Defense factors in clam extrapallial fluids

The extrapallial fluid of clams is located between the external epithelium of the mantle and the inner face of the shell. The pallial line separates the fluid into central and peripheral extrapallial fluids (CEF and PEF, respectively). These fluids are the sites of biomineralization, but their role in other biological processes, such as defense, remains unknown. A comparative study of some characteristics of the cellular and humoral fractions of the extrapallial fluids and hemolymph of the Manila clam Ruditapes philippinarum was carried out. Both CEF and PEF contained abundant hemocytes comparable to those found in hemolymph. Total and viable hemocyte counts were high especially in the CEF. Extrapallial fluid cell lysate and supernatant showed significant bacteriolytic (lysozyme-like) activity. The CEF was found to be bacteriologically sterile. This sterility may result from the presence in the CEF of significant defense factors in the form of numerous macrophage-like cells and high lysozyme activity. These findings are of interest in the case of bivalves susceptible to shell diseases such as R. philippinarum, which is susceptible to a bacterial disease caused by Vibrio tapetis.