Mussel sampling. Mussels from the NW coast of the Iberian Peninsula belong to the Mytilus galloprovin ci.alis Lm.k form (Sanjuan et al. 1990, 1994). In Novem ber 1998, adult mussels were collected from 2 different environments: a raft-cultured population in an inner cultivation area in the Ria de Arosa (NW coast of the Iberian Peninsula), and a mid-intertidal population in a wave-exposed rocky shore in the same region. Mussels in each of the 2 environments are exposed to very dif ferent physical (temperature, desiccation and turbu lence) and ecological (food availability and predation) conditions. While mussels from the raft-cultured popu lation are permanently submerged in a sheltered and food-rich environment with water temperatures rang ing from 12°C in winter to 19°C in summer (Fuentes et al. 2000), mussels from the wave-exposed mid-inter tidal population withstand daily periods of aerial expo sure during which feeding is impeded and tempera
ture reaches extreme values of near oac in winter and
30°C in summer (Fuentes unpubl. data). The sampled mussels were transported live to the laboratory, where they were dissected. The foot of each mussel was frozen and lyophilised, and then stored at -70°C until electrophoresis