Abstract
The present study aimed at examining a possible role of low salinity in causing mass mortalities of the Manila clam Ruditapes philippinarum in culture areas. Laboratory experiments were conducted on the effect of low salinity on survival, hemolymph osmolality and tissue water content at 25 °C. Also, seawater salinity was monitored from June 2003 through October 2005 in a culture area in Isahaya Bay, Kyushu, Japan.
No clam died during 96 h exposure to 15 psu, while substantial mortalities occurred below 10 psu. When clams were exposed to salinities below 20 psu, hemolymph osmolality declined gradually and became iso-osmotic with the external media after 48 h. In contrast, forced opening of valves resulted in a rapid reduction of hemolymph osmolality. Thus, clams appear to maintain hemolymph osmolality by closing the valves when exposed to low ambient salinities. Tissue water content increased from 78.3-80.6% to 81.4-88.3% during 72-96 h exposure to 22.5-10 psu. Salinity remained over 25 psu when mass mortalities occurred in 2003 and 2004 in the culture area. These results led us to conclude that low salinity was not a major cause of the mass mortality in the two years.
