和歌川河口域における汽水性希少巻貝ワカウラツボ個体群の変動

—2000～2010年—

抄録

The abundance and size structure of a population of an endangered brackish-water snail, Iravadia (Fairbankia) sakaguchii, was surveyed annually in August and September in the Waka River Estuary, Wakayama, middle Japan. The number of snails at the three surveyed stations, comprising a total area of ca. 9.5 m², was greatest in 2000 (1738 snails) and decreased in most successive years to 2009 (7 snails), but increased slightly in 2003 and again in 2010 (27 snails). Few or no small-sized snails were present in any year, which suggests that little or no recruitment of planktonic larvae was taking place. When the population density increased, large-sized snails were responsible, reflecting the immigration of snails from other nearby populations. This phenomenon indicates the importance of maintaining a patchy population distribution for the conservation of this species.