Genetic heterogeneity among adult and recruit red sea urchins, Strongylocentrotus franciscanus

Allozyme electrophoresis was used to characterize genetic variation within and among natural populations of the red sea urchin Strongylocentrotus fran ciscanus. In 1995 to 1996, adult urchins were sampled from twelve geographi cally separated populations, seven from northern California and five from s outhern California (including Santa Rosa Island). Significant population he terogeneity in allelic frequencies was observed at five of six polymorphic loci. No geographic pattern of differentiation was evident; neighboring pop ulations were often more genetically differentiated than distant population s. Northern and southern populations were not consistently distinguishable at any of the six loci. In order to assess within-population genetic variat ion and patterns of recruitment, large samples were collected From several northern California populations in 1996 and 1997, and were divided into thr ee size classes, roughly representing large adults (>60 mm), medium-sized i ndividuals (31 to 60 mm, "subadults") and individuals <2 yr of age (less th an or equal to 30 mm test diam, referred to as "recruits"). Comparisons of allelic counts revealed significant spatial and temporal differentiation am ong size-stratified population samples. Recruit samples differed significan tly from adult samples collected at the same locale, and showed extensive b etween-year variation. Genetic differentiation among recruit samples was mu ch higher in 1997 than in 1996. Between-year differences within populations were always greater for recruits than for adults. Potential explanations f or the differentiation of recruit samples include pre- and post-settlement natural selection and high interfamily variance in reproductive success or "sweepstakes" recruitment. Unless recruit differentiation can be attributed to an improbable combination of strong and spatially diverse selection, su ch differentiation across northern California populations indicates that th e larval pool is not well mixed geographically (even on spatial scales <20 km), despite long planktonic larval duration.