Size at first reproduction of the sea urchin Strongylocentrotus purpuratus in a central California kelp forest

A subtidal population of Strongylocentrotus purpuratus limited to small individuals (mostly 5 30 mm test diameter) by sea otter predation was examined to determine the size of reproductive sea urchins.Although a previous study found that most individuals of S. purpuratus examined were not reproductively mature until larger than 30 mm diameter, in this population most individuals 16 mm and larger were mature.The possible influence of habitat on the size at first reproduction is discussed.Often, due to energetic or mechanical constraints, a predator will only exploit prey of a particular size range.For this reason, prey species may have a size refuge from predation.The refuge may be large body size, such as that which protects the mussel Myrilus californianus and the barnacle Balanus cariosus from predation by the gastropod Thais spp.(Dayton 1971) and M. californianus from predation by the sea star Pisaster ochraceus (Paine 1976), or small body size, as is the refuge of Myfdus edulis (VanBlaricom 1988) and several temperate eastern Pacific sea urchin species from predation by the sea otter Enhydra lutris (Estes & VanBlaricom 1988, Estes et al. 1989).In habitats lacking suitable spatial refuges, sea otter predation can lead to a complete elimination of the larger size classes of sea urchins (Estes et al. 1978, Kenner in press).This predation could either lead to loss of the reproductive segment of the population, or, because large sea urchins have considerably more reproductive potential than small sea urchins (gonad size is limited by test volume, which is a cubic function of diameter), greatly reduce the population's reproductive output.Little information is available concerning the size at first reproduction for species of sea urchins in the field.Of those species for which there is information, several show some variation with habitat in the size or age at