Abstract
We investigated the histological and morphological processes of sexual maturation in the common estuarine polychaete Simplisetia erythraeensis (Annelida: Nereididae). Seasonal changes in their sexual maturity were also examined at Omoi River estuary in Kagoshima Bay, Japan. Histological sections of sampled individuals revealed that S. erythraeensis is gonochoristic (because specimens had only either oocytes or cell clusters of spermatogenesis) and undergoes extraovarian oogenesis at sexual maturity (because oogenesis was observed in the coelom of mature specimens). Oocytes in mature females possessed a nucleus, a yolk, and a rough-surfaced endoplasmic reticulum-like structure. Female sexual maturation occurred in two stages: F1 and F2. The female F1 stage occurred from March to August, and the F2 stage occurred from July to August when males were filled with cell clusters of spermatogenesis. In addition, the inside portion of the muscular layers of females disappeared with sexual maturity (epitokous metamorphosis) and was replaced by amoebocyte associations around the oocytes (presumably to store nutrition for the oocytes), which enabled the species to spend the majority of its energy for monotelical reproduction. No significant changes were observed in parapodia of F2 in response to epitokous metamorphosis, namely, S. erythraeensis does not seem to engage in reproductive swarming.
