Invertebrate fauna associated with kelp enhances reproductive output of the green sea urchin Strongylocentrotus droebachiensis

Kelp beds are species rich communities providing food and shelter for a wide range of invertebrate and fish species. As the dominant subtidal grazer in the northwest Atlantic, the green sea urchin Strongylocentrotus droebachiensis can severely limit the distribution and abundance of kelps and influence the structure of the rocky subtidal community. Urchins form large mobile feeding aggregations (termed fronts) along the margins of kelp beds that destructively graze kelps and, incidentally, the associated invertebrate fauna. Urchins along the leading edge of fronts climb upon and consume kelp fronds and trailing urchins graze the remaining stipes and holdfasts. The fronds are often encrusted with an epiphytic bryozoan Membranipora membranacea, while the holdfasts harbour many sessile and cryptic invertebrates, such as mussels and brittle stars. The effects on urchin growth (as test diameter and body weight) and reproduction (as gonad index) of feeding on kelp fronds or holdfasts, both with or without the natural associated fauna, were examined in a 95-day experiment in the laboratory. Female urchins had a greater gonad index than males in all diet treatments at the end of the experiment. Females fed holdfasts with associated fauna had a greater gonad index that those fed cleaned holdfasts, but there was no difference between these treatments for males. For urchins fed fronds, there was no significant difference in gonad index of either sex between treatments with and without M. membranacea, although urchins fed at a lower rate on bryozoan-encrusted fronds. Test diameter increased by 1-7 mm (based on tank means) during the experiment, but there was no effect of diet or sex on growth rate. The finding that a diet of kelp with naturally associated fauna can significantly enhance gonad production in S. droebachiensis has important implications for the development of optimal aquaculture and harvesting practices.