Cryptic lineages and high population genetic structure in the exploited marine snail Hexaplex trunculus (Gastropoda: Muricidae)

Although the banded murex Hexaplex trunculus (Linnaeus, 1758) has no dispersal stage, it is widely distributed in a relatively broad range of habitats. These features make it a particularly suitable model to reconstruct the history of the fragmentation of its geographical range. We investigated its genetic structure from the eastern Mediterranean to the Atlantic coast, by sequencing a nuclear intron (i29) and a mitochondrial marker (cox1). We found strong genetic differentiation between all population pairs, congruent with the absence of a dispersing larval stage. A deep phylogeographical break separated two parapatric lineages, a western lineage (I) and an eastern one (II). The two lineages were separated by a vicariance event dated to the Pliocene by both markers, analysed independently. They co-occur in southern Italy and in the Siculo-Tunisian straight, where some individuals display recombined genotypes (lineage I for one marker, lineage II for the other), suggesting that introgression occurred in sympatric populations. We were unable to determine whether the vicariance was across the STS or located further east across the Peloponnese Arc, but the presence of lineage I in the northern Adriatic is more parsimoniously explained by a more eastern vicariance. Lineage I displayed a stronger signal of demographic expansion than lineage II, and its expansion was estimated to be more recent. This result, which has been reported in other marine species, suggests less drastic conditions for the eastern (and possibly also the central) Mediterranean benthos than for western populations during past climatic oscillations. The parapatric distribution still observed today suggests that human exploitation of this snail, which dates back to Antiquity, did not result in efficient introductions among basins, although present-day introductions were recently reported in the Bay of Biscay.