the sediment through a 

 0.5-mm mesh. For each live clam (marked with two 

 dots or not), an obvious mark parallel to the entire 

 shell circumference was visible on the shell at or near 

 the size the animal was on 26 May 1984 (see "Re- 

 sults" section). This mark permitted us to estimate 

 a shell growth index for every live clam: 



Relative growth index = 

 [{final length - initial length) / initial length] x 100. 



Index values greater than 100% indicate at least a 

 doubling in growth. 



Shells from this 1984 field experiment were not 

 saved, but results from subsequent field trials in 

 eastern Maine with hatchery-reared softshell clam 

 juveniles (Beal, 1994) revealed that this shell mark 

 remains visible for periods up to 1.5 years after plant- 

 ing. We used scanning electron microscopy (SEM) to 

 describe the gross shell structure across the distinc- 

 tive pre- and postplanting regions of the outer shell 

 of juvenile, hatchery-reared clams. We used the 

 valves of cultured clams (initial size similar to 

 those reported above) that had been reared during 

 1989 at the Beals Island Regional Shellfish Hatch- 

 ery (BIRSH), Beals, Maine, planted at the Narrows 

 in May 1990 and recovered in October 1990. We 

 also used SEM to examine the outer shell struc- 

 ture of wild juvenile clams (ca. 15 mm) taken from 

 a nearby mudflat in Roque Bluffs, Maine, in Sep- 

 tember 1992. 



Results 



When viewed macroscopically, every live hatchery- 

 reared clam taken on 1 September 1984 (regardless 

 whether or not it had been marked with two dots) 

 had a distinct line on its shell surface that appeared 

 to correspond to its size at planting on 26 May 1984. 

 Scanning electron microscopy (SEM) of the gross 

 shell structure of wild juvenile clams (Fig. 1, A and 

 B) revealed sharp ridges (=lines viewed macroscopi- 

 cally) approximately 15-microns apart punctuated by 

 a relatively smooth region. In sharp contrast, the 

 valves of cultured clams (Fig. 1. C and D) consistently 

 revealed three distinct zones: an inner region from 

 the umbo to a distinct area (line) of demarcation that 

 was approximately 50-microns in width (Fig. ID) and 

 an outer zone from that line to the ventral margin. 

 The inner zone was characterized by a pitted, amor- 

 phous surface with interrupted ridges, whereas the 

 outer zone of the cultured clams (Fig. IC) appeared 

 similar to the entire surface of wild clams. Observa- 

 tions from experimental or stock enhancement 

 outplantings of cultured clams from BIRSH in nu- 

 merous coastal communities in Maine since 1987 

 indicate that the shell mark may be present for up 

 to 1.5 years after planting (Beal, 1994). 



To test whether the obvious line, or shell mark, 

 could be used quantitatively to distinguish initial size 

 at planting from subsequent shell growth, we exam- 

 ined closely the 57 surviving clams of the 200 that 

 had been marked with two dots on 25 May 1984. We 

 used calipers to estimate the initial size of the clam 

 delimited by the obvious surficial shell mark (line). 

 Next, we compared, for each clam, this estimate of 

 initial (i.e. predicted) shell length with the one re- 

 corded on 25 May. If the hypothesis that the line is 

 formed at or near the time of transplanting is cor- 

 rect, then the mean difference between actual (i.e. 

 recorded) and estimated size should not be signifi- 

 cantly different from zero. Results of a one-sample, 

 two-tailed /-test demonstrated that the average dif- 

 ference was not significantly different from zero (- 

 0.047 mm; ^=-1.505; P=0.138; n=57). An alternative 

 statistical approach is to plot the predicted initial 

 shell length (y-axis) against the actual initial shell 

 length (.v-axis; Fig. 2) and to determine whether the 

 straight line (y= -0.229 -f- 1.017.v; r~=0.97) is signifi- 

 cantly different from the line y = x. The slope of the 

 least squares equation for that line was not signifi- 

 cantly different from unity (F=1.38; df=2, 55; P=0.26), 

 and the intercept was not significantly different from 

 zero (F=0.81; df=l, 55; P=0.37). 



The mean relative growth index for the period 26 May 

 to 1 September for the 57 marked survivors was 86.9 ± 

 2.26% SE (Fig. 3A). Mean final length of the same in- 



