Large-scale genetic patchiness among NE Atlantic populations of the brittle star Ophiothrix fragilis

The brittle star Ophiothrix fragilis (Abilgaard, 1789) constitutes a heterogeneous mor- phological group that can be subdivided into 4 varieties. The species is also characterized by high demographic variability. The present study explores the possibility of genetic patchiness arising due to admixture of varieties or recruitment heterogeneity. We sequenced a portion of the mitochondrial cytochrome oxidase subunit I (mtCOI) gene and genotyped 7 allozymes from 21 populations of the 2 most common varieties. While mtCOI analyses showed that all the varieties grouped together with virtually no divergence among them (<1%), a clear divergence (18.6%) was evident between the northern Atlantic populations and individuals collected in the Mediterranean and along the Galician coast. Conversely, canonical analysis indicated that classification by variety explained a substantial part of the allozyme variance. Within O. f. pentaphyllum, no clear pattern of geographic structure was observed around the British Isles, with neighbouring populations often showing more genetic differ- entiation with one another than with distant populations. This either reflects admixture of cryptic lineages/species, diversifying selection due to contrasting habitats, or spatial and/or temporal admix- ture of genetically differentiated cohorts produced from isolated populations. The latter hypothesis is reinforced by the fact that significant haplotype frequency differences were recorded between larvae, juveniles and adults within the same locality. Providing that time-isolated clades co-occur within populations, the variation in the number and intensity of recruitment events must impact local genetic structure extensively and induce chaotic genetic patchiness at a large spatial scale.