Ephemeral Occurrence of the Echiuran Listriolobus brevirostris (Annelida: Echiura) in Osaka Bay between 1995 and 2002; a New Record for Japan, Probably Resulting from Human-mediated Introduction

抄録

The primarily Indo-West Pacific echiuran Listriolobus brevirostris Chen and Yeh, 1958 was collected, sometimes abundantly, from benthos samples from northeastern Osaka Bay (from 6.4–17.5 m depth) between 1999 and 2001, or recorded in the field between 1995 and 2002. Since 2003, it has remained uncollected. These records were shown to represent the first occurrence of L. brevirostris in Japanese waters, being the first reliable evidence so far determined of the introduction of an exotic echiuran species, probably in ballast water, to any locality worldwide. Aquarium observations suggested a completely infaunal adult life mode, including feeding activity. Monthly changes in size distribution of specimens collected in 1999 indicated successful summer survival under oxygen-depleted bottom conditions in Osaka Bay. Factors behind the ephemeral occurrence of L. brevirostris in Osaka Bay were briefly considered in relation to annual fluctuations of dissolved oxygen content in bottom water, although the driving forces behind such occurrence remain to be established.