Survival and behavior patterns associated with hypoxia at different life stages of the pen shell Atrina cf. japonica.

Pen shell (Atrina cf. japonica) resources have been devastated in Ariake Bay, Japan, and to facilitate the recovery of this species, there is an urgent need to fully understand the factors contributing to its high levels of mortality. Pen shells living in natural waters grow through successive life stages, and environmental factors may affect these bivalves differently at different stages. Accordingly, to elucidate the causes of mortality in natural waters, it is necessary to gain an understanding of the quantitative effects of environmental factors on bivalves at each life stage. In this study, we sought to determine the differential effects of hypoxic conditions on 1-year-old (average shell length: 93.9 mm) and 2-year-old (146.5 mm) pen shells bred under artificial conditions. We exposed shells of each age group to six different dissolved oxygen (DO) concentrations for 96 h and monitored their behavior and survival rate. Based on the survival status, we estimated the lethal DO concentrations that induced 50%, 95%, and 5% mortality (LC50, LC95, and LC5, respectively) at each age. We found that for 1-year-old shells, the LC50 values at 48, 72, and 96 h were 0.51, 0.74, and 0.84 mg/L, respectively, whereas the corresponding values for 2-year-old shells were 0.74, 1.27, and 1.80 mg/L. Furthermore, we found that for 1- and 2-year-old shells, the estimated ranges from LC95 to LC5 at 48, 72, and 96 h were 0.39-0.68, 0.62-0.88, and 0.64-1.12 mg/L, and 0.31-1.75, 0.77-2.09, and 1.29-2.53 mg/L, respectively. Under low DO concentrations (0.47 to 1.93 mg/L and 0.49 to 3.30 mg/L for 1- and 2-year-old shells, respectively), we observed pen shells with more than half of their shell length protruding above the substrate. In addition to age and body size, the 1- and 2-year-old pen shells used in the present study also differed with respect to reproductive status, with 7.6% of 1-year-old and 96.7% of 2-year-old shells considered to be fully ripe. Collectively, our observations indicate that 2-year-old pen shells are less tolerant to hypoxic conditions than are 1-year-old pen shells, and we suspect that the differences in hypoxic tolerance could be attributable to differences in the physiological status of the pen shells during gonadal development. We believe the findings of this study will make an important contribution to enhancing our understanding of the effects of hypoxia on the viability of A. cf. japonica in natural waters.

Extracellular polysaccharide-protein complexes of a harmful alga mediate the allelopathic control it exerts within the phytoplankton community.

The goal of this study was to examine the significance of allelopathy by the raphidophyte Heterosigma akashiwo in a multispecies phytoplankton community in the field. Towards this aim, we sought allelochemicals of H. akashiwo, which had allelopathic effect both in laboratory experiments and in the field. As an initial step, we showed that the allelopathic effects of H. akashiwo filtrate were both species-specific and dependent upon the cell density of the target species. Secondly, we found for the first time that extracellular, high-molecular weight allelochemicals [that is, polysaccharide-protein complexes (APPCs)] were produced by a marine phytoplankton species, H. akashiwo. Thirdly, we indicated that the purified APPCs selectively inhibited the growth of the diatom Skeletonema costatum that is a major competitor of H. akashiwo, and thereby tended to promote the formation of monospecific H. akashiwo blooms. Furthermore, we demonstrated that the inhibitory effect of APPCs on the growth of the diatoms was determined by binding to the cell surface of the target species. Finally, we succeeded in the detection of APPCs in the field samples at concentrations exceeding their experimentally determined action threshold during the H. akashiwo bloom. Strategies for ecosystem control, including mitigation of harmful algal blooms (HABs), should take into account that red-tide organisms like H. akashiwo are already part of complex webs involving inter-specific allelopathic inhibition and ecosystem control during their dense blooms.