Microgeographic genetic variation of populations of Idotea chelipes (Crustacea: Isopoda) in lagoons of the southern English coast

Allozyme variability was examined using starch-gel electrophoresis between sub-populations of Idotea chelipes over a small geographic distance (approx. 104 km), in lagoons within the Solent area (Fort Gilkicker Moat; Ashlett Pond) and at one site in Dorset (the Fleet Lagoon). Genetic identity values indicate that all three populations are conspecific (I = 0.967 to 0.995), and mean heterozygosity per locus for all three populations was 8%. Significant levels of genetic differentiation (mean FST = 0.119***; p< 0.001) were detected over this small spatial scale, but this mainly resulted from the slow esterase locus (mean FST (no esterase) = 0.072***; p< 0.001). Genetic differentiation was moderate given the low geographic distances between populations with only one to two migrants per deme per generation (Nem(FST) = 1.85) from the Fleet to Gilkicker. There was no significant deviation from the genotypes expected under the Hardy-Weinberg equilibrium despite an overall slight excess of heterozygotes (mean FIS = -0.01). Pairwise FST values indicated that there was limited genetically effective migration between the sampled lagoons. This genetic differentiation may have been promoted by the presence of the Southampton Water and Solent estuarine system that may act as a physical barrier to gene flow for this species. The previous use of the esterase (ET2) locus as a sub-specific biochemical marker for Idotea chelipes is placed in doubt by this study.