The Seto Inland Sea in Japan is well known world-widely as one of the famous enclosed coastal sea, which is surrounded on all sides by the three large islands of Honshu, Kyushu and Shikoku, and is the largest inland sea in Japan (area ; 22,000Km2, total coastline ; 6,600Km, average depth ; 38m). In order to obtain the information about the environmental condition in the Seto Inland Sea, the quantity and distribution of macrobenthos, especially Annelida which is considered to indicate the sedimental condition, were investigated from the 425 sedimental samples as the Investigation for Environmental Management of the Seto Inland Sea (IEMS), which conducted from 1991 to 1996.
The results obtained can be concluded as follows:
1) The macrobenthos mainly observed in the Seto Inland Sea were Lumbrineris longiforia, Paraprionospio spp., Prionospio ehlersi and Theora fragilis, which were recognized as the indicator species of organic pollution.
2) 16species of Spionidae were observed, i.e. Paraprionospio sp., Prionospio ehlersi, P. cirrifrea, Spiophanes sp. and so on.
3) The species number and the individual number of Annelida were decreased with increase of muddy ratio, IL, COD, T-N, T-P and TOC.
4) The species number and the individual number of Annelida were decresed with increase of Cd, Cu, Ph, Zn and Mn.
5) Organic pollution was suggested especially in inner bay area because of dominations of some species of Annelida, Spionidae.
