Impact of season and rearing site on the physiological and immunological parameters of the Manila clam Venerupis (=Tapes, =Ruditapes) philippinarum

Juvenile clams were distributed in four rearing sites selected for their varied ecological characteristics to assess the effects of environmental conditions on the physiological and immunological parameters, and Brown Ring Disease (BRD) status. Clams were sampled every 3 months for 15 months at each site. Brittany rearing sites, especially the Bay of Brest, showed the worst performances in terms of immunological and physiological indices and disease status, while the best were obtained in Marennes ponds. When the health of the clams was compared to assess seasonal effects, the winter clearly was a stressful period. A combination of bad rearing site and winter conditions led to major mortalities in the Bay of Brest in February. In other sites, winter mortalities were low. Condition index, total haemocyte count and haemocyte size were greatly affected by seasonal variation whereas haemocyte complexity and lysozyme content were more affected by the location of the site. Growth and haemolymph protein content were affected by both season and sites. Linear regressions between and within the physiological and immunological parameters indicated that large haemocyte size was related to low total haemocyte count (THC) and low haemocyte mortality. This relationship suggests a reduction in the cell division rate. Total haemocyte count and protein and lysozyme concentrations were positively correlated to the condition index. Seeded clams showed very low BRD prevalence in all sites and for all seasons; however, high prevalence was observed in natural stocks from one of the studied sites (Gulf of Morbihan), suggesting that hatchery-seeded clams may be more resistant to BRD and may be worthy of further studies. (C) 2004 Elsevier Science B.V. All rights reserved.