INTERSPECIFIC COMPARISONS OF GENETIC POPULATION-STRUCTURE IN MEMBERS OF THE JAERA ALBIFRONS SPECIES COMPLEX

Marked genetic differentiation in the intertidal isopod, Jaera albifro ns (Crustacea: Isopoda) has been shown to occur on a scale of just a f ew metres on British shores. Allozyme electrophoresis at 21 enzyme-cod ing loci has been employed to examine genetic structure in other UK me mbers of the complex (Jaera forsmani, J. ischiosetosa, J. praehirsuta) , and explore the relationship between genetic diversity and perceived niche-width. Comparisons were made with the nonsibling species J. nor dmanni. Three subpopulations of each species taken from each of two sh ores on Anglesey, UK (subpopulations N=30) were assayed for electropho retic variability. Data from 11 polymorphic loci (P-0.95) demonstrated marked genetic differentiation in all populations of J. albifrons and J. praehirsuta, and on one shore for each of J. ischiosetosa and J. n ordmanni, with J. praehirsuta (G(ST)=0.207) and J. albifrons (G(ST)=0. 121) showing the highest genetic differentiation. In contrast, J. fors mani exhibited population homogeneity on both shores studied. Genetic diversity ranged markedly across species (H-0=0.165-0.040), with the t wo most widely distributed species, J. albifrons (H-0=0.135) and J. is chiosetosa (H-0=0.165) exhibiting the highest genetic variability, pro viding support for the niche-width variation hypothesis. Data indicate that although habitat fragmentation and direct development is associa ted with microgeographic differentiation in Jaera spp., localized fact ors such as habitat continuity and exposure to water movements determi nes the magnitude of such effects.