Very low mitochondrial diversity and genetic homogeneity in the starfish Echinaster sepositus along the Tunisian coast

Evidence for panmixia despite barriers to gene flow in the hooked mussel, Ischadium recurvum (Mytilidae; Brachidontinae) along the North American coastline

The discovery of phylogeographic patterns within broadly distributed marine species can be particularly challenging because absolute physical barriers to dispersal can be inconspicuous. Genetic boundaries often lie where ocean currents meet, forming sharp physical and ecological gradients, which may act as barriers to successful migrants. In eastern North America, coastal species often show phylogeographic differentiation associated with two recognized genetic barriers: the Gulf/Atlantic and the Virginia/Carolina discontinuities. We examined 185 specimens of the intertidal hooked mussel Ischadium recurvum collected from 15 locations along the eastern coastline of North America to examine phylogeographic, migration and historical demographic patterns associated climate change associated with Pleistocene glacial patterns. Hypothesis testing using Bayes factors in Migrate-n rejected the presence of phylogeographic breaks consistent with either maritime discontinuity and favoured a panmictic population model. The migration rate from the Gulf to the Atlantic was approximately three times higher than the migration from the Atlantic to the Gulf whereas the Carolina-Virginia migration rates were nearly equal. The summary statistics (Tajima's D, Fu's Fs) were significant and the demographic analyses (mismatch distributions, Bayesian skyline plot) were consistent with patterns of population expansion following glacial retreat during the Pleistocene epoch.

Low genetic diversity and recent demographic expansion in the red starfish Echinaster sepositus (Retzius 1816)

Understanding the phylogeography and genetic structure of populations and the processes responsible of patterns therein is crucial for evaluating the vulnerability of marine species and developing management strategies. In this study, we explore how past climatic events and ongoing oceanographic and demographic processes have shaped the genetic structure and diversity of the Atlanto-Mediterranean red starfish Echinaster sepositus. The species is relatively abundant in some areas of the Mediterranean Sea, but some populations have dramatically decreased over recent years due to direct extraction for ornamental aquariums and souvenir industries. Analyses across most of the distribution range of the species based on the mitochondrial cytochrome c oxidase subunit I gene and eight microsatellite loci revealed very low intraspecific genetic diversity. The species showed a weak genetic structure within marine basins despite the a priori low dispersal potential of its lecithotrophic larva. Our results also revealed a very recent demographic expansion across the distribution range of the species. The genetic data presented here indicate that the species might be highly vulnerable, due to its low intraspecific genetic diversity.

Mitochondrial DNA panmixia in the little brown seastar Echinaster spinulosus suggests a recent population expansion

In North America, the Atlantic-Gulf discontinuity depicts genetic breaks between populations distributed along the Florida peninsula. This study examines 83 specimens of E. spinulosus obtained from seven locations along the Florida coast to examine phylogeographic patterns, population structure and examine historical demographic patterns. Analyses of 511 bp from the cytochrome oxidase 1 (COI) mitochondrial gene inferred no significant genetic structure for E. spinulosus along either side of the Florida coast nor was there a genetic discontinuity associated with a "Gulf-Atlantic" split. Population-based statistical analyses revealed no significant degree of genetic differentiation (Fst, p < 0.05) among populations, thus indicating substantial gene flow. Similarly, summary statistics (Fu's Fs, mismatch distributions) were significant indicating that there has been a population increase in the recent past. Recent range expansion, possibly due to a leading edge effect, combined with some level of contemporary larval mixing seems to be the main source of panmixia in this species.