瀬戸内海におけるムシロガイ科腹足類3種に共生するヒドロ虫類の生態学的研究

広島県竹原市近海において潮間帯に生息する腹足類アラムシロにエボシクラゲ属の1種，潮下帯に生息する腹足類ムシロガイ，キヌボラにそれぞれタマクラゲ，ミサキアミネウミヒドラのポリプが共生し，これらの調査期間中の共生率はそれぞれ9.1%，93.8%，83.8％であることが明らかとなった。これらの組合せには例外がみられず，宿主特異性は高かった。エボシクラゲ属の1 種の生殖体は6，9月，タマクラゲは8月のみ出現していたが，ミサキアミネウミヒドラは4–7，9，11，12月と頻繁に出現していた。また，ムシロガイ，キヌボラに比べて，アラムシロが底質中に潜行している時間は長い傾向が認められた。3種のポリプのうち，エボシクラゲ属の1種のみ厚い囲皮を備えるが，この形態は宿主が潮間帯に生息するため，干出による乾燥への適応であるかもしれない。また，このことが本種の共生率が低い原因の一つとして考えられる。

Polyps of three hydrozoan species associated with three gastropod species of the family Nassariidae were observed in the Seto Inland Sea, Japan: (1) Leuckartiara sp. with Nassarius festivus occurring in the intertidal zone as well as (2) Cytaeis uchidae with N. livescens and (3) Stylactaria misakiensis with N. japonicus occurring in the sublittoral zone. Each hydrozoan species was highly host-specific, without any exceptional combination. The host utilization rates of the three hydrozoan species were markedly different; as such, the host utilization rate of Leuckartiara sp. occurring in the intertidal zone was less than 10%, whereas that of C. uchidae and S. misakiensis occurring in the subtidal zone ranged from 84% to 94%. Sexual reproduction was limited to summers in Leuckartiara sp. and C. uchidae, but it occurred yearround in S. misakiensis. The burying behavior of host shells differed among the three species; specifically, the intertidal N. festivus was buried in the sediments for over 20 h a day, while the subtidal C. uchidae and S. misakiensis were buried for less than 7 h a day. The ecological differences among the examined hydrozoan species associated with specific nassarrid hosts likely strengthen their niche segregation.