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The hidden side of a major marine biogeographic boundary: a wide mosaic hybrid zone at the Atlantic-Mediterranean divide reveals the complex interaction between natural and genetic barriers in mussels. AbstractThe Almeria-Oran Front (AOF) is a recognised hotspot of genetic differentiation in the sea, with genetic discontinuities reported in more than 50 species. The AOF is a barrier to dispersal and an ecological boundary, and both facts can explain the position of these genetic breaks. However, the maintenance of genetic differentiation is likely reinforced by genetic barriers. A general drawback of previous studies is an insufficient density of sampling sites at the transition zone with a conspicuous lack of samples from the southern coastline. We analysed the fine-scale genetic structure in the mussel Mytilus galloprovincialis with a few ancestry-informative loci previously identified from genome scans. We discovered a 600 km wide mosaic hybrid zone eastward of the AOF along the Algerian coasts. This mosaic zone provides a new twist to our understanding of the Atlantic-Mediterranean transition because it demonstrates the two lineages can live in sympatry with ample opportunities to interbreed in a large area, but hardly do. This implies some form of reproductive isolation must exist to maintain the two genetic backgrounds locally cohesive. The mosaic zone ends with an abrupt genetic shift at a barrier to dispersal in the Gulf of Bejaia, Eastern Algeria.Simulations of endogenous or exogenous selection in models that account for the geography and hydrodynamic features of the region support the hypothesis that sister hybrid zones could have been differentially trapped at two alternative barriers to dispersal and/or environmental boundaries, at Almeria in the north and Bejaia in the south. A preponderantly unidirectional north-south gene flow next to the AOF can also maintain a patch of an intrinsically maintained genetic background in the south and the mosaic structure, even in the absence of local adaptation. Our results concur with the coupling hypothesis that suggests natural barriers mostly explain the position of genetic breaks while their maintenance must additionally require genetic barriers.

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Hexaplex trunculus (Linnaeus, 1758) is a gonochoric marine gastropod. Previous studies demonstrated that the biocide TBT (tributyltin) induced a sexual abnormality known as imposex (superimposition of male sexual characters onto females) in this whelk. Our study showed imposex in 19 stations out of 20 along the Tunisian coast. The frequency of imposex ranged from 0 to 100%. Among the 19 sites where the condition was found, 8 were considered as highly affected by imposex (VDSl > 3.7), 6 were moderately affected (VDSI > 1.3), and 4 were slightly affected (VDSI > 0). The most affected population was obseiwed in the Bizerta Channel where the highest boating traffic was recorded; no imposex features were found in the Sea of Zarat where boating traffic was veiy low. Significant differences in imposex levels were obtained among sites with low, moderate, and high boating traffic. All the imposex indices values (1%, RPSI, RPLl, VDSI, FPL, and VDL) were significantly more elevated at sites with high boating traffic compared with sites with low and moderate boating traffic. Malformations of the penis were observed only in five stations and in very low rates, but where imposex rates were high. The incidence of penis malformation in males was significantly related to the boating traffic, 1%, and VDSI. However, in females, a correlation was obtained only for the RPLL The present study provides data on imposex level and penis malformations in H. trunculus from the Tunisian Coast that could be used as a starting point for future monitoring programs and for temporal trend surveillance related to TBT pollution in Tunisia where the use of TBT is not yet banned.

We have assessed for the first time the current status of tributyltin (TBT) contamination in Hexaplex trunculus along Tunisian coastal waters. Two sampling campaigns have been performed in July 2004 and July 2007 at seven different sites. The snails were analyzed for imposex status, i.e. Imposex incidence, Relative Penis Length Index (RPLI) and Vas Deferens Sequence Index (VDSI), followed by the quantification of butyltins in their tissues. Imposex was detected in six populations from the 2004 samples and in five populations from the 2007 samples. The imposex incidence ranged from 0 (no imposex) to 100%, the RPLI from 0 to 56.7 and the VDSI from 0 to 4.4. TBT was also detected in six populations in 2004 and five populations in 2007. The concentrations ranged from 1.5 to 73.7 ng Sn g(-1) dw in 2004 samples and from 1.2 to 23.9 ng Sn g(-1) dw in 2007 samples. Temporal comparison of the data indicates some evidence of imposex recovery in most affected populations. Improvements in reduced body concentrations of TBT in both sexes were also seen in most stations. These TBT concentrations were only significantly correlated to imposex indices in samples from 2007 (r > 0.8, n = 7, p < 0.05). The present results will serve as a reference for long-term monitoring of butyltin contamination in the Tunisian coast, where TBT restrictions were not yet implemented.