両側回遊種ミゾレヌマエビにおけるデモグラフィの河川流程間変異

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  • Upstream and Downstream Variation in Demography of the Amphidromous Shrimp Caridina leucosticta
  • リョウガワ カイユウシュ ミゾレヌマエビ ニ オケル デモグラフィー カセン リュウテイカン ヘンイ

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To examine upstream dispersal and the resultant upstream-downstream variation in demography of the amphidromous shrimp Caridina leucosticta Stimpson, 1860, we collected shrimp from six sites along the lower to middle reaches of the Murasaki River in Kitakyushu city monthly over one year. Spatio-temporal variation in carapace length distribution revealed that recruitment occurred from August to November. Newly recruited individuals finished their upstream migration by winter. The population density decreased gradually after winter, but the rate of decrease was uniform among sites. This suggests that there is neither intensive migration up- or downstream nor variation in survivorship among sites after overwintering. The shrimp grew rapidly from April to August, but growth rates weremuch higher in females than males. Females that migrated to upstream sites kept growing for longer periods than those remaining in downstream ones, resulting in larger body sizes and therefore higher fecundity for the former. Moreover, a higher proportion of ovigerous females was found in upstream sites during the peak of the reproductive season (June to October), which suggests a higher fertility upstream. Such intrapopulation variation in growth and fertility seems likely to have been caused by environmental differences among sites. The proximate factors for this were not clear, but the fact that growth and reproduction performances farther upstream were higher suggests that the upward migration of this shrimp is adaptive.

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