We report that an eelgrass bed Zostera marina L. at the Aland Islands, nort
hern Baltic Sea, is dominated by a. single genotype which extends over an a
rea of approximately 160 x 40 m. In total, 47 individuals were sampled and
genotyped for 6 highly polymorphic microsatellite loci. Forty-four of the r
amets revealed the same 6-locus genotype, including 3 identical heterozygou
s loci. They were thus assigned to the same genet (= clone). To our knowled
ge, it represents the largest marine plant identified thus far. Based on es
timates of horizontal rhizome growth rates, this clone may be more than 100
0 yr old. The remarkable phenotypic plasiticity of a single genotype which
dominates this site illustrates that there is no simple one-to-one relation
ship between genetic diversity and population persistence in changing and s
tressful environments.