Human‐mediated transport creates secondary contacts between genetically differentiated lineages, bringing new opportunities for gene exchange. When similar introductions occur in different places, they provide informally replicated experiments for studying hybridisation. We here examined 4,279 Mytilus mussels, sampled in Europe and genotyped with 77 ancestry‐informative markers. We identified a type of introduced mussels, called “dock mussels,” associated with port habitats and displaying a particular genetic signal of admixture between M. edulis and the Mediterranean lineage of M. galloprovincialis. These mussels exhibit similarities in their ancestry compositions, regardless of the local native genetic backgrounds and the distance separating colonised ports. We observed fine‐scale genetic shifts at the port entrance, at scales below natural dispersal distance. Such sharp clines do not fit with migration‐selection tension zone models, and instead suggest habitat choice and early‐stage adaptation to the port environment, possibly coupled with connectivity barriers. Variations in the spread and admixture patterns of dock mussels seem to be influenced by the local native genetic backgrounds encountered. We next examined departures from the average admixture rate at different loci, and compared human‐mediated admixture events, to naturally admixed populations and experimental crosses. When the same M. galloprovincialis background was involved, positive correlations in the departures of loci across locations were found; but when different backgrounds were involved, no or negative correlations were observed. While some observed positive correlations might be best explained by a shared history and saltatory colonisation, others are likely produced by parallel selective events. Altogether, genome‐wide effect of admixture seems repeatable and more dependent on genetic background than environmental context. Our results pave the way towards further genomic analyses of admixture, and monitoring of the spread of dock mussels both at large and at fine spacial scales.

Growth of the mussel, Mytilus edulis, was compared for the first time under three culture regimes, longline located in open sea, pole and on-bottom both situated on an intertidal flat, in the Pertuis Breton, on the Atlantic coastline of France. Mussel sampling was performed on a monthly basis over 1 year while monitoring of hydrobiological parameters was conducted on a biweekly basis. Fluctuation of environmental parameters showed a similar pattern on both sea and shore locations with generally higher concentration levels for the intertidal area compared with the open sea, i.e. 4.07 and 3.17 mg l −1 for particulate organic matter, 3.16 and 2 µg l −1 for chlorophyll-a, 58.43 and 38.72 mg l −1 for inorganic-N, respectively. A clear seasonal growth pattern was observed, being similar for all three cultural conditions. A gradient of length and weight growth appeared as a function of the culture type. Longline mussels exhibited the highest performance while Bottom-type culture showed the lowest. An emersion time of approximately 26% was estimated from the temperature record of the Pole station during the period of maximal growth. This could partly explain the reduced growth in length on Pole compared to Longline. While growth was faster in Longline culture and condition index were better for Pole culture, further data on the carrying capacity of the area are needed for the establishment of a mussel culture extension policy.

Clearance rate (CR) was measured in blue mussels Mytilus edulis L. from Aiguillon Bay and the Oosterschelde using 3 different methods: the flow-through method, the bio-deposition method and the indirect or clearance method. CR differed significantly as a function of the method used and of the origin of the mussels. CR measured with the bio-deposition method were significantly lower than rates measured with the other methods. Results for the flow-through method depended, however, on how CR was calculated. CR using the flow-through and indirect methods was on average 10.0 l g -1 h -1 in mussels from Aiguillon Bay and 5.3 l g -1 h -1 in mussels from the Oosterschelde. The significantly lower CR of mussels from Oosterschelde was related to condition index and gill area, but could not entirely be explained by these factors.KEY WORDS: Clearance rate · Intercalibration · Method · Mussels · Gill area · ConditionResale or republication not permitted without written consent of the publisher † This paper is written in memory of Serge Bougrier, who died tragically during the project.

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