Appendix B. A description of the test for potential effects of enclosures on clam responses.
At two haphazardly chosen experimental sites (Argyle and Shaw) I dug a 0.18-m2 patch of sediment 16 cm deep and 1 m from each end of the experimental array. I removed all clams found in this sediment, refilled the hole to 5 cm from the surface, added 20 measured and marked individuals of both Protothaca and Venerupis, and finished filling the hole. This density of clams mirrored that of both species in the moderate Venerupis density treatment. At the end of the experiment I recovered all the marked clams from these control patches that I could find—86% of Protothaca and 60% of Venerupis. Although clams should theoretically be harder to recover in control plots since there were no enclosure walls to reduce migration, recovery of each species within control plots at both sites was within 7–18 percentage points of clams in topless enclosures. I also quantified dry tissue growth of control clams and for each species compared changes in dry tissue mass between clams in control plots and enclosures at each site using t tests. Protothaca did not exhibit significant differences in growth between control and enclosed clams (P > 0.5). Control clams for Venerupis at both sites exhibited slightly greater growth (0.2 g) than their analogues in the topless enclosures of the experiment (Shaw: t = 3.0, df = 12, P = 0.011; Argyle: Welch-Satterthwaite t = 8.75, df = 11, P < 0.0001). Even though enclosures depressed Venerupis growth slightly, this influence should conservatively bias relative comparisons between the species by handicapping Venerupis, the superiorly performing species, within the experimental enclosures.