福岡県曽根干潟におけるカブトガニ幼体の密度と底質の物理環境特性

2017年7月25–27日の大潮最干時刻に福岡県北九州市曽根干潟において，大野川河口，貫川河口，貫川河口−朽網川河口中間点，朽網川河口の底質上に存在するカブトガニ幼体密度を目視よって測定した．幼体は貫川河口付近から沖に向って約300mまでの範囲及び貫川河口−朽網川中間地点にて確認され，その密度は0.063–1.000個体 /m²であった．出現した幼体の前体幅は17.0–60.5 mm（n = 27）であり，第4–8脱皮齢に相当する．粒度分析の結果より，大野川河口は細砂，貫川河口は細粒砂～中粒砂，貫川河口−朽網川河口中間地点は極細粒砂，朽網川河口は極粗粒砂に分類される底質であることが分かった．各調査地点において底質硬度・全硫化物量・強熱減量を測定したところ，いずれも幼体密度との相関はみられなかった．貫川河口域の底質の粒度より算出した中央粒径値は，2016年7月16日に同地点で採取した底質の中央粒径値とは大きく異なっており，貫川河口域での底質環境の変化が示唆された．

The density of juveniles of the tri-spined horseshoe crab Tachypleus tridentatus (Leach, 1819) was measured at the lowest tide during the period from 25 to 27 July 2017 at the Sone Estuary in Kitakyushu City, Fukuoka Prefecture. It ranged from 0.063 to 1.000 indiv./m². The highest density was recorded at the mouth of Nuki River. The range of carapace width of juveniles was 17.0–60.5 mm (n = 27), corresponding to the 4th to 8th molts. No juveniles were found at the mouth of the Ono River. Median diameter of sedimentary particles at the habitat area ranged from 0.020 to 0.740 mm. Hardness, total sulfide content and ignition loss were simultaneously measured at each station, but no significant correlation was detected with the density. The median particle size was significantly different from that calculated at the same stations on 16 July 2016, suggesting that the sedimentary environments for the habitat of juveniles in Sone estuary were drastically changed within a year.