Infestation of the clam Venus verrucosa by Sipunculoidea and the lithophagus bivalve, Gastrochaena dubia

From August 2003 to July 2004, specimens of the bivalve Venus verrucosa (L.) were collected monthly in the channel connecting the lagoon of Bizerte (Tunisia) to the Mediterranean Sea. During the winter, 4% of the specimens had tiny perforations on the outer and inner faces of the valves; the perforations were connected to an intra-valve network of galleries, caused by 10-12 mm Sipunculoidea. Of specimens collected in February-March, 4% were infested with the lithophagous bivalve, Gastrochaena dubia, that lived within a cavity in the V. verrucosa valves. The cavity communicated to the outside through a calcareous tube developed by the G. dubia near the exit of the V. verrucosa siphons, ndicating parasitism that can cause progressive perforation of the valve and lead to the death of the host

Monitoring of coastal pollution using shell alterations in the false limpet Siphonaria pectinata

International audienceLipid peroxidation level (LPO), shell biometry, shape, elemental content, and microstructure were studied in three populations of Siphonaria pectinata in the complex lagoon-channel of Bizerte across a coastal pollution gradient (northern Tunisia). LPO was found in higher concentrations in harbour populations, and shells had centred apex and were flattened. Shells were also thicker, particularly in the inner layer, with many fibrous inter-beds formed. Difference in crystallization pattern was observed in numerous shells from all three populations, being more common in harbours. From the control station to the contaminated stations, shell elemental changes were observed, with a decrease in Ca, P, Sr, and S and an increase in Cl, Cd, Cu, Fe, and K. All of these findings suggested that shell alterations could be used as a good biomarker for coastal contamination

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

International audienceThe 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; both can determine 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 using 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 that the two lineages can live in sympatry with ample opportunities to interbreed in a large area, but they hardly do so. This implies that 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 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 that natural barriers can explain the position of genetic breaks, while their maintenance depends on genetic barriers

Data from: 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

The Almeria-Oran Front (AOF) is a recognised hotspot of genetic differentiation in the sea. It is a barrier to dispersal and an ecological boundary, which explain the position of 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 genetic structure in the mussel Mytilus galloprovincialis with ancestry-informative loci. 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 but hardly interbreed. This implies some form of reproductive isolation must exist to maintain the two genetic backgrounds locally cohesive. The zone ends with an abrupt genetic shift at a barrier to dispersal in the Gulf of Bejaia. Simulations in models that account for hydrodynamic features of the region support the hypothesis that sister hybrid zones could have been differentially trapped at two alternative barriers to dispersal or environmental boundaries. 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. 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

Unexpected mosaic distribution of two hybridizing sibling lineages in the teleplanically dispersing snail Stramonita haemastoma suggests unusual postglacial redistribution or cryptic invasion

International audienceMolecular approaches have proven efficient to identify cryptic lineages within single taxonomic entities. Sometimes these cryptic lineages maybe previously unreported or unknown invasive taxa. The genetic structure of the marine gastropod Stramonita haemastoma has been examined in the Western Mediterranean and NorthEastern Atlantic populations with mtDNA COI sequences and three newly developed microsatellite markers. We identified two cryptic lineages, differentially fixed for alternative mtDNA COI haplogroups and significantly differentiated at microsatellite loci. The mosaic distribution of the two lineages is unusual for a warm-temperate marine invertebrate with a teleplanic larval stage. The Atlantic lineage was unexpectedly observed as a patch enclosed in the north of the Western Mediterranean Sea between eastern Spain and the French Riviera, and the Mediterranean lineage was found in Macronesian Islands. Although cyto-nuclear disequilibrium is globally maintained, asymmetric introgression occurs in the Spanish region where the two lineages co-occur in a hybrid zone. A first interpretation of our results is mito-nuclear discordance in a stable post-glacial hybrid zone. Under this hypothesis, though, the location of genetic discontinuities would be unusual among planktonic dispersers. An alternative interpretation is that the Atlantic lineage, also found in Senegal and Venezuela, has been introduced by human activities in the Mediterranean area and is introgressing Mediterranean genes during its propagation, as theoretically expected. This second hypothesis would add an additional example to the growing list of cryptic marine invasions revealed by molecular studies. KEYWORDS biological invasion, cryptic species, hybrid zone, introgression, Stramonita haemastoma, Western Mediterranean Se

Shell alterations in Hexaplex trunculus collected in the vicinity of an impacted zone by industrial marine discharges (Gabès, Southern Mediterranean)

International audienceUsing the gastropod Hexaplex trunculus, a comparative study of the shell characteristics (morphometrics, microstructure and elemental composition) was carried out in the vicinity of Gabès city (Chatt Essalem, CE) which is known as seriously affected by industrial pollution, and two control zones (Ghanouche, GH and Teboulbou, TB). Gastropods from CE had larger, thicker and heavier shells, associated with bigger apertures and slighter soft bodies. Scanning electron microscope (SEM) revealed the presence of three shell layers having a crossed lamellar structure. The inner layer was thinner, while the median layer was thicker in snails from CE, suggestive of the ability of H. trunculus to remodel the shell microstructure accordingly. Moreover, the median and inner layers were marked by the presence of fibrous interbeds that were numerous and thicker in shells from CE. In this site, shell elemental alteration was also observed with a decrease in Ca, P, S, K and O contents against an increase in C, Cl and Mg compared to control sites. This investigation was in agreement with previous studies and further suggests the deleterious effect of coastal pollution generated by various industrial units implemented around Gabès city on marine organisms