Field and laboratory observations on the hypoxic impact on survival and distribution of short-necked clam Ruditapes philippinarum larvae in Tokyo Bay, central Japan

抄録

There are few descriptions of the impact of hypoxic conditions on short-necked clam Ruditapes philippinarum larvae in the natural environment, while a number of reports describe catastrophic mortality of benthic clams caused by coastal upwelling of hypoxic water in the eutrophicated bays in Japan. We conducted seasonal observations of larval density in the water column along with measurement of DO at coastal and off-shore stations in Tokyo Bay during 2001–2003, and also experimental exposure of the clam larvae to low DO to examine the impact of hypoxic water on larval survival. Under hypoxic conditions (<1.0 mgO₂ L⁻¹) in the bay, a marked decline in larval density was consistently detected in the bottom layer at coastal stations adjacent to adult habitats and at off-shore stations of the central bay. In the low DO exposure experiment, the ratio of swimming larvae decreased with exposure period. Larval survival declined within 12 h, and most of the larvae at early developmental stage (shell length <170 μm) died within 24 h under severe hypoxic conditions (<0.2 mgO₂ L⁻¹) at 25°C. These results indicate that larvae being transported to the hypoxic zone may have reduced swimming ability and may inevitably sink to the lower layer and die shortly thereafter. In Tokyo Bay, anoxic or hypoxic conditions in the lower layer during May–October are continuous, and this time period overlaps with the entire spawning period of the short-necked clam. Hypoxic water exerts critical stress on larval survival, and may result in decline of juvenile recruitment to the benthic populations of short-necked clam in Tokyo Bay.