In situ population dynamics of Aurelia coerulea polyps in Tokyo Bay

Abstract Long-term changes in the abundance, occurrence period and distribution of Aurelia coerulea polyps were monitored in the innermost (35°37.2′ N, 139°46.4′ E) and mouth regions (35°8.2′ N, 139°37.4′ E) of Tokyo Bay, Japan. The monitoring was performed from January to September 2010 and from January to December 2011, respectively. In the innermost of the bay, the population of A. coerulea polyps was maintained from January to April. However, it rapidly declined in May; by June, it had completely disappeared, losing its attachment space to other sessile organisms such as mussels. Strobilae were observed from January to April, with a peak in March. In contrast, the population of A. coerulea polyps was maintained throughout the year in the mouth of the bay. Strobilae were observed from February to May, with the highest abundance in April The ephyra liberation time was longer in the mouth part than in the innermost of the bay. However, the total number of liberated ephyrae was larger in the innermost part than in the mouth of the bay. These differences were attributed to the substantial variability in water temperature and salinity at the innermost of the bay. The scale of ephyra liberation seems to have been determined by the instability of the water temperature, salinity in the area and the degree of invasion of other sessile organisms in the area. This might be related to the observed medusae blooms in the innermost of the bay.