ECOLOGICAL STUDIES ON THE JELLY-FISH, AURELIA AURITA (LINNE), IN URAZOKO BAY, FUKUI PREFECTURE-VIII. DIEL VERTICAL MIGRATION OF THE MEDUSA IN EARLY FALL, 1969

Abstract

During the 24 hours from 13:00 on September 9th to 12:00 on September 10th 1969, the diel vertical migration of the population of Aurelia medusae was studied. Sampling of the medusae was made at intervals of about 2 hrs. by means of simultaneous horizontal hauls at the depths 2.5, 7.5 and 12.5 meters of three planktonnets (100cm in mouth diameter, 220cm in length, and stretched with gauze of 10mm meshes), the mouth-ring of a net being fixed directly to respective wire-ropes each with a 30 kg weight. The nets were hauled for ten minutes at a speed of0.5 meter/sec. in the outer-western part of Urazoko Bay, a small cove of Wakasa Bay near the middle of the Japan Sea coast of Honshu Island. The results obtained are summarized as follows: 1. Collected medusae ranged from 4 to 12cm in diameter and with the mean 7.0± 1.88cm and were found immature. They were distributed fairly dominantly in the middle layer around 7 meter deep in the daytime in fair weather, but more than a half of the population seemed to move up to the surface layer around 2 meter deep just before the sunset. After that time medusae began gradually to go down into the middle layer till they reached the bottom layer around 12 meter deep in dark hours. After the sunrise in the next rainy morning, the main part of the population rose from the bottom up to the surface layer. 2. A close correlation was again confirmed between the vertical distribution of medusae and the underwater illumination, while other environmental factors such as the air and water temperature, the wave strength and chlorinity did not seem to have any significant influence on the vertical distribution of medusae. They were collected in the surface layer when the light intensity was around 10^3 Lux, but sank down in either cases when it was fairly decreased below or increased above 10^3 Lux.