Gametogenic asynchrony of mussels Mytilus in a mixed-species area: Implications for management
ABSTRACT Marine mussels Mytilus edulis, Mytilus galloprovincialis, Mytilus trossulus and their hybrids have recently been reported to occur sympatrically in both wild and farmed populations in Scotland. The presence of M. trossulus has led to significant economic losses at some aquaculture sites. Interest in understanding the reproductive cycles of these mussels in a mixed-species aquaculture area, and the potential to use this information to favour cultivation of the preferred species M. edulis, has motivated a seasonal study of the maturation stages of M. edulis, M. trossulus and M. trossulus × M. edulis hybrids, the most frequent genotypes at one site in Loch Etive, Scotland. To investigate larval abundance of these genotypes, plankton samples were collected simultaneously at the same site over the year and analysed using a real-time PCR assay for the identification of Mytilus species-specific alleles. The main spawning events of M. trossulus and M. trossulus × M. edulis hybrids occurred slightly later, but also more frequently and over a longer period than those of M. edulis. M. trossulus alleles were detected in plankton samples throughout the year, always in larger numbers than M. edulis alleles, except during February. Overall, results suggest that the settlement of M. trossulus and/or M. trossulus × M. edulis hybrids may occur during most of the year. This implies that management of the timing of the deployment of settlement ropes to significantly favour the settlement and production of M. edulis is unlikely to be successful. The real-time PCR methodology has provided valuable information on the temporal patterns of abundance of alleles of each species at the planktotrophic stage of development, and demonstrated the potential to identify and quantify Mytilus larvae in plankton samples.