Reproductive season and life span of an estuarine polychaete, Simplisetia erythraeensis (Annelida, Nereididae), in Southern Japan

Abstract

The reproductive season and life span of the common estuarine polychaete Simplisetia erythraeensis (Annelida: Nereididae) was investigated by monthly sampling during the years 2015 and 2016 in two tidal flats at two different regions in southern Japan: Shigetomi in Kagoshima Bay, Kyusyu Island, a warm-temperate region and Ichi on Amami-Oshima Island, Ryukyu Islands, a maritime subtropical region. At both sites, one or two cohorts were distinguished simultaneously using monthly body-width histograms. Recruitment of new cohorts occurred from August in Kagoshima Bay and from July on Amami-Oshima Island. Ovigerous females were observed from March to August in the former area and from April to July in the latter area, with the highest female maturity rate (number of females with oocytes/total number of adults larger than the smallest mature size) in July in both areas. The maximum value of the mean oocyte size of each female increased with increasing female maturity rates. The density of each cohort decreased from May onward because of the death of adults after reproduction, which resulted in the complete disappearance of the cohort in early October. Our results showed that the life span of S. erythraeensis was one year, and the reproductive period occurred in summer.