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INTRODUCTION
The endemic species Pinna nobilis (Linneaus, 1758) is the largest bivalve of the Mediterranean Sea; the shell exceeds 1 m in length (Richardson et al., 1999; García-March et al., 2002). Fan mussels have a triangular shape (García-March et al., 2007b) and live partially buried in the substrate (García-March et al., 2002; Richardson et al., 2004). Generally, the sediment covers between one-third (Zavodnik, 1967; García-March et al., 2007b) and one-fifth (Richardson et al., 1999) of the shell, depending on substrate type, its compactness (Zavodnik, 1967) and shell size (Richardson et al., 1999). Fan mussels anchor themselves in the sediment attaching byssus threads to solid structures buried in the substrate (García-March et al., 2007b).
Pinna nobilis is a long-lived species; Butler et al. (1993) suggested a maximum age of 20 years, although a 27-year-old specimen was found (Galinou-Mitsoudi et al., 2006). The species shows one of the fastest shell growth rates (up to 1 mm d-1) recorded for bivalves (Katsanevakis, 2007b). In spite of the high variability of growth curves in the studied Mediterranean populations (Vicente et al., 1980; Richardson et al., 1999; Siletic & Peharda, 2003; Galinou-Mitsoudi et al., 2006; Rabaoui et al., 2007), Pinna nobilis is fast-growing during the first three years, thereafter it shows a much slower growth rate (Katsanevakis, 2005 and literature therein). Pinna nobilis has a highly variable recruitment, both on spatial and temporal scale (Katsanevakis, 2007a). Its larval stage lasts 5-10 days (De Gaulejac & Vicente, 1990) and settlement occurs mostly during late summer and early autumn (Richardson et al., 1999; Katsanevakis, 2007b). Specimens with a shell height up to 20 cm are considered juveniles (Siletic & Peharda, 2003). Knowledge of mortality rates during various ontogenetic phases is scarce: for instance, no data are available on the survival rate of the planktonic stages, although information does exist on the decrease of post larval mortality rate with age, due to the lower vulnerability of large individuals to predation (Fiorito & Gherardi, 1999; García-March et al., 2007b). Higher mortality rates occur during the first year; indeed Katsanevakis (2007b) suggested a 'refuge size' for shell width of about 8...