Freeman, Aaren¹, ¹

ABSTRACT- The Japanese green alga Codium fragile ssp. tomentosoides has invaded several marine regions worldwide. In New England this invasive alga forms subtidal beds to depths of 8 meters, often replacing endemic kelp beds. However in the San Juan Islands, Washington, a highly similar native subspecies of C. fragile forms beds confined to the intertidal zone. In order to test the hypothesis that subtidal grazing may restrict C. fragile to the intertidal zone in Washington but not in New England, at sites in the Pacific and Atlantic I experimentally transplanted the algae within and outside of their natural vertical distribution to quantify grazing pressure. Endemic C. fragile transplanted to the subtidal zone in the San Juan Islands experienced significant herbivory, primarily by the pacific red sea urchin Strongylocentrotus franciscanus. In New England, the non-native C. fragile transplanted to cages containing green sea urchins, S. droebachiensis, were not grazed. Complementary laboratory experiments performed to determine the amounts of C. fragile consumed by various local herbivores indicated that the pacific red sea urchin S. franciscanus consumed significantly more C. fragile than native kelp species, while the green sea urchin, S. droebachiensis (both Pacific and Atlantic) consumed kelp and C. fragile at similar rates. These results imply that the ability of the native urchin grazers to consume the two similar algae may influence regional differences in the vertical distribution of C. fragile and the success of invasive palatable algae.

KEY WORDS: invasive, algae, herbivory, Codium fragile