Journal of Oceanography, Vol. 66 (No. 3), pp. 329-336, 2010
Haruto Ishii* and Kiyonori Katsukoshi
Tokyo University of Marine Science and Technology, Kounan, Minato-ku, Tokyo 108-8477, Japan
(Received 3 June 2008; in revised form 15 December 2009; accepted 18 December 2009)
Abstract: Natural aggregations of Aurelia aurita polyps were found by direct observation during SCUBA diving on the surface of a pylon in the innermost part of Tokyo Bay, Japan, and changes in polyp abundance with depth were examined from November 2000 to December 2001. Most polyps were observed on substrates in the layer within 1 m from the bottom. Substrates in the upper layer were mainly occupied by other sessile organisms such as the blue mussel Mytilus galloprovincialis, implying that A. aurita polyps may be exposed to interspecific competition for space with other organisms, especially during spring and summer. The bottom layer, especially within 1 m from the bottom, was characterized by summer hypoxia and low recruitment and growth of other sessile organisms such as mussels, resulting in abundant settlement and high survival of A. aurita polyps, which are tolerant of near-hypoxic conditions. It is considered that the occurrence of hypoxic waters due to eutrophication contributes directly to the recruitment of polyps and the consequent production of A. aurita medusae in the following year.