Feeding, growth and reproduction of sea urchins (Strongylocentrotus droebachiensis) on single and mixed diets of kelp (Laminaria spp.) and the invasive alga Codium fragile ssp tomentosoides

Since its introduction to Nova Scotia in the early 1990s, the invasive gree n alga Codium fragile ssp. tomentosoides has spread rapidly to become a dom inant and persistent component of seaweed assemblages in the rocky subtidal zone. To examine the potential of sea urchins (Strongylocentrotus droebach iensis) to regulate Codium, and the potential of the alga to support urchin growth and reproduction, we fed urchins Codium and/or kelp (Laminaria spp. , a high quality and preferred native food) in the laboratory for I I month s. Urchins showed a strong and active preference for kelp: they consumed mo re Laminaria than Codium (as dry weight) in single and mixed diet treatment s. Urchins fed only Codium consumed 3.5 times more of the alga than those o n a mixed diet, but did not increase their feeding rate in winter when kelp consumption was greatest. Laminaria was consumed at the same rate in singl e and mixed diets, indicating that the presence of an alternative food had no effect on kelp consumption. Survival and growth (change in test diameter ) were lowest on the Codium diet, although the differences among diets were not statistically significant. Urchins on the Codium diet showed no gonada l production over the annual reproductive cycle, compared to a marked rise in gonad index on the Laminaria and mixed diets. Our results indicate that Codium is an unattractive, unpalatable and poor quality food, which is not readily consumed by urchins. Thus we predict that urchins at moderate densi ties will be much more likely to graze kelp than Codium in mixed stands, cr eating a mosaic of Codium patches and barren areas. At high densities, howe ver, urchins are likely to destructively graze all seaweeds, although dense stands of the invasive alga may prolong the widespread formation of barren s.