An inverse correlation between dispersal ability and genetic differentiation among
populations of a species is frequently observed in the marine environment. We investigated the
population genetic structure of the free-living marine nematode Pellioditis marina. A total of 426 bp
of the mitochondrial cytochrome oxidase subunit 1 (COI) gene were surveyed on a geographical
scale of approximately 100 km during spring 2003. Nematodes were collected from 2 coastal locations
in Belgium, and from 2 estuaries and a saltwater lake (Lake Grevelingen) in The Netherlands.
Molecular variation was assessed with the single-strand conformation polymorphism (SSCP) method.
In total, 32 different haplotypes were observed, and sequence divergence among 452 individuals
ranged from 0.2 to 10.6%. We discovered 4 distinct mitochondrial lineages, with low divergences
within the lineages (0.2 to 1.6%) and high divergences between the lineages (5.1 to 10.6%). The
nuclear ribosomal ITS (internal transcribed spacer) region showed concordant phylogenetic patterns,
suggesting that nematode species diversity may be considerably underestimated. Analysis of
molecular variance (AMOVA) indicated a strong genetic differentiation among populations. The
Lake Grevelingen population was clearly differentiated from all other populations, but genetic
structuring was also significant within the Westerschelde and was correlated with gradients in
salinity and pollution. The observed population genetic structure is in accordance with the limited
active dispersal capacity of P. marina, but is at variance with its significant potential for passive
dispersal. We therefore suggest that autecological characteristics, including short generation time,
high colonization potential and local adaptation, may be at the basis of this nematode’s population
genetic structure.
