Mactra veneriformis, Ruditapes philippinarum, and Meretrix lusoria are dominant clams in Japanese tidal flats. Juveniles and adults of these species were reared in the laboratory and the effects of salinity on their sand burrowing activity and growth- and clearance-rates were examined. Juveniles of each species (shell length 10–16 mm) filtered water actively in the salinity range of 11.8–34.6 psu, with clearance rates not affected by salinity. The clams showed positive growth in the above salinity range, but growth rates were retarded at 11.8 psu, probably because of increased respiration at low salinity levels. In 6.1-psu seawater, adult Ma. veneriformis dug into the sand after a few days acclimation, Me. lusoria did not and R. philippinarum died. At 10.8 psu, adult Ma. veneriformis filtered water actively, Me. lusoria dug into the sand but did not filter water, and R. philippinarum neither dug into sand nor filtered water. These results indicate that the clam species examined are euryhaline but that the response to low-salinity water (≤11 psu) by adult clams (shell length 31–37 mm) differed among species: Ma. veneriformis is the most adaptable to low-salinity water, followed (in order) by Me. lusoria and R. philippinarum. These differences, however, are not consistent with the distribution patterns of these clam species in the Shirakawa tidal flat (Japan), where salinity varies spatially and temporally.
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Vol. 24 • No. 4