東京湾多摩川河口域におけるハサミシャコエビおよびニホンスナモグリの浮遊幼生の分布特性（短報）

抄録

Distributions of planktonic larvae of the burrowing mud shrimp Laomedia astacina and the ghost shrimp Niphonotripaea japonica were observed in the estuary of Tama River in Tokyo Bay by longitudinal section samplings in August, 2010 and 2011 and by a 24-hour-sampling at the river mouth in the August, 2006. Larvae of L. astacina were abundant in the higher salinity layer (S > 20), with the exception of the hypoxic layer (DO < 1 mg L^-1), in the estuarine waters. The high proportion of zoea 3–4 stages in the total larvae of L. astacina and the low outflow flux at the river mouth suggest that the planktonic phase of this species is completed within estuarine waters. Larvae of N. japonica, including developed stages, were distributed consistently in the layer of the bay waters deeper than 3 m, excluding the hypoxic layer, in both 2010 and 2011. Larvae of this species occurred in the estuarine waters as well as L. astacina in 2011, but disappeared mostly from estuarine waters in 2010 when the hypoxic layer was well-developed near the bottom. The absence of stages past the zoea 2 stage of N. japonica in the estuarine waters and the high outflow flux of the larvae to the bay at the river mouth suggest that the main larval habitat is not estuarine waters but bay waters. The lower limits of the vertical distribution of planktonic larvae were influenced by the development of bottom hypoxia in both species. Especially in the estuarine waters, bottom hypoxic water may be an important factor controlling the retention and the outflow of these larvae.