The Brown Ring Disease in the Manila Clam, Ruditapes philippinarum: II. Microscopic Study of the Brown Ring Syndrome

Abstract

Brown ring disease in the manila clam, Ruditapes philippinarum, is a bacterial shell disease characterized by a brown conchiolin deposit adhering to the inner surface of the shell. A Vibrio sp. termed VP1 has been identified as the etiologic agent of the disease. The pathogen has been detected by immunofluorescence techniques in the layers of the deposit. Microscopic modifications of the inner surface of the shell were examined using light microscopy and scanning electron microscopy. In naturally diseased clams, the structure of the deposit appeared very heterogeneous and is made up of a stratification of periostracum, organic fibrous layers, and calcified shell layers. Experimental reproduction 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 types of deposits were distinctly visible: small pustules generally localized on the surface of the inner shell layer and a continuous film observed 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 wider, thicker, and invaded by many bacteria. Microscopic observations showed that since 1 week after challenge with VP1, the experimentally induced signs were similar to the naturally occurring ones. A hypothesis as to the formation of the periostracal deposit is suggested.