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Beiras a, * , N. Fern andez a , J.J. Gonz alez b , V. Besada b , F. Schultze b a Departamento de Ecolox  ıa e Biolox  ıa Animal, Facultade de Ciencias, Universidade de Vigo, E-36200 Vigo, Galicia, Spain b Instituto Espa~ nol de Oceanograf  ıa (IEO), Cabo Estay, Canido, E-36200 Vigo, Galicia, Spain Accepted 19 October 2001 National programmes of surveillance of the marine environment usually involve the chemical analysis of sediments and sessile organisms, typically filter-feeding bivalves such as mussels (e.g., Anonymous, 1990; O’ Connor, 1998). These matrices act as a surrogate for the pollutant concentrations in the water with important advantages. First, they integrate variations in time, and secondly, they accumulate most pollutants at orders of magnitude higher than those found in the water column. Therefore their use makes analytical determination of potential toxicants both more reliable and technically feasible. Nevertheless, little effort is commonly invested in studying the correlation between the levels of pollu- tants present in the different environmental matrices, despite the fact that if this correlation is low the choice of matrix will affect the conclusions drawn from the moni- toring programme. Some authors have warned about the lack of correlation between biological and mineral matrices (Arjonilla et al., 1994; Hayes et al., 1998). Mercury is the most toxic trace metal for marine fauna, and the only one responsible for cases of human lethal intoxication in marine ecosystems. It is listed as a priority pollutant by the international agencies in charge of ma- rine environmental protection and its analysis is included in most monitoring programmes. Different species of marine mussels have been selected as indicator organisms world wide because of their abundance, ubiquity, sessile nature, long life span, high filtration rates, and especially, 0025-326X/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0025-326X(01)00280-6 * Corresponding author. Tel.: +34-986-812-648; fax: +34-986-812- 556. E-mail address: rbeiras@uvigo.es (R. Beiras). Baseline / Marine Pollution Bulletin 44 (2002) 340–349 345