Ikeda taenioides (Ikeda, 1904) (Annelida: Echiura: Ikedidae) is the largest spoon worm in the world and has been recorded only from Japanese coasts. This species was described by Ikeda in 1904 and then collected from various localities in Japan by the middle of the 20th century. However, this species has dramatically decreased in number because of habitat destruction and human activities; it has not been recorded recently, except for in Kagoshima Bay, the Tohoku district, and the Seto Inland Sea. Thus, a field survey is required to reevaluate its current distribution. In this study, we collected proboscises of I. taeniodes using a yabby pump in a mud flat in Tanabe Bay (Wakayama Prefecture) located in the southwestern part of the Kii Peninsula, Japan. We also sampled a proboscis of I. taenioides using a Smith-McIntyre grab sampler at a depth of 25 m in the mouth of Maizuru Bay (Kyoto Prefecture), the Sea of Japan. The former and latter samples update the records of I. taenioides from the Kii Peninsula and Sea of Japan for the first time in 70 and 51 years, respectively. Moreover, we also observed a proboscis, most likely I. taeniodes, coming out of a burrow entrance at a water depth of 21 m off Kuninao, Amami-Oshima Island (Kagoshima Prefecture), Japan, which may update the southern limit of the distribution of this species. The specimens from Kii Peninsula and Maizuru Bay were genetically identified to be I. taenioides by comparing with those from the Seto Inland Sea and Tohoku district by a COI marker. Our findings suggest that I. taenioides may be still broadly distributed along the coasts of Japanese Islands, from Mutsu to Ryukyu Islands, both in the Pacific and Sea of Japan sides.
Temporal changes in benthic amphipod assemblages after the dike closure in Isahaya Bay were investigated using amphipod samples collected from the central part of the Ariake Sea between 1997 and 2002. Amphipod densities decreased every year from June 1997 (5,207 ind. m−2) to June 2001 (413 ind. m−2) but increased abruptly in June 2002 (22,944 ind. m−2). Non-metric multidimensional scaling (nMDS) ordination of sampling occasions showed that amphipod assemblages were divided into three groups (A: June 1997 and June 2002; B: November 1998, June 1999 and June 2000; C: November 2000, June 2001 and November 2001). Group A was dominated by species of Corophiidae and Photidae, and group B and C were dominated by species of Ampeliscidae. In terms of total amphipod density, the order was as follows: group A>group B>group C. These differences among groups A–C indicate that corophiid and photid species contribute significantly to the fluctuation of amphipod density and assemblages in the central part of the Ariake Sea. In fact, an explosion of the amphipod density that occurred in June 2002 was attributed to rapid increases in the populations of Crassicorophium sp. A, Crassicorophium crassicorne and Photis reinhardi. Judging from the changes in the bottom sediment grain size, we conclude that the grain-size reduction on the east side of the study area led to a rapid increase in the density of these opportunistic species.
Eleven genera of chironomid larvae were identified in sediments of the Isahaya Bay reservoir in western Kyushu, Japan from 1998 to 2018. Since 2005, the dominant genera collected at designated sampling sites were Chironomus and Microchironomus. Larval densities of these two genera were highest in 2011 and 2012. Consequently, adult emergence was also highest in these years. Increased reproduction in these two genera is a result of eutrophication, silty sediment deposition, and algal bloom in the reservoir.
The inner region of Isahaya Bay was isolated from the Ariake Sea, due to the construction of a dike, on April 14, 1997. We monitored water quality and oxidation-reduction potential (ORP) of bottom sediments in the reservoir for 21 years, from March 1997 to June 2018. We also analyzed the variation in community structure of macrobenthic fauna using sediment samples collected at 16 permanent stations and 8 temporary stations. After the seawater inflow was shut-off, mean individual densities of macrobenthic animals rapidly declined from 1997 to 1999, and then only a few species, such as Potamocorbula sp. and Sinocorophium sinensis greatly increased in their populations from 2001 to 2002. However, from 2003 to 2018, most of them decreased rapidly in the inner region of Isahaya Bay. Our results suggested that, interannual variation in macrobenthic fauna is affected by variation in water quality and bottom sediments.