Blue mussels Mytilus edulis and M. galloprovincialis are the bivalves with the highest production output in Europe (491 000 tons/year; FAO, 2006). To date, European mussel culture relies entirely on wild seed. In the Netherlands, 65 000 tons of seed are needed to produce 100 000 tons of mussels (Kamermans and Smaal, 2002). Seed is fished from natural beds in the Wadden Sea by dredging (scraping mussels from intertidal hard surfaces). Fishing the wild stocks of shellfish in this way is criticized because of the possible damage caused to bottom habitats and resulting food shortages for shellfish-eating birds. In addition to these environmental constraints, the supply of seed shows large fluctuations. It is extremely difficult to collect sufficient amounts of mussel seed each year. As a result, two Dutch companies recently started pilot hatcheries for the production of mussel seed. Spain is the second world producer of mussels and the top European producer, with an output of 300 000 tons per year (M. galloprovincialis), of which 99% is produced in Galicia, where mussels are an important economic and social resource. Cultures in Spain also depend on a seed supply from natural settlement on rocks. This source of seed is becoming less adequate every year, making the development of mussel seed production in the hatchery an essential complement.