|Budget Amount *help
¥2,000,000 (Direct Cost : ¥2,000,000)
Fiscal Year 1995 : ¥600,000 (Direct Cost : ¥600,000)
Fiscal Year 1994 : ¥1,400,000 (Direct Cost : ¥1,400,000)
(1) Spawning response of oyster under the field and experimental condition
The actual aspects of reproductive cycle and spawning response of oyster, Crassostrea gigas in Matsushima Bay were investigated The spawning of the oyster in Tona and Nonoshima populations in Matsushima Bay was, not synchronous, and was possibly induced by different types of stimuli. Although the oyster which were put in the experimental apparatus, not in natural condition, were observed to spawn very frequently during the breeding season. In addition, spawning of the oyster was induced dose-dependently by the exposure to sperm-suspended seawater or serotonin injection. These facts suggest that the oyster can respond very sensitively to some environmental stimuli and can spawn repeatedly, resulting in synchronous spawning at least in a small population.
(2) Neurosecretory phenomena in the central nervous system (CNS)
Using the neurosecretory staining methods, chrom hematoxylin-phloxine (CH/P) and alcian blue 8GX an
d alcian yellow GXS (AB/AY), the nerve cells in the CNS were classified into five types other than the type of the cells with no specific stainability, indicating the possibility that there were some type s of neurosecretory cells in the cerebral ganglion (CG) and visceral ganglion (VG). The number of cells showing positive reactions to CH and/or AB/AY in the CG was much more than that of the VG.Serotonin immunoreactive cells were also observed in the main part of the CG,and not in VG.The electron microscopical observation revealed the presence of possible neurosecretory cells and suggested axonal transport of granules from the nerve cells. However the relation between neurosecretory cells by electron microscopical observation and those by histochemical observation is not clear.
The number of the CH-positive cells in the right CG showed a clear seasonal change. The annual cycle of neurosecretory activity was synchronized with reproductive cycle. These facts suggsted that the CG may play important roles in the physiological control of the gonadal development and spawning in the oyster.