Haemocyte parameters associated with resistance to brown ring disease in Ruditapes spp. clams

Brown ring disease (BRD) is a shell disease caused by Vibrio tapetis. This pathogen disturbs the periostracal lamina causing the appearance of a brown conchiolin deposit on the inner face of the shell, within the extrapallial space. Although differences in resistance to BRD have been documented, the ir relationship to possible defense functions has never been investigated. In this study, Row cytometry was used to analyze cellular parameters in asy mptomatic and experimentally infected Ruditapes philippinarum from France a nd the west coast of the USA. Parallel analyses were made on Ruditapes decu ssatus. the native European clam, which is highly resistant to BRD. In the haemolymph and extrapallial fluid of animals without BRD, total haemocyte c ounts, the percentage of granulocytes. and the phagocytic activity against latex beads or V. tapetis by the haemocytes were significantly higher in Am erican R. philippinarum than in French R. philippinarum. In most cases, lev els in R. decussatus were the highest of all three groups. Four weeks follo wing challenge with V. tapetis, BRD prevalence reached 52 in American clams and 100% in French specimens, but only 37% in R. decussatus. In symptomati c animals, phagocytosis of V. tapetis increased significantly in the resist ant species of clam. R. decussatus, was unchanged in US clams, and decrease d significantly in FR specimens when compared to asymptomatic individuals f rom each population. Ingestion of V. tapetis by haemocytes in the extrapall ial fluid, which is in contact with the periostracal lamina, could be the m ain defense mechanism used to counter the pathogen, Our results suggest tha t resistance to BRD may well be related to the concentration of granular ha emocytes and the phagocytic activity of haemocytes. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.