SETTLEMENT-PATTERNS OF RED AND PURPLE SEA-URCHINS (STRONGYLOCENTROTUS-FRANCISCANUS AND STRONGYLOCENTROTUS-PURPURATUS) IN CALIFORNIA, USA

Weekly settlement of red and purple sea urchins (Strongylocentrotus franciscanus and S. purpuratus) was documented at sites in northern and southern California (USA) for 4 settlement seasons from 1990 to 1993. Newly settled sea urchins (<0.5 mm) were collected from standardized settlement surfaces (scrub brushes), identified and counted, There was marked seasonality in settlement. For both species, most settlement occurred during one to several episodes between February and July of each year. Settlement tended to be more regularly annual in southern than in northern California and settlement densities generally were higher in the south. This pattern is consistent with greater retention of water in the Southern California Bight and more energetic offshore advection of water in northern California, S. purpuratus showed geographic coherence in settlement whereas S, franciscanus did not. For purple sea urchins, northern sites formed a group, a second group was formed by sites near Santa Barbara plus a site in Orange County, and a third group was formed by 3 sites at San Diego.