Population Structure, Recruitment and Mortality of Two Sea Urchins (Strongylocentrotus franciscanus and S. purpuratus) in a Kelp Forest

Abstract

The red sea urchin S. franciscanus and the purple sea urchin S. purpuratus are important members of southern California USA kelp bed communities. Their population structures were studied in terms of patterns of recruitment and mortality at 3 sites in the Point Loma kelp forest near San Diego, California. Recruitment was annual and substantial at all sites. Several predators consume adult S. purpuratus, but only the spiny lobster Panulirus interruptus and the California sheephead fish Semicossyphus pulcher (a large labrid) prey on large S. franciscanus. Urchin test collections, small scale urchin distribution patterns and grazing mortality of the giant kelp Macrocystis pyrifera suggest that lobsters and sheephead, both of which are harvested, control urchin populations. In the presence of these predators, the size-frequency distributions of S. franciscanus populations are bimodal, apparently because juveniles 40 mm) are protected by the spine canopies of adults, urchins of intermediate size (50-80 mm) are very vulnerable to predators and large adults 90 mm) attain a partial refuge in size. The population structure of S. purpuratus is unimodal, evidently because this species is less dependent on the spine canopy association and its short spines and smaller adult test dimensions do not allow it a refuge in size from predation.