Specimens of the common intertidal nemerteans Lineus ruber and L. viridis were collected fromsites along the west and Southwest coasts of Britain,northern France and North America. Allele frequenciesof up to 13 putative enzyme loci were estimated forall populations of L. ruber and L.viridis. Estimates of genetic variation were low forpopulations of L. ruber (Hobs 0.008–0.052)but were higher for populations of L. viridis(Hobs 0.068–0.153). Exacttests for conformity of observed genotype frequenciesto those expected under Hardy-Weinberg equilibriumfailed to detect significant deviations for L.ruber or L. viridis. F-statistics wereaffected by small sample size and low expected valuesin some populations, but, FST wassignificantly different from zero for most lociexamined for both Lineus ruber and L.viridis. This indicated a significant degree ofpopulation structuring for both species (only amoderate level of gene-flow). Intraspecificcomparisons of genetic distance and genetic identityshowed little evidence of genetic differentiationbetween populations separated by large geographicdistances (1000s of km). There was no apparentrelationship between genetic distance betweenpopulations and the geographic distance separatingthem. Possible explanations for this lack of geneticdifferentiation between populations of L. ruberand L. viridis are discussed. These include alack of variation in the enzyme loci sampled caused bypopulation dynamics, balancing selection in the enzymeloci sampled, large introductions between populationsand passive dispersal.
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