Hitch-hiking on scallops: Grazing avoidance by macrophytes

Epibenthic macroalgae were observed to grow more abundantly on the upper surface of the Japanese scallops Patinopecten yessoensis (Bivalvia: Pectinidae) than on other available substrate. A field experiment to examine the hypothesis that colonization of live scallops protects macrophytes from grazing by sea urchins Strongylocentrotus nudus and S. intermedius was performed. Algal growth in the areas with urchins was compared with that in the enclosures without urchins. Each enclosure contained two live scallops, two empty shells, and one rock similar to scallops in size and shape. Twenty live scallops, 16 empty shells and ten rocks were marked and left outside the enclosures. After three months there was no difference in macrophyte mean percentage cover on live scallops inside and outside the enclosures. Percentage cover of macrophytes on dead scallop shells and rocks was significantly lower outside the protective enclosures as compared to unprotected areas. The results support the hypothesis that scallops serve as mobile refugia for macroalgae from urchin grazing.