To assess the potential of herbivory in regulating the invasive green alga Codium fragile ssp. tomentosoides, field and laboratory experiments were conducted with the green sea urchin Strongylocentrotus droebachiensis in Nova Scotia, Canada. In the field, urchins at different densities (0, 50 and 100 urchins m -2 ) were caged on boulders covered with a canopy of either the kelp Laminaria longicruris, Codium, or a mixture of both species for 13 wk. In the treatment with Laminaria only, ∼90% of the canopy was removed within 34 and 75 d in cages with 100 and 50 urchins m -2 , respectively. In contrast, Codium cover decreased by ∼20% at both levels of urchin density in the treatment with Codium only, and did not differ significantly from the control (no urchins) at the end of the experiment. In the mixed canopy treatment, urchins showed a preference for Laminaria, consuming 90% of kelp cover within 39 and 54 d (at 100 and 50 urchins m -2 , respectively), while Codium cover increased gradually. Urchins grazed turf-forming red algae in all treatments, although in treatments with Laminaria, intensive grazing of turf only occurred once kelp was completely consumed. In the laboratory, urchins fed single diets of Laminaria or Codium for 8 wk had similar grazing rates (∼0.20 dry weight g urchin ). These experimental results indicate that urchins prefer kelp but will consume Codium when other algal foods are not available. We predict that urchin aggregations encountering mixed stands of kelp and Codium will initially graze the kelp and turf algae, creating patches of Codium that ultimately will be consumed as well.

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