Does the invasive alga Codium fragile ssp. tomentosoides support growth and reproduction of the green sea urchin, Strongylocentrotus droebachiensis?
Lyons, Devin*,1, Scheibling, Robert1, 1 Dalhousie University, Halifax, NS, Canada
ABSTRACT- The rocky subtidal community off the Atlantic coast of Nova Scotia has historically undergone a cyclical transition between Laminaria-dominated kelp beds and sea urchin-dominated barrens. Since the introduction of the invasive alga Codium fragile ssp. tomentosoides, a third community state has emerged: Codium-dominated algal beds. We conducted a ten-month feeding experiment in the laboratory, which matched the quantity and quality of food available to urchins (Strongylocentrotus droebachiensis) in each of these community states. Feeding rate, growth, reproduction, and survival of urchins fed either a high ration of kelp, a low ration of kelp, or a high ration of Codium were measured. On average, feeding rate on Codium was 1.8 times greater than that on kelp, when both algae were provided ad libidum. However, urchins in the high kelp treatment developed larger tests and gonads than those in the Codium treatment. Urchins in the low kelp treatment did not increase in test diameter or gonad index over the course of the experiment. Food ration had no effect on survival. Our results contrast with previous studies in two ways. First, they contradict a previous laboratory study, which suggested that Codium cannot support growth and reproduction in S. droebachiensis. Second, they demonstrate that Codium can be consumed at a higher rate than kelp in monospecific diets, although it has been found to be a much less preferred food in choice experiments conducted in the field and the laboratory. The mechanisms that govern this preference remain unknown.
Key words: invasive species, plant-herbivore interactions, Codium fragile ssp. tomentosoides, Strongylocentrotus droebachiensis
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