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The venerid clam Ruditapes philippinarum is the most prominent suspension-feeding bivalve inhabiting muddy intertidal seagrass beds in Arcachon Bay (SW France). It is exploited by fishermen, and Arcachon Bay ranks number one in France in terms of production and total biomass of this species. Previous studies revealed a decrease in the standing stock of R. philippinarum since 2003 and unbalanced length-frequency distributions with a lack of juveniles and of adults > 40 mm. Consequently, the population dynamics of this bivalve were studied at four intertidal sites and one oceanic site in Arcachon Bay. As clam size structure did not allow classical dynamics computations, field monitoring was coupled with field experiments (tagging-recapture) over two years. Monitoring of condition index and gonadal maturation stages highlighted a high variability in spawning number and intensity between sites. Recruitment events in the exploited area varied spatially but with uniformly low values. Von Bertalanffy Growth Function (VBGF) parameters (K, L ∞) were determined using Appeldoorn and ELEFAN methods. In the exploited sites in the inner lagoon, K was relatively high (mean = 0.72 yr − 1) but L ∞ was low (mean = 41.1 mm) resulting in a moderate growth performance index (Φ′ = 2.99). Growth parameters were not correlated with immersion time and L ∞ was different between sites. Comparison of mortality coefficients (Z) between cage experiments and field monitoring suggested that fishing accounts for 65-75% of total adult mortality. Low recruitment, a low growth rate and a normal mortality rate led to low somatic production (4.1 and 8.7 g Shell-Free Dry Weight (SFDW) m − 2 yr − 1) and an annual P/B ratio from 0.44 to 0.92 yr − 1. Under current conditions, the possibility of a sustainable population in Arcachon Bay will strongly depend on recruitment success and fishing management.

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We describe an emerging pathology, brown muscle disease (BMD), which specifically affects the Manila clam Ruditapes philippinarum in Arcachon Bay (France). BMD induces a transformation of the posterior adductor muscle, which becomes infused by conchiolin and calcified, reducing the ability of clams to bury. The disease affects both types of muscular tissue, with striated muscle becoming affected to a higher degree than smooth muscle. Two indices were created to quantify the symptoms: the Muscle Print Index, used for empty and live shells, and the Final Disease Index, utilized for live clams only. Histological sections were made and observed under light microscopy to examine the muscular damage and to investigate a causal agent. Sections revealed an important inflammatory response with a large invasion of hemocytes into tissues and a heavy necrosis of muscular fibers. Additionally, molecular biology analyses were carried out to search for bacteria and protozoan agents using generic primers. In both histological and molecular assays, bacteria and protozoans were discounted. We monitored 4 sites scattered around the bay over 2 yr. The mean prevalence was <12% without seasonal variation in 3 sites against 30% and a winter peak in 1 site. The latter site was accurately surveyed and revealed that clams at the sediment surface (abnormal position) were affected 3 times more frequently than buried clams (normal position). KEY WORDS: Brown muscle disease · Clam · Ruditapes philippinarum · Adductor muscle · Pathology · Light microscopy Resale or republication not permitted without written consent of the publisherDis Aquat Org 80: [219][220][221][222][223][224][225][226][227][228] 2008 tracal lamina and causes an obvious abnormal conchiolin deposit on the inner side of the shell, forming a characteristic brown ring (Paillard 1992, Paillard & Maes 1995a. This disease led to mass mortalities in Brittany (NW France) in 1987 and decimated stocks of cultured clams (Paillard 2004). Subsequently, BRD was observed along the French Atlantic coast and in other countries including Spain, Ireland, England, Italy and Korea (Paillard 2004, Park et al. 2006). Perkinsosis, caused by the protozoan Perkinsus sp. affects numerous molluscan species all over the world and can lead to mass mortalities (Azevedo 1989, Burreson & Ragone Calvo 1996, Goggin 1996, Da Ros et al. 1998, Park & Choi 2001, Leite et al. 2004, Cremonte et al. 2005, Villalba et al. 2005. In Korea, this parasite has been the cause of a severe decrease in clam populations since 1993 (Park & Choi 2001). Previous studies in Arcachon Bay revealed that clams could be infected by brown ring disease, perkinsosis (Lassalle et al. 2007) and also by digenean trematodes (de Montaudouin et al. 2000). However, prevalence and infection intensity were relatively low.Mortalities have been evident in Arcachon Bay in recent years, particularly during winter. A preliminary study revealed a previously undescribed necrosis of the posterior adductor muscle. Macroscopic diagnosis under...