Demographic patterns of the purple sea urchin Strongylocentrotus purpuratus along a latitudinal gradient, 1985-1987

The abundant center model of geographic distribution has been tested for the purple sea urchin by others who found highest densities and reproduction towards the southern end rather than in the center of the range and so rejected the model. A question is whether size, growth, and survival data would yield other or contrary results. Intertidal densities and size structures were measured for purple sea urchins along the Pacific coast of North America during 1985-1987 from 29.93 to 50.47 degrees N, covering about 65% of the reported geographic range; these historical data were used to explore latitudinal patterns. Recruitment was based on the fractions of individuals in the smallest mode of the size distributions. Growth was determined by tagging with tetracycline and survival was estimated using size distributions and growth. Density of purple sea urchins was highest between 35 and 37 degrees N and the samples with the highest numbers of recruits occurred between 34 and 38 degrees N. Maximum diameter was largest at 43 to 44 degrees N and smallest at about 34 degrees N. There was no latitudinal pattern to growth or survival. Patterns of population traits did not fit simple models or previously published results. The interplay of coastal topography and currents are suggested as the primary determinants of density and recruitment. No general models for describing geographic distributions predict the observed patterns. The southern range limit is best explained by thermal tolerance and the northern limit by development times of larvae at low temperatures.