Density-related reproductive trade-offs in the green sea urchin, Strongylocentrotus droebachiensis

Green sea urchin, Strongylocentrotus droebachiensis (O.F. Muller), populati ons are being depleted rapidly in the Gulf of Maine and there is justified concern that the potential of this free-spawner to produce larvae may be se verely inhibited. We evaluated the opposing effects of different population densities on gonad development and fertilization success, using population surveys and fertilization experiments. We determined gonad indices (gonad mass/body mass) over a range of population densities (0.1 to 250 ind. m(-2) ) at seven sites in coastal Maine, USA, sampled at two depths (5 and 15 m). At shallow sites, we found that gonad indices declined by 50% over the 150 0-fold range in adult population density. At 15 m deep locations, gonad mas s was consistently low and did not vary significantly with density. Pattern s of macroalgal abundance suggest urchins at high density and in deeper wat er were food limited. Because macroalgal cover covaries inversely with sea urchin density, we designed field experiments to determine the interaction between sea urchin density and kelp canopy on fertilization success. On squ are arrays we manipulated the spacing of simulated urchins, but held their numbers constant (five sperm-filled syringes interspersed with four Nitex m esh egg containers permeable to sperm). These experiments, simulating the o bserved range of natural density, suggested that (1) fertilization rates de creased many times faster than individual gamete production increased over the same range in density, and (2) kelp increased fertilization success at high density when eggs were within 25 cm of a sperm source, but not when sp aced I m apart. Additional laboratory fertilization experiments at ambient temperatures (3 to 5 degrees C) indicated that diluted sperm were viable fo r <1 h, but egg viability was virtually unchanged for >8 h. In short, to th e individual the reproductive benefits of aggregating appear to outweigh th e costs; and while sperm may be limiting at low population density, eggs ma y remain viable long enough to be fertilized by sperm from more distant mal es.