汽水棲巻貝ワカウラッボ(腹足綱:ワカウラッボ科)の分布に関係する要因

抄録

Factors affecting the spatial distribution of the brackish water snail Iravadia (Fairbankia) sakaguchii were investigated in the field in the Waka River Estuary (34°10'N, 135°10'E) of central Japan and in the laboratory. The snails occurred under stones partially buried on intertidal muddy flats. A tracking survey of marked snails released beneath stones revealed that many of the snails moved among the stones. Observations in tide-simulating tanks indicated that most of the snails came out from beneath the stones and were active during both daytime and nighttime submergence. Salinity tolerance tests in the laboratory showed that the snails died within 6 days at 0 psu, 7 days at 5 psu, and 12 days at 10 psu, whereas no snails died during 13 days of exposure to higher salinity conditions (20-35 psu). Neither immigration nor retention rates of snails differed significantly between partially buried stones and non-buried stones artificially placed in the field. This indicates that the extent of burial of the stones is not critical for habitat selection by the snails. Preference tests conducted in the laboratory and in the field revealed that the snails prefer mud to sand under the stones. Retention and survival rates were lower for snails released with stones in the lower intertidal zone than for those released with stones in the middle intertidal zone, and this suggests the importance of tidal exposure in habitat selection of I. (F) sakaguchii.