THE BROWN RING DISEASE IN THE MANILA CLAM, RUDITAPES-PHILIPPINARUM .2. MICROSCOPIC STUDY OF THE BROWN RING SYNDROME

Brown ring disease in the manila clam, Ruditapes philippinarum, is a b acterial shell disease characterized by a brown conchiolin deposit adh ering to the inner surface of the shell. A Vibrio sp. termed VP1 has b een identified as the etiologic agent of the disease. The pathogen has been detected by immunofluorescence techniques in the layers of the d eposit. Microscopic modifications of the inner surface of the shell we re examined using light microscopy and scanning electron microscopy. I n naturally diseased clams, the structure of the deposit appeared very heterogeneous and is made up of a stratification of periostracum, org anic fibrous layers, and calcified shell layers. Experimental reproduc tion of the conchiolin deposit allowed us to follow the development of the microscopic modifications of the inner surface of the shell over a 2-month period. One week after inoculation of the pathogen, two type s of deposits were distinctly visible: small pustules generally locali zed on the surface of the inner shell layer and a continuous film obse rved on the surface of the prismatic shell layer. These first deposits were made up of periostracal and degraded fibrous layers. From 1 to 4 weeks after challenge with VP1, the deposit becomes progressively wid er, thicker, and invaded by many bacteria. Microscopic observations sh owed that since 1 week after challenge with VP1, the experimentally in duced signs were similar to the naturally occurring ones. A hypothesis as to the formation of the periostracal deposit is suggested. (C) 199 5 Academic Press, Inc.