The venerid bivalve Phacosoma japonicum (Reeve, 1850) shows north-south gradients in the annual shell growth and sexual maturity patterns among the Japanese populations. However, one southern population from Ariake Bay, Kyushu, does not fit with this general trend, and life-history traits of this population are comparable to those of the northern population in Hakodate Bay, Hokkaido. In this study, to understand the genetic and environmental factors responsible for the life-history traits of this species, transplant experiments and analyses of monthly shell growth and reproductive cycles were conducted on the populations from Ariake Bay and its neighboring regions. A total of 128 living individuals were transplanted from the population in Tokyo Bay (central Japan) and those in Ariake and Kagoshima Bays (southwestern Japan) to Aburatsubo Cove, Sagami Bay (central Japan). Follow-up studies of the transplanted individuals for 3 years revealed that both annual and seasonal shell growth patterns of most individuals did not differ from those of animals in the habitats of origin. This fact suggests that the mode of shell growth of this species is not only controlled by environmental factors but also has some genetic background. Analysis of the seasonal patterns of growth and reproductive cycles revealed that shell growth and gonad development were active from winter to early spring in individuals from Ariake Bay, whereas growth and reproduction occurred from late spring to summer in individuals from Tokyo and Kagoshima Bays. Seasonal changes in water temperature and salinity and also population density were similar among Ariake Bay and its neighboring regions. However, phytoplankton becomes most abundant in winter in Ariake Bay, in contrast to most other bays of central and southern Japan, including Tokyo and Kagoshima Bays, where phytoplankton flourishes in summer. These facts suggest that the growing seasons for both shell growth and reproduction of this species are strongly influenced by the annual pattern of food availability, and the geographical variations of annual shell growth and sexual maturity patterns can be explained by the difference in mean water temperature during the growing season among them.
|出版ステータス||Published - 1999 1月 4|
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