粗粒・細粒砕屑物による急速埋没に対するナメクジウオBranchiostoma belcheriの耐性

概要

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Although populations of the lancelet Branchiostoma beicheri are said to have been declining in Japan, many specimens of the lancelet were collected from shallow marine, relatively mud-free, sand-to gravel-grade clastic bottoms, off Naka island, western part of the Seto Inland Sea. Tolerance of the lancelet to rapid burial with coarse-(1.0-2.0mm in diameter) and fine-grained (less than 0.125mm in diameter) clastics was determined through laboratory experiments. All the individuals were killed by only 3cm thick burial with the “exotic” fine-grained clastics, but some could tolerate up to 19cm thick burial with the “native” coarse-grained ones. This suggests that the lancelet has low tolerance to a rapid burial with fine-grained clastics. Coastal development such as offshore mining and dredging, which agitated the seafloor and induced mud-draping around there, is one of possible causal factors for population decline of the lancelet.